ORIGINAL PAPER

Fish assemblage patterns in the Elbe estuary: guild composition,spatial and temporal structure, and influence of environmentalfactors

Dennis Eick & Ralf Thiel

Received: 26 August 2013 /Revised: 19 February 2014 /Accepted: 1 April 2014# Senckenberg Gesellschaft für Naturforschung and Springer-Verlag Berlin Heidelberg 2014

Abstract There is still a great lack of information regardingspatial and temporal gradients of quantitative guild composi-tion of the fish fauna and their relation to environmentalvariables in European estuaries. To fill parts of this gap inknowledge, fish species densities in the Elbe estuary wereestimated with high resolution using samples taken by acommercial stow net vessel monthly between April 2009and October 2010. Altogether, 61 fish species as well ascyprinid hybrids were recorded. A high number of thesespecies belong to the marine life cycle categories of marinestragglers and marine estuarine-opportunists. However, allmarine species contributed only 0.4 % to mean total fishabundance, whereas the anadromous species reached a pro-portion of more than 98 %. The total contribution of freshwa-ter species to the overall catch was 1.4 %. The mean total fishabundance was about 362,989 individuals 10−6 m−3 betweenApril 2009 and October 2010. According to their abundanceproportions, the fish fauna was dominated by the followingthree species: Osmerus eperlanus (96.1 %), Alosa fallax(1.9 %), and Gymnocephalus cernua (0.9 %). Twenty-ninepercent of the species had a boreal distribution and 27.4 %belonged to the lusitanian distribution category, whereas aminority of 3.2 % of the species were of Atlantic origin. Anadditional 40.4 % of the species were freshwater specieswhich were divided into the following five zoogeographicalcategories: European (19.3 %), Euro-Siberian (8.1 %),Holarctic (6.4 %), Nordic (3.3 %) and Pale-Arctic (3.3 %).The species composition underwent interannual changescomparing both years of investigation at each of thesampling sites in the Elbe estuary. Using the Bio-Envanalysis, the maximum correlation between fish assemblage

structure and environmental variables was obtained for aparameter combination comprising salinity, water depth, wa-ter temperature, and oxygen concentration.

Keywords Elbe estuary . Fish assemblage . Spatial structure .

Temporal structure . Guild composition . Environmentalfactors

Introduction

Estuaries are coastal waters comprising a complex of differenthabitat types, where salt- and freshwater meet, where abruptchanges in salinity, temperature, oxygen, and turbidity exist,and where high levels of productivity may occur (Methvenet al. 2001; Elliott and Hemingway 2002). High levels ofproductivity in estuarine parts are often related to environ-mental variables like salinity, water depth, water temperature,turbidity, and oxygen concentration, because they are knownto affect the spatial organisation of fish assemblages in estu-aries (Thiel et al. 1995; Jaureguizar et al. 2004; Maes et al.2005; Baptista et al. 2010). Density, diversity, and biomass offish show high variations in estuaries, because the above-mentioned environmental factors place considerable physio-logical demands on fish (Whitfield 1999). Furthermore, it isknown that fish species which are tolerant towards thesefactors use estuaries as important nursery grounds,overwintering areas, migration routes, feeding sites, and asrefuge areas (Claridge et al. 1986; Blaber 1997; Elliott andHemingway 2002; Franco et al. 2008).

However, there is still a great lack of detailed informationregarding spatial and temporal gradients of the quantitativecomposition of the fish fauna and their relation to environ-mental variables in European estuaries. Accordingly, progressin estuarine fish research needs future studies on detailed

D. Eick (*) : R. ThielUniversity of Hamburg, Biocenter Grindel and Zoological Museum,Martin-Luther-King-Platz 3, 20146 Hamburg, Germanye-mail: Dennis.Eick@uni-hamburg.de

Mar BiodivDOI 10.1007/s12526-014-0225-4

quantitative analyses of fish assemblage (Elliott andHemingway 2002).

In consideration of the different uses of estuaries by fishspecies, there are a number of different categories of life cyclestrategies into which fish species can be grouped using guildclassification schemes. Guild classification schemes were de-veloped as useful tools for describing and understanding thefunctional structure of extensive ecosystems (Garrison andLink 2000). Their usability was shown in a number of estua-rine fish composition studies, e.g. by Elliott and Dewailly(1995), Thiel et al. (2003), Pombo et al. (2005), Elliott et al.(2007), Franco et al. (2008), and Potter et al. (2013).

According to these studies, the fish fauna of northernhemisphere estuaries is often dominated by marine species(Elliott and Dewailly 1995; Elliott and Hemingway 2002;McLusky and Elliott 2004; Thiel 2011), which are subdividedinto marine stragglers and marine estuarine-opportunists, thelater ones dominating in a number of northern hemisphereestuaries (Potter et al. 1997). However, estuaries also play animportant role for diadromous species by supplying a route fortheir migration between spawning and feeding grounds (Thieland Potter 2001). The characterisation of the fish fauna struc-ture by means of the guild composition is a very useful tool todetermine the spatial and temporal utilisation of the availableresources, especially of estuarine habitats and food (Francoet al. 2008).

Although Elliott and Hemingway (2002) alreadyrecommended that future studies should consider theabundance of each species in guild assessments, there is stilllittle information available, especially on changes ofquantitative contributions made by the number ofindividuals of different guilds along and across estuaries.Thiel and Potter (2001) presented guild compositions alongthe Elbe estuary and Thiel et al. (2003) analysed guild com-positions along and across the Elbe estuary based on data thatwere sampled between the end of the 1980s and mid-1990s.However, no recent studies on guild composition in theestuarine area of the Elbe exist, nor for many otherEuropean estuaries.

The geographical distribution categories also serve as a toolfor understanding the functional structure of coastal marineecosystems, whereby the focus lies on the latitudinal/longitudinal distribution of marine, estuarine, diadromous,and freshwater fish species. According to Yang (1982),Wheeler et al. (2004), and Engelhard et al. (2011), the NorthSea fish species can be grouped into Boreal (distributioncentred north of the English Channel), Atlantic (widely dis-tributed in the Atlantic Ocean), and Lusitanian (distributioncentred south of the English Channel) species. Freshwater fishspecies can be grouped into five zoogeographical categoriesaccording to Illies (1978), Banarescu (1991; 1998), Jungwirthet al. (2003), and Griffiths (2006). Using different geograph-ical distribution categories, it is possible to estimate the impact

of changing environmental influences and effects of climatechange on fish assemblages in coastal areas as well as inestuaries (Dulvy et al. 2008; Tulp et al. 2008; Ter Hofstedeet al. 2010; Nicolas et al. 2011). However, effects of climatechange on fish fauna assemblages in most European estuarieshave not been assessed yet using geographical distributioncategories.

The species composition of the fish fauna in the Elbeestuary undergoes pronounced cyclical annual changes, whichare attributable to annual variation in the sequential immigra-tion and emigration of marine estuarine opportunist species,diadromous species, and the movements of freshwater speciesduring winter and early spring (Thiel and Potter 2001; Thielet al. 2003). However, limited information exists on interan-nual reproducibility of cyclical changes of the fish faunaassemblages and long-term stability of structuring environ-mental factors of the ichthyofaunal composition in Europeanestuaries. Such data are important to increase the understand-ing of the patterns and factors that influence the compositionof estuarine ichthyofaunas.

In a first step of the present study, the densities of thedifferent fish species along and across the Elbe estuary wereestimated. Assignment of the fish fauna of the Elbe estuary togeographical distribution categories and guilds of marine lifecycle categories was then performed on the basis of thenumbers of species as well as the numbers of individuals.The calculated data were then used to check if the numberof marine species declined in an upstream direction along theestuary. Furthermore, the data were used to test the hypothesisthat the densities of marine fish species are higher at down-stream sampling sites, whereas those of freshwater speciesshow a contrary trend. Additionally, the density data wereused to test the hypothesis that densities of anadromous andcatadromous fish species show no trends along the estuary.

Furthermore, the densities of each fish species at eachsampling site of the estuary were compared using a hierarchi-cal agglomerative clustering and nonmetric multidimensionalscaling ordination. These methods were used to test the hy-pothesis that there is a progressive change in species compo-sition along and across the estuary. The interannual stability ofcyclical seasonal changes of the species composition at eachstation was then analysed using frequency of occurrence.Finally, several environmental factors were checked regardingtheir influence on the structure of the fish fauna assemblage.

Material and methods

Study area

The study was performed in the Elbe estuary in northernGermany (53°50′N; 9°20′E), one of the largest Europeanestuaries (Elliott and Hemingway 2002). The Elbe estuary is

Mar Biodiv

partially mixed and belongs to the coastal-plain estuaries witha strong tidal action and nomarked halocline (Thiel and Potter2001; Elliott and McLusky 2002). The tidal Elbe has beensubject to many anthropogenic impacts since the mid-nineteenth century, for example, channelization for shipping,increasing inlet of sewage, water supply, and abstraction ofcoolant for power plants (Thiel et al. 2003; Kerner 2007; Illing2009). These modifications had strong influences on the tidalrange and the current velocity. The tidal range between highand low water is 3.4 m (Uncles et al. 2002); the currentvelocity has maximum values from 1 to 2 m/sec. Anotherimpact initiated by the channelization was the reduction ofsub- and inter-tidal shallow water areas (Riedel-Lorjé andGaumert 1982; Elliott and Hemingway 2002).

Sampling strategy

Fish were sampled monthly fromApril 2009 to October 2010,except for November 2009 and January and February 2010.Altogether, 703 hauls (one haul in August 2009 was notperformed due to strong winds) were made with a commercialstow net vessel. The stow net, with dimensions of 12×10/8×60 m, was used at 11 stations between the cities of Cuxhavenand Hamburg in the main channel as well as in the marginalareas. Here, the stations were grouped into four differentgeographical areas in accordance with Thiel (2001) (Fig. 1).

The net had a mesh size in 8 mm cod end, measured fromcentre to centre of knots, and a square opening of 135 m2

(deep main channel) and 108 m2 (shallow marginal areas).The western main channel stations (WMS) 1–3 and the east-ern main channel stations (EMS) 4, 6, 8 and 11 had a mini-mum water depth of 9 to 11 m and a maximum depth of 14 to20 m, the northern marginal area stations (NMS) 5 and 7 andthe southern marginal area stations (SMS) 9 and 10 werecharacterised by aminimum depth of 5 to 8m and a maximumdepth of 9 to 12 m.

At each station, two flood and two ebb tide hauls, each ofabout 90 min duration, were performed. Sampling at each siteinvolved firstly anchoring the boat and then placing the stownet on its right side so that the opening faced the direction ofthe water flow. Water temperature (°C), salinity (PSU), pH,and oxygen concentration (mg/l) were measured at the begin-ning and end of each haul using a portable multi-probe U-50(Horiba®). Both values of each parameter were averaged foreach haul with the exception of salinity. To verify our ownrecorded data, we compared our data of oxygen concentration,temperature and salinity with external data from the officialsources of FGG Elbe (2013). Additionally, river run-off datawere taken from FGG Elbe (2013).

Flow velocity (m/sec) during the entire haul was measuredusing three mechanical flow meters (Hydro-Bios® and/orGeneral Oceanics®) installed between the upper and lower

Fig. 1 Study area with sampling sites and stream kilometres; circle=western main channel stations; triangle=eastern main channel stations; square=-northern marginal area stations; rhombus=southern marginal area stations and stream kilometres: X=stream kilometres

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beam of the net opening in three different depths (1, 4, and7 m). For further details regarding the sampling strategy seeEick (2012).

Captured subadult and adult fish were identified to specieson board immediately, counted, weighed (wet weight, accu-racy 1 g) and measured. Fish larvae and young of the years(YOY) were identified in the laboratory following Muus andNielsen (1999), Kottelat and Freyhof (2007), and Froese andPauly (2013).

Guilds of life cycle categories

To allow an exact comparison of our actual data with dataprovided earlier by Thiel et al. (1995), Thiel and Potter (2001),and Thiel et al. (2003), the assignment of each species wasrestricted to one of six overall life cycle categories followingThiel and Potter (2001), Thiel et al. (2003), Elliott et al.(2007), Franco et al. (2008), and Thiel (2011). However, thedefinitions of the used guild categories are mainly based onthe most recent publication on this topic by Potter et al.(2013), considering life cycle categories of fish species inestuaries worldwide.

Marine species occurring in the Elbe estuary arecategorised either as marine stragglers, which spawn at sea,typically enter estuaries sporadically and in low numbers andare most common in the lower reaches where salinities typi-cally do not decline far below 35, and marine estuarine-opportunists, which spawn also at sea, but regularly enterestuaries in substantial numbers, particularly as juveniles,but use, to varying degrees, coastal marine waters as alterna-tive nursery areas. Estuarine fish species (solely estuarine,according to Potter et al. 2013) are defined as species foundonly in estuaries. Coregonus maraena is treated as an estua-rine species following Gerson (2013) and Franco et al. (2008).Although Franco et al. (2008) listed Coregonus oxyrinchusinstead of Coregonus maraena, these are only different scien-tific names for the same species. However, the taxonomy ofCoregonidae is still unclear because a lot of different species,subspecies or local populations exist (Kottelat and Freyhof2007; Hansen et al. 2008; Jacobsen et al. 2012). As a result ofthis confused taxonomic status, an exact estuary guild classi-fication is not available. Strontium and calcium analysis of theotoliths from Coregonus maraena indicate that a lot of spec-imens of the Elbe population stay in the freshwater–mesohaline area of the Elbe estuary and do not performanadromous migrations (Gerson 2013). In contrast to thesefindings, several other authors, e.g. Freyhof and Schöter(2005), describe it as an anadromous species. Summarizingall these facts, the classification of Coregonus maraena as anestuarine species seems to be adequate.

Diadromous species occurring in the Elbe estuary are sep-arated into anadromous species, which grow up in the seabefore migration into rivers to spawn, and catadromous

species, which start their life in fresh water and subsequentlymigrate to sea to spawn. Finally, freshwater species (freshwa-ter stragglers as well as freshwater estuarine-opportunists,according to Potter et al. 2013) are species that spawn infreshwaters and are found in low numbers in estuaries, andwhose distribution is usually limited to low salinity, upperreaches of estuaries, or which are found regularly and inmoderate numbers in estuaries and whose distribution canextend well beyond the oligohaline sections of thesesystems, respectively.

Geographical distribution categories

Classification of marine, estuarine and diadromous fish spe-cies of the North Sea into geographical distribution categorieswas performed according to Yang (1982), Dulvy et al. (2008),and Engelhard et al. (2011). According to these studies, thereare the following three distinct categories: boreal fish specieswhich occur mostly between the Norwegian Sea and theBritish Islands, lusitanian species which have a north–southdistribution range between the northwest North Sea (theBritish Islands) and the Iberian Peninsula/North Africa, andAtlantic fish species which occur in the Atlantic Ocean aroundEuropean coasts. Each freshwater fish species was groupedaccording to Illies (1978), Banarescu (1991; 1998), Jungwirthet al. (2003), and Griffiths (2006), into one of the followingfive categories: European=fish species which occur in Europe,Euro-Siberian=fish species which have a European andSiberian distribution, Holarctic=northern hemisphere species,Nordic=fish species which occur in northern areas of Europe,Pale-arctic=fish species which have a European, Siberian, andEast-Asian distribution.

Data analyses

Seasonal changes of the fish fauna composition at all stationsduring the 2 years of investigation were calculated based onthe frequency of occurrence of the 12 most abundant species.Frequency of occurrence was calculated separately for eachspecies as the percentage of total sampling from all samplescollected at each station.

Fish densities (individuals per one billion m3 of water)were calculated for all species using the counted number ofindividuals and computing the filtered water volume from theopening area of the stow net and the flowmeter measurements.Mean density for each species was calculated as the sumof all densities of the relevant fish species per stationdivided through the sum of hauls performed at thisstation. The percentage contributions of each fish speciesto the total fish density at each sampling site, to the lifecycle categories, and to the geographical distributioncategories were also calculated.

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The analysis of fish fauna composition per station wasdone with the density of each species by using the PRIMERpackage (Clarke and Gorley 2006). For this purpose, thedensities of the fish species were fourth-root transformedand then classified by hierarchical agglomerative clustering(CLUSTER), and nonmetric multidimensional scaling (MDS)ordination. The Bray-Curtis similarity measure was used toproduce the association matrix. Analysis of similarities(ANOSIM) was performed using the PRIMER packageto test for differences between the species compositionsat the sampling sites. Similar compositions are indicatedby pair-wise R-stat values close to zero. The significantlevel for pair-wise comparisons was adjusted with theBonferroni correction.

A Bio-Env analysis (PRIMER package) was performed tocorrelate the multivariate analysis of fish assemblage withenvironmental variables based on Spearman’s rank correlationcoefficient. Environmental data using a matrix based onEuclidean distance was fourth-root transformed and normal-ized. Bio-Env examines the influence of environmental vari-ables like bank distance, flow velocity, water depth, watertemperature, oxygen concentration, salinity, and Secchi depthon the distribution pattern of fish assemblages by acceptingthat stations with analogue environmental variables accom-modate analogue fish assemblage, provided that the relevantenvironmental variables influence the composition of the fishassemblage (Deubel 2000).

Results

General composition of the fish fauna

The mean total fish abundance was about 362,989 indi-viduals 10−6 m−3 between April 2009 and October 2010(Tables 1 and 2). Sixty-one fish species as well as cyprinidhybrids were recorded. According to their abundance propor-tions, the fish fauna was dominated by the following tenspecies: O. eperlanus (96.1 %), A. fallax (1.9 %), G. cernua(0.9 %), Abramis brama (0.2 %), Blicca bjoerkna (0.2 %),Clupea harengus (0.1 %), Platichthys flesus (0.1 %), Sprattussprattus (0.1%), Sander lucioperca (0.1 %), andGasterosteusaculeatus (<0.1 %).

A high proportion of 40.2 % (25 species) of all recordedspecies belonged to the marine life cycle categories of marinestragglers and marine estuarine-opportunists (Table 3).However, all marine species contributed only 0.4 % to meantotal fish abundance, whereas the anadromous species reachedan abundance proportion of more than 98 %, mainly due toextremely high proportions of O. eperlanus (96.1 %). Thetotal contribution of freshwater species to the overall catchwas 1.4 %.

Twenty-nine percent of the species had a boreal distributionand 27.4 % belonged to the lusitanian distribution category,whereas a minority of 3.2 % of the species were of Atlanticorigin. An additional 40.4 % were freshwater species, with asubdivision into the following five categories: European(19.3 %), Euro-Siberian (8.1 %), Holarctic (6.4 %), Nordic(3.3 %) and Pale-Arctic (3.3 %) (Table 3). In terms of indi-vidual numbers, the boreal category also reached the highestpercentage values with 96.3 %, which is mostly based on theextremely abundant O. eperlanus. Of the caught individuals,2.3 % belonged to the lusitanian category, 1.3 % were mem-bers of the freshwater category, and less than 0.1 % belongedto the Atlantic distribution category.

Spatial structure of the fish fauna

Classification of the densities of each of the species collectedfrom the 11 sampling sites separated the samples into twolarge clusters which were marked with A and B (Fig. 2). Thefirst cluster (A) comprised the downstream sampling sites 1–4with higher salinity values than measured at more upstreamstations. The other large cluster (B) contained the upstreamstations 5–11. Here, salinity values were below 0.5.

On the next level, the two large clusters A and B could bedivided once more. Cluster A was divided into C and D,whereby C consisted of the two stations 1 and 3. Cluster Dalso comprised two stations, stations 2 and 4 (Fig. 2).

The second cluster (B) was subdivided into two additionalclusters, clusters E and F. Cluster E comprised station 11 andcluster G, which comprised cluster H and station 7. Cluster Hcomprised stations 9 and 10. Cluster F was subdivided intocluster I and station 5. Cluster I comprised stations 6 and 8(Fig. 2).

Following multidimensional scaling ordination of the samedensity data as used for classification, sampling sites 1 and 3as well as sites 2 and 4 were separated from all other stations(Fig. 3). The differences between downstream stations 1–4belonging to cluster A were much greater than between theupstream stations belonging to cluster B, except for station 5.Although the ANOSIM analysis highlighted significant dif-ferences between stations (R=0.06, ≤ 0.001, for further detailssee Table 4), the low global R-value indicates a high overlapof the community structure between them. However, there aresome significant similarities or differences that were not iden-tified by cluster analysis andMDS. In contrast to the results ofthese analyses, ANOSIM analysis indicated high similaritiesalso between elements of the two groups A and B. Forexample, the fish community in station 1 is significantlydifferent from that of all others stations including that of theother downstream stations. Accordingly, it cannot be seen as amember of the same group as identified in the cluster analysisor in the multidimensional scaling ordination. Great differ-ences were observed between the communities in stations 6

Mar Biodiv

Tab

le1

Listo

fspeciescaught

onboth

tides

at11

stations

intheElbeestuarybetweenApril2009

andOctober

2010,incombinationwith

theirlifecyclecategories

(for

abbreviation,seeTable3).M

ean

abundance(N

)and

percentage

contributionby

abundance.The

relativeabundancerepresentsthesumof

thedensities

ofbothtides

oneach

samplingoccasion.A

lsogivenarethetotalnum

bero

fspecies,the

meanabundanceandthetotaln

umbersof

fish

individualscaught

oneach

samplingoccasion

Species

LCC

Com

bined

Station1

Statio

n2

Statio

n3

Statio

n4

Statio

n5

Statio

n6

N%

N%

N%

N%

N%

N%

N

Osm

erus

eperlanus

A348952.4

96.10

40,016.49

81.10

58,064.72

96.50

47,284.79

94.20

73,653.53

98.90

120,998.66

92.80

152,843.63

Alosa

falla

xA

7,177.82

1.90

58.24

0.10

43.98

<0.1

83.54

0.20

59.22

0.10

65.92

0.10

959.94

Gym

nocephalus

cernua

F3,409.80

0.90

8.15

<0.1

240.85

0.40

177.03

0.40

475.85

0.60

7,246.35

5.60

268.46

Abram

isbram

aF

705.02

0.20

18.42

<0.1

0.17

<0.1

18.80

<0.1

84.52

0.10

22.54

Blicca

bjoerkna

F637.58

0.20

<0.10

<0.1

38.23

<0.1

0.17

<0.1

29.94

<0.1

91.25

0.10

23.28

Clupeaharengus

O518.36

0.10

3,555.22

7.20

1,020.58

1.70

825.26

1.60

42.16

0.10

250.32

0.20

0.20

Platichthysflesus

O455.24

0.10

604.16

1.20

253.94

0.40

169.74

0.30

50.12

0.10

1,435.15

1.10

179.52

Sprattu

ssprattu

sO

371.67

0.10

2,213.82

4.50

343.45

0.60

1,456.26

2.90

53.06

0.10

16.05

<0.1

Sander

lucioperca

F286.88

0.10

0.90

<0.1

1.66

<0.1

1.35

<0.1

1.53

<0.1

11.43

<0.1

7.96

Gasterosteusaculeatus

A162.96

<0.1

204.03

0.40

143.35

0.20

63.49

0.10

100.36

0.10

151.91

0.10

186.21

Syngnathus

rostellatus

E158.71

<0.1

1,702.62

3.40

2.25

<0.1

38.32

0.10

0.15

<0.1

Liparislip

aris

O84.83

<0.1

881.96

1.80

0.22

<0.1

49.67

0.10

Anguilla

anguilla

C21.19

<0.1

1.76

<0.1

5.39

<0.1

1.81

<0.1

3.36

<0.1

5.78

<0.1

5.15

Leuciscusidus

F15.93

<0.1

0.39

<0.1

<0.10

<0.1

0.29

<0.1

2.48

<0.1

0.14

Pom

atoschistusminutus

O6.62

<0.1

3.69

<0.1

4.05

<0.1

1.67

<0.1

1.08

<0.1

3.48

<0.1

<0.10

Gasterosteusgymnurus

A4.78

<0.1

5.11

<0.1

6.72

<0.1

1.70

<0.1

1.15

<0.1

1.10

<0.1

4.63

Lam

petraflu

viatilis

A4.62

<0.1

6.59

<0.1

60.20

<0.1

6.23

<0.1

1.64

<0.1

3.49

<0.1

6.40

Pleuronectesplatessa

O3.34

<0.1

34.04

<0.1

0.61

<0.1

2.09

<0.1

Pom

atoschistusmicrops

E2.29

<0.1

0.19

<0.1

0.55

<0.1

0.69

<0.1

0.18

Ballerusballerus

F2.08

<0.1

Perca

fluviatilis

F1.72

<0.1

0.14

<0.1

0.30

<0.1

0.19

<0.1

<0.10

<0.1

0.39

<0.1

0.19

Salmotrutta

trutta

A1.70

<0.1

0.66

<0.1

2.61

<0.1

1.77

<0.1

2.88

<0.1

0.78

<0.1

4.06

Soleasolea

O1.69

<0.1

12.09

<0.1

2.28

<0.1

4.26

<0.1

Aspiusaspius

F1.48

<0.1

<0.10

<0.1

0.14

<0.1

0.70

<0.1

0.19

Coregonus

maraena

E0.61

<0.1

<0.10

<0.1

0.13

<0.1

<0.10

<0.1

0.87

<0.1

0.20

<0.1

0.83

Rutilu

srutilus

F0.45

<0.1

<0.10

<0.1

0.13

<0.1

<0.10

Gadus

morhua

O0.33

<0.1

0.69

<0.1

2.93

<0.1

Merlangiusmerlangus

O0.31

<0.1

0.86

<0.1

0.13

<0.1

2.40

<0.1

<0.10

<0.1

Cyprinuscarpio

F0.30

<0.1

<0.10

<0.1

0.29

<0.1

<0.10

<0.1

0.22

<0.1

0.12

<0.1

0.45

Salmosalar

A0.18

<0.1

<0.10

<0.1

0.36

<0.1

<0.10

<0.1

0.22

<0.1

0.16

<0.1

0.11

Agonuscataphractus

S0.14

<0.1

1.40

<0.1

<0.10

<0.1

<0.10

<0.1

Pungitiu

spungitius

F<0.10

<0.1

0.10

<0.1

0.14

<0.1

Zoarcesviviparus

S<0.10

<0.1

<0.10

<0.1

Salmotrutta

fario

F<0.10

<0.1

<0.10

Salvelinus

fontinalis

F<0.10

<0.1

Barbusbarbus

F<0.10

<0.1

Rhodeus

amarus

F<0.10

<0.1

<0.10

<0.1

Chelonlabrosus

O<0.10

<0.1

0.15

<0.1

Squaliu

scephalus

F<0.10

<0.1

Gaidropsarusvulgaris

S<0.10

<0.1

<0.10

<0.1

Ciliatamustela

O<0.10

<0.1

<0.10

<0.1

Mar Biodiv

Tab

le1

(contin

ued)

Species

LCC

Com

bined

Station1

Statio

n2

Statio

n3

Statio

n4

Statio

n5

Statio

n6

N%

N%

N%

N%

N%

N%

N

Callio

nymus

lyra

S<0.10

<0.1

<0.10

<0.1

Entelurus

aequoreus

S<0.10

<0.1

<0.10

<0.1

Trachurustrachurus

S<0.10

<0.1

0.16

<0.1

0.37

<0.1

Hybride

F<0.10

<0.1

<0.10

<0.1

Carassius

carassius

F<0.10

<0.1

<0.10

<0.1

0.14

<0.1

Coregonus

albula

F<0.10

<0.1

<0.10

<0.1

Ammodytes

tobianus

S<0.10

<0.1

<0.10

<0.1

Limanda

limanda

O<0.10

<0.1

<0.10

<0.1

<0.10

<0.1

Pom

atoschistuslozanoi

O<0.10

<0.1

0.33

<0.1

<0.10

<0.1

Petromyzon

marinus

A<0.10

<0.1

0.16

<0.1

<0.10

<0.1

<0.10

<0.1

<0.10

<0.1

<0.10

<0.1

<0.10

Chondrostom

anasus

F<0.10

<0.1

Lotalota

F<0.10

<0.1

<0.10

<0.1

Oncorhynchusmykiss

F<0.10

<0.1

<0.10

<0.1

Chelid

onichthyslucerna

O<0.10

<0.1

<0.10

<0.1

Microstom

uskitt

S<0.10

<0.1

<0.10

<0.1

0.13

<0.1

<0.10

<0.1

Engraulisencrasicolus

S<0.10

<0.1

<0.10

<0.1

Myoxocephalus

scorpius

S<0.10

<0.1

0.63

<0.1

<0.10

<0.1

Scophthalmus

maximus

S<0.10

<0.1

0.33

<0.1

<0.10

<0.1

<0.10

<0.1

<0.10

<0.1

Alburnusalburnus

F<0.10

<0.1

Silurusglanis

F<0.10

<0.1

Vimba

vimba

F<0.10

<0.1

Num

berof

species

62.00

42.00

34.00

37.00

28.00

25.00

24.00

Meanabundance

362,988.69

49,315.19

60,202.13

50,176.06

74,491.03

130,371.23

154,514.65

Totaln

umbers

35,078,986.21

1,152,659.43

1,119,856.70

973,851.35

1,374,906.48

1,345,712.81

2,577,522.77

Num

berof

samples

703.00

64.00

64.00

64.00

64.00

64.00

64.00

Statio

n6

Station7

Statio

n8

Statio

n9

Statio

n10

Station11

%N

%N

%N

%N

%N

%

98.90

212,575.12

90.20

371,018.40

98.50

1,066,989.93

98.80

1,491,235.52

94.90

221,643.88

96.70

0.60

2,753.10

1.20

2,273.04

0.60

1,239.54

0.10

70,941.43

4.50

1,474.37

0.60

0.20

17,213.71

7.30

2,688.56

0.70

4,049.03

0.40

3,303.17

0.20

1,832.07

0.80

<0.1

366.77

0.20

258.23

0.10

4,396.00

0.40

1,051.17

0.10

1,543.97

0.70

<0.1

388.11

0.20

421.12

0.10

2,639.77

0.20

1,473.52

0.10

1,926.99

0.80

<0.1

0.13

<0.1

0.10

1,443.42

0.60

123.58

<0.1

139.26

<0.1

563.15

<0.1

14.38

<0.1

<0.1

601.33

0.30

11.33

<0.1

507.56

<0.1

1,515.51

0.10

514.34

0.20

0.10

186.21

0.10

24.57

<0.1

272.00

<0.1

420.09

<0.1

70.62

<0.1

Mar Biodiv

Tab

le1

(contin

ued)

Statio

n6

Station7

Statio

n8

Statio

n9

Statio

n10

Station11

%N

%N

%N

%N

%N

%

<0.1

10.63

<0.1

11.63

<0.1

16.69

<0.1

84.14

<0.1

87.80

<0.1

<0.1

16.28

<0.1

0.53

0.10

127.06

<0.1

20.95

<0.1

7.19

<0.1

<0.1

53.97

<0.1

0.51

<0.1

3.57

<0.1

<0.10

<0.1

0.23

<0.1

<0.1

1.88

<0.1

0.35

<0.1

25.11

<0.1

1.66

<0.1

3.12

<0.1

<0.1

3.49

<0.1

9.89

<0.1

0.66

<0.1

3.02

<0.1

3.35

<0.1

<0.1

1.17

<0.1

<0.10

<0.1

19.82

<0.1

2.28

<0.1

0.35

<0.1

<0.10

<0.1

8.33

<0.1

13.54

<0.1

1.18

<0.1

<0.1

2.87

<0.1

0.93

<0.1

2.00

<0.1

10.05

<0.1

1.95

<0.1

<0.1

1.68

<0.1

0.85

<0.1

1.26

<0.1

1.05

<0.1

1.12

<0.1

<0.1

4.36

<0.1

0.34

<0.1

5.03

<0.1

3.47

<0.1

2.04

<0.1

<0.1

1.16

<0.1

0.49

<0.1

0.29

<0.1

1.49

<0.1

1.14

<0.1

<0.1

<0.10

<0.1

0.27

<0.1

1.33

<0.1

2.89

<0.1

0.21

<0.1

<0.1

0.49

<0.1

0.52

<0.1

0.39

<0.1

0.39

<0.1

0.19

<0.1

<0.1

0.39

<0.1

<0.10

<0.1

0.52

0.10

<0.10

<0.1

<0.10

<0.1

<0.1

<0.10

<0.1

<0.10

<0.1

<0.10

<0.1

0.22

<0.1

<0.10

<0.1

Mar Biodiv

Tab

le1

(contin

ued)

Statio

n6

Station7

Statio

n8

Statio

n9

Statio

n10

Station11

%N

%N

%N

%N

%N

%

0.72

<0.1

<0.10

<0.1

<0.10

<0.1

<0.10

<0.1

<0.1

<0.10

<0.1

0.16

<0.1

<0.10

<0.1

0.11

<0.1

<0.10

<0.1

0.28

<0.1

0.19

<0.1

<0.10

<0.1

<0.10

<0.1

<0.10

<0.1

<0.10

<0.1

0.16

<0.1

<0.10

<0.1

<0.10

<0.1

26.00

25.00

25.00

27.00

30.00

235,600.23

376,845.78

1,080,446.02

1,570,652.00

229,131.04

2,181,880.72

4,965,973.56

2,721,881.13

11,562,972.98

5,101,768.28

64.00

64.00

64.00

63.00

64.00

Mar Biodiv

Tab

le2

Num

bersof

speciesandmeanabundanceof

each

lifecyclecategory

andgeographicaldistributio

ncategory,w

iththeirpercentage

contributio

natall11stations

intheElbeestuarybetweenApril

2009

andOctober

2010

Ecologicalg

uilds

Com

bined

Station1

Station2

Station3

Station4

Station5

Station6

N%

N%

N%

N%

N%

N%

N%

Byspecies:

Marinestraggler(S)

1117.7

1126.1

38.8

513.5

13.5

Marineestuarine-opportunist(O)

1422.5

1330.9

926.4

1129.7

517.8

416.0

312.6

Estuarine

(E)

34.8

37.1

38.8

25.4

27.1

28.0

28.4

Anadrom

ous(A

)8

12.8

819.0

823.5

821.6

828.6

832.0

833.6

Catadromous(C)

11.6

12.3

12.9

12.7

13.5

14.0

14.2

Freshw

ater

(F)

2540.4

614.2

1029.4

1027.0

1139.2

1040.0

1042.0

Totaln

umbers

6242

3437

2825

24

Byindividuals:

Marinestraggler

0.34

<0.1

2.84

<0.1

0.27

<0.1

0.58

<0.1

<0.1

<0.1

Marineestuarine-opportunist

1,439.51

0.4

7,307.04

14.8

1,625.29

2.7

2,514.47

5.0

146.50

0.2

1,705.01

1.31

180.10

0.1

Estuarine

161.62

<0.1

1,702.83

3.5

2.93

<0.1

38.42

<0.1

1.02

<0.1

0.89

<0.1

1.01

<0.1

Anadrom

ous

356,304.83

98.1

40,291.37

81.7

58,267.84

96.8

47,441.59

94.6

73,819.08

99.1

121,222.10

93.0

154,005.04

99.7

Catadromous

21.19

<0.1

1.76

<0.1

5.39

<0.1

1.81

<0.1

3.36

<0.1

5.78

<0.1

5.15

<0.1

Freshw

ater

5061.53

1.4

9.34

<0.1

300.41

0.5

179.20

0.3

521.01

0.7

7,437.46

5.7

323.34

0.2

Meantotalabundance

362,988.69

49,315.19

60,202.13

50,176.06

74,491.03

130,371.23

154,514.65

Totaln

umbers

35,078,986.21

1,152,659.43

1,119,856.70

973,851.35

1,374,906.48

1,345,712.81

2,577,522.77

Geographicald

istributioncategories

Com

bined

Station1

Station2

Station3

Station4

Station5

Station6

N%

N%

N%

N%

N%

N%

N%

Byspecies:

Boreal

1829.0

1842.8

1338.2

1437.8

828.6

832.0

833.3

Atlantic

23.2

24.8

25.9

25.4

27.1

28.0

28.3

Lusitanian

1727.4

1638.1

926.5

1129.7

725.0

520.0

416.6

European

1219.3

37.2

411.8

38.1

414.2

416.0

416.8

Euro-Siberian

58.1

24.7

411.8

410.8

414.2

416.0

416.8

Holarctic

46.4

12.9

12.7

23.6

Nordic

23.3

12.7

14.2

Pale-A

rctic

23.3

12.3

12.9

12.7

13.6

28.0

14.2

Totaln

umbers

6242

3437

2825

24

Byindividuals:

Boreal

349,730.90

96.3

44,673.53

90.6

59,309.81

98.5

48,236.17

96.1

73,802.66

99.1

121,406.53

93.1

153,046.02.

99.0

Atlantic

21.38

<0.1

1.85

<0.1

5.74

<0.1

1.84

<0.1

3.58

<0.1

5.94

<0.1

5.27

<0.1

Lusitanian

8174.88

2.3

4630.47

9.41

586.17

1.0

1758.85

3.6

163.78

0.2

1521.30

1.2

1140.02

0.7

European

2429.52

0.7

4.65

<0.1

120.16

0.2

53.76

0.1

189.44

0.3

2974.96

2.3

129.33

0.1

Euro-Siberian

1012.33

0.2

3.14

<0.1

120.16

0.2

71.68

0.2

189.44

0.3

2974.96

2.3

129.33

0.1

Holarctic

809.84

0.2

30.04

<0.1

17.92

<0.1

94.72

0.1

Nordic

404.92

0.1

17.92

<0.1

32.33

<0.1

Mar Biodiv

Tab

le2

(contin

ued)

Pale-A

rctic

404.92

0.1

1.55

<0.1

30.04

<0.1

17.92

<0.1

47.36

<0.1

1,487.48

1.1

32.33

<0.1

Meantotalabundance

362,988.69

49,315.19

60,202.13

50,176.06

74,491.03

130,371.23

154,514.65

Totaln

umbers

35,078,986.21

1,152,659.43

1,119,856.70

973,851.35

1,374,906.48

1,345,712.81

2,577,522.77

Ecologicalg

uilds

Station7

Station8

Station9

Station10

Station11

N%

N%

N%

N%

N%

Byspecies:

Marinestraggler(S)

Marineestuarine-opportunist(O)

311.5

312.0

28.0

27.4

26.6

Estuarine

(E)

27.7

28.0

28.0

27.4

26.6

Anadrom

ous(A

)8

30.8

832.0

832.0

829.6

826.6

Catadromous(C)

13.8

14.0

14.0

13.7

13.3

Freshw

ater

(F)

1246.2

1144.0

1248.0

1451.8

1756.6

Totaln

umbers

2625

2527

30

Byindividuals:

Marinestraggler

Marineestuarine-opportunist

1,497.61

0.6

124.2

<0.1

142.82

<0.1

563.22

<0.1

14.61

<0.1

Estuarine

2.34

<0.1

0.54

<0.1

20.11

<0.1

3.77

<0.1

1.49

<0.1

Anadrom

ous

215,495.28

91.5

373,327.32

99.1

1,068,529.11

98.9

1,562,602.94

99.5

223,196.57

97.4

Catadromous

10.62

<0.1

11.63

<0.1

16.69

<0.1

84.14

<0.1

87.80

<0.1

Freshw

ater

18,594.37

7.9

3,382.08

0.9

11,737.29

1.1

7,397.93

0.5

5,830.57

2.5

Meantotalabundance

235,600.23

376,845.78

1,080,446.02

1,570,652.00

229,131.04

Totaln

umbers

2,181,880.12

4,965,973.56

2,721,881.13

11,562,972.98

5,101,768.28

Geographicald

istributioncategories

Station7

Station8

Station9

Station10

Station11

N%

N%

N%

N%

N%

Byspecies:

Boreal

726.9

832.0

728.0

725.9

723.3

Atlantic

27.7

28.0

28.0

27.4

26.6

Lusitanian

519.2

416.0

416.0

414.8

413.3

European

726.9

520.0

624.0

933.3

1033.3

Euro-Siberian

415.5

416.0

416.0

414.8

413.5

Holarctic

14.0

13.3

Nordic

14.0

13.3

Pale-A

rctic

13.8

14.0

14.0

13.7

13.3

Mar Biodiv

and 9 to 11, which are less similar than indicated by the clusteranalysis and MDS.

The progressive change that occurred in the fish faunacommunity in the upstream direction corresponds to decreasesin the abundance of marine species and increases in theabundance of freshwater species, as well as in the numbersof species along this axis (see also Tables 1 and 2). Here,differences in the abundance of marine and freshwater fishspecies helped to distinguish between the fauna compositionat all stations. For example, station 1 with a total number of 42species differed from all other stations, which could be basedon the fact that higher numbers of marine estuarine-opportunists like, C. harengus, P. flesus, and S. sprattus, andmarine stragglers like Ammodytes tobianus,Callionymus lyra,Ciliata mustela, Engraulis encrasicolus, Entelurusaequoreus, Gaidropsarus vulgaris, and Zoarces viviparousappeared here. Station 6 with 24 species is more similar tothe upstream stations 9–11, probably caused by highernumbers of the marine estuarine-opportunist P. flesusand anadromous fish species like O. eperlanus andA. fallax occurring here. The greater numbers of anadro-mous and freshwater fish species like O. eperlanus, A.fallax, G. cernua, A. brama, B. bjoerkna and S. lucioperca atthe upstream stations 9 (25 species), 10 (27 species), and 11(30 species) seems to be the reason for the estimated smalldifferences (see also Table 1 and 2).

Spatial differences in presence and abundance of species

Species of marine stragglers like A. tobianus, C. lyra,C. mustela, E. encrasicolus, E. aequoreus, G. vulgaris, andZ. viviparous were only caught at station 1, whereas othermarine stragglers like Agonus cataphractus,Microstomus kitt,Myoxocephalus scorpius, Scophthalmus maximus, andTrachurus trachurus were found further upstream, up to sta-tion 3 (Tables 1 and 2).

Marine estuarine-opportunists such as Gadus morhua,Limanda limanda, Liparis liparis, Merlangius merlangus,Pomatoschistus lozanoi, and Solea soleawere also found onlyat the three most downstream stations, but species likeC. harengus, P. flesus, Pomatoschistus minutus, andS. sprattus tended to stay in the middle part of the Elbe estuary(up to station 5). In contrast to marine species, freshwaterspecies like G. cernua and S. lucioperca were less abundantdownstream at stations 1–4 and very abundant more upstreamat stations 5–11.

For anadromous fish species like A. fallax andG. aculeatus, a preference of a specific section of the Elbeestuary was not estimated, whereby higher abundances ofA. fallax were estimated at station 10. The smelt,O. eperlanus, was the most abundant anadromous specieswith proportions of total fish abundances increasing upstreamT

able2

(contin

ued)

Totaln

umbers

2625

2527

30

Byindividuals:

Boreal

212,742.96

90.3

371,054.84

98.5

1,067,289.34

98.8

1,491,662.93

95.0

221,723.30

96.8

Atlantic

11.02

<0.1

11.69

<0.1

17.21

<0.1

84.20

<0.1

87.84

<0.1

Lusitanian

4251.88

1.8

2397.18

0.6

1402.18

0.1

71506.95

4.6

1489.33

0.6

European

10846.71

5.0

1537.3

0.4

5868.6

0.5

4755.78

0.3

3429.70

2.0

Euro-Siberian

6198.12

2.2

1229.84

0.3

3912.4

0.4

2113.68

0.1

1371.88

0.3

Holarctic

307.46

0.1

342.97

0.1

Nordic

978.1

0.1

342.97

0.1

Pale-A

rctic

1,549.53

0.5

307.46

0.1

978.1

0.1

528.42

<0.1

342.97

0.1

Meantotalabundance

235,600.23

376,845.78

1,080,446.02

1,570,652.00

229,131.04

Totaln

umbers

2,181,880.12

4,965,973.56

2,721,881.13

11,562,972.98

5,101,768.28

Mar Biodiv

and ranging from 81.1 % at station 1 to 98.9 % at stations 4and 6 (Tables 1 and 2).

To summarize the results regarding the longitudinal direction,station 1 showed the highest abundance of marine fish species.Most of them were marine estuarine-opportunists likeC. harengus, L. liparis, and S. sprattus. Station 3 showed asimilar species composition like station 1, but L. liparis occurredhere in relatively low density. Stations 2 and 4 had similar fishcompositions, but, compared to stations 1 and 3, higher abun-dances of freshwater species like G. cernua and also higherabundances of anadromous species like O. eperlanus (Tables 1and 2). However, the other seven upstream stations could be

distinguished by higher densities ofO. eperlanus. Here, the mainstream stations 6, 8, and 11 showed higher densities than stations5, 7, 9, and 10 of the marginal areas.

Comparisons of the frequency of occurrence for specieswith values>5 % in the lateral direction between the mainstream and the marginal areas indicated that in shallow sideareas with lower currents freshwater species like A. brama,B. bjoerkna, Leuciscus idus, and S. lucioperca occurred morefrequently than in the main channel. In contrast, C. harengus,S. sprattus, and other marine fish species were found moreoften in the main stream, showing a preference for deeperareas with higher current velocities (Fig. 4).

Fig. 2 Classification of thedensity of each fish species ateach station in the Elbe estuarybetween 2009 and 2010.WMS=western main channelstations; EMS=eastern mainchannel stations; NMS=northernmarginal area stations;SMS=southern marginal areastations

Fig. 3 Multidimensional scalingordination of the density of eachfish species at each station in theElbe estuary between 2009 and2010. Circle=Groups of stationsthat do not differ from each other.Triangle=Station for whichANOSIM analyses indicatedsignificant differences from otherstations (see also Table 4)

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Spatial contribution by different ecological guildsand geographical distribution categories

The number of marine straggler species decreased in an up-stream direction, from 11 species at station 1 to five at station3, three at station 2, and only one at station 4. Species ofmarine estuarine-opportunists decreased from 13 at station 1to 11 at station 3, nine at station 2, five at station 4, four at

station 5, three at stations 6 to 8, and two species at stations 9to 11 (Table 3). In contrast to the marine species, the numberof freshwater species decreased in a downstream direction.Seventeen species were recorded at station 11, followed bystations 10, 9 and 7 with 12 species each, 11 species at station4, and ten species at all other stations except station 1 withonly six species. The only catadromous species, Anguillaanguilla, which was found throughout the whole study period,

Fig. 4 Frequency of occurrence (%) for 25 fish species with values>25 %

Table 3 R-stat values and significant levels in the pair-wise comparison between the species composition of samples at the 11 stations in the Elbe estuaryin 2009/2010 using ANOSIM

Station 1 2 3 4 5 6 7 8 9 10

1

2 0.430***

3 0.330*** −0.018 ns

4 0.575*** 0.002 ns 0.044 ns

5 0.705*** 0.304*** 0.358*** 0.297***

6 0.583*** 0.028 ns 0.089** −0.023 ns 0.283***

7 0.745*** 0.292*** 0.384*** 0.303** −0.011 ns 0.246***

8 0.698*** 0.103** 0.165*** −0.004 ns 0.344*** −0.013 ns 0.314***

9 0.684*** 0.122** 0.185*** 0.0556* 0.414*** 0.035 ns 0.340*** 0.037 ns

10 0.637*** 0.09** 0.159*** 0.053* 0.355*** 0.013 ns 0.255*** 0.032 ns −0.031 ns

11 0.712*** 0.112** 0.191*** 0.047* 0.417*** 0.005 ns 0.345*** −0.005 ns −0.025 ns −0.029 ns

* P<0.05

** P<0.01

*** P<0.001

ns not significant

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was caught at all stations. All of the three estuarine specieswere only found at stations 1 and 2, at all other station onlytwo of them were found. Eight anadromous species werecollected at all stations (Table 3).

Regarding the numbers of individuals, the Elbe estuary wasdominated by the anadromous fish species at all stations(Table 3). The abundance of marine stragglers and marineestuarine-opportunists decreased in an upstream directionfrom station 1 to station 11 (Table 3).

The percentage of anadromous species increased from81.7 % at station 1 to 96.8 % at station 2 and to 99.7 % at

station 6. Stations 7 to 11 also showed high numbers ofindividuals and percentages between 91.5 and 99.1 %. Thecontribution of freshwater species increased from<0.1 % atstation 1, to 0.5 % at station 2, 0.7 % at station 4, 5.7 % atstation 5, and 7.9 % at station 7. Then their contributiondeclined to 2.5 % at station 11 (Table. 3). Marine estuarine-opportunists were detected in higher quantities only at stations1 to 3 (5.0–14.8 %), then at station 5 with 1.3 %. Theydeclined further upstream to<0.1 % at stations 8 to 11.Higher densities of estuarine species were only found atstation 1 with a proportion of 3.5 %. Marine stragglers and

Fig. 5 Our records of water temperatures with their standard deviationsin comparison with data provided by FGG Elbe (2013) at the times ofeach fishing campaign in the Elbe estuary during the 2 years ofinvestigation

Fig. 6 Our records of oxygen concentration with their standard devia-tions in comparison with data provided by FGG Elbe (2013) at the timesof each fishing campaign in the Elbe estuary during the 2 years ofinvestigation

Table 4 List of the 15 dominant fish species and their interannual comparison between 2009 and 2010. Ser. No.=Serial number

Ser. no. Species Percent [%] 2009 Ser. no. variation Species Percent [%] 2010

1 Osmerus eperlanus 96.68 1 –>1 Osmerus eperlanus 93.21

2 Alosa fallax 2.21 3 –>2 Gymnocephalus cernua 1.61

3 Gymnocephalus cernua 0.81 12 –>3 Abramis brama 1.21

4 Clupea harengus 0.10 15 –>4 Blicca bjoerkna 1.10

5 Sprattus sprattus 0.05 2 –>5 Alosa fallax 0.71

6 Syngnathus rostellatus 0.05 7 –>6 Platichthys flesus 0.61

7 Platichthys flesus 0.03 10 –>7 Sander lucioperca 0.46

8 Liparis liparis 0.02 5 –>8 Sprattus sprattus 0.36

9 Gasterosteus aculeatus < 0.01 4 –>9 Clupea harengus 0.35

10 Sander lucioperca < 0.01 9 –>10 Gasterosteus aculeatus 0.25

11 Anguilla anguilla < 0.01 8 –>11 Liparis liparis 0.03

12 Abramis brama < 0.01 11 –>12 Anguilla anguilla < 0.01

13 Leuciscus idus < 0.01 13 Leuciscus idus < 0.01

14 Pomatoschistus minutus < 0.01 6 –>14 Syngnathus rostellatus < 0.01

15 Blicca bjoerkna < 0.01 15 Gasterosteus gymnurus < 0.01

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the catadromous species A. anguilla never contributed morethan 0.1 % to the whole number of individuals (Table 3).

The distributions of boreal und lusitanian species decreasedin upstream direction between stations 1 and 11. The number

of boreal species declined from 18 at station 1 to 14 at station3, 13 at station 2, eight at stations 4, 5, 6, and seven to eight atstations 7, 9, 10, 11 (Table 3). A similar trend was found in thelusitanian species. Here, the numbers decreased from 16 at

Fig. 7 Salinity of the ebb andflood tides at the times of eachfishing campaign in the Elbeestuary during the 2 years ofinvestigation

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station 1 to 11 at station 3, nine at station 2, seven at station 4,five at stations 5 and 7, and four at stations 6 and 8 to 11. Thetwo Atlantic species A. anguilla and Salmo salar were caughtat all stations. Freshwater fish species decreased in a down-stream direction between stations 11 and 1. The number ofindividuals of freshwater species showed the same results. Indetail, most freshwater fish species belonged to the Europeanguild and they decreased in downstream direction. The secondimportant guild with five species was the Euro-Siberian guild,which appeared at all stations with only two species at station1 and four species at all other stations. From the Holarctic andNordic guilds only one species appeared at each of the 11stations. One of the two Pale-Artic species was caught at allstations, but only at station 5 both species were collected(Table 3).

Interannual comparison of seasonal changes of the fish faunacomposition

The seasonal species composition underwent interannualchanges comparing both years of investigation at each of thesampling sites in the Elbe estuary (Table 5). Here,O. eperlanus was the dominant species in 2009, followed byA. fallax and G. cernua. The marine estuarine-opportunistsC. harengus and S. sprattus followed in fourth and fifthposition. In contrast, O. eperlanus and G. cernua were alsodominant in 2010, but in that year the freshwater speciesA. brama and B. bjoerkna followed in third and fourthposition.

Another example was the marine estuarine-opportunistL. liparis which appeared regularly at stations 1 to 3 betweenspring and autumn 2009, whereas the species appeared only atstation 1 in June and October 2010.

Other examples are A. brama and B. bjoerkna which werefound at stations 4 to 6 from April to October 2009, and atstations 7 to 11 in each month in 2009, whereas they were alsofound at stations 1 to 3 from August to October 2010. Furtherexamples are C. harengus and S. sprattus with fewer occur-rences at station 2, and in the case of S. sprattus, also at station

3 in 2010. Records of S. lucioperca, in contrast, were higher in2010 compared to 2009, when this species was also found atstations 1 to 3.

Influence of environmental variables

Generally, water temperature was lower in 2009 than in2010. A comparison of our water temperature data withexternal values of official sources (FGG Elbe 2013)indicated that the course of both data sets is very similar(Fig. 5). Single differences probably arise from differenttime points and localities used for the measurements. Thelowest water temperatures occurred between December2009 and April 2010, highest in July and August 2010(Fig. 5). Oxygen concentrations measured during oursampling ranged between 5 and 10 mg/l, were lowestin July 2009 and 2010 and highest in March 2010. Thesevalues were also in line with the values provided byofficial sources (FGG Elbe 2013, Fig. 6). Although floodand ebb tide values of salinity showed greater differ-ences, a typical longitudinal trend of decreasing salinitywas estimated (Fig. 7). These typical longitudinal trendscan also be seen by official data (FGG Elbe 2013),whereby the lowest salinity was measured at theHamburg harbour (0.2) and the highest was measuredat Cuxhaven (22.5).

Another important parameter with potential power to affectspecies abundance is the river run-off. Here, we evaluated datafrom official site (FGG Elbe 2013), whereby the highest riverrun-off in 2009 and 2010 was measured between autumn andspring.

Therefore, the ichthyofaunal composition in the Elbeestuary, especially the proportion of life cycle categories,was strongly influenced by the longitudinal salinity gra-dient. Using the Bio-Env analysis, the maximum corre-lation between the fish assemblage structure and envi-ronmental variables was obtained for a parameter com-bination comprising salinity, water depth, water temper-ature, and oxygen concentration (r=0.5).

Table 5 River run-off data (m3/s) from the water gauge “Neu Darchau” between 1989 and 1993 as well as 2009 and 2010, according to FGG Elbe(2013)

Year\Month Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

1989 1,193.67 642.53 794.64 571.63 568.74 256.83 318.96 242.67 252.56 328.64 437.36 612.38

1990 578.25 561.92 990.87 481.10 444.45 361.43 275.41 203.03 273.26 288.67 379.33 520.00

1991 740.64 394.03 479.64 451.76 406.64 320.29 322.53 366.19 219.90 227.74 293.43 351.25

1992 749.5 640.10 880.64 1,124.93 560.41 289.96 320.48 230.19 242.76 248.70 353.6 569.41

1993 602.06 639.03 709.45 653.43 367.61 365.96 370.19 394.83 393.30 365.93 441.16 826.00

2009 468.74 499.75 1,575.67 1,075.13 517.29 434.5 750.35 389.67 258.06 363.25 603.03 681.74

2010 775.64 589.25 1,386.77 1,178.40 753.45 983.46 393.41 1,035.54 921.86 1,304.58 1,032.10 1,442.25

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Discussion

Catch efficiency of the stow net

A stow net fishery provides an effective sampling method forthe Elbe estuary as well as for all other tidal affected areas ofthe world like shown, e.g., by Breckling and Neudecker(1994), Maes et al. (2001), Thiel and Potter (2001), andMacbeth et al. (2005). A wide range of different fish specieswith regard to size, weight, behaviour, and habitat specifica-tions could be caught with stow nets including both small andlarge species (Thiel and Potter 2001). Although it must beassumed that the abundance of very small species (e.g.Gobiidae) or species which live in shallow shore habitats(e.g. Lota lota) is underrepresented in stow net catches(Breckling and Neudecker 1994), stow net catches recentlyprovided first records even of very small species of Gobiidae,e.g. of P. lozanoi in the Elbe estuary (Eick 2012) and ofGobiosoma bosc in the Weser estuary (Thiel et al. 2012).Comparisons with other fishing methods (e.g. trawl net) indi-cated that the stow net fished the entire water column effi-ciently by collecting pelagic as well as benthic fish species,and also caused reduced damage to small individuals(Breckling and Neudecker 1994; Zhang 2005; Zhang et al.2008).

Contribution of life cycle categories and geographicaldistribution categories in relation to environmental factors

The contribution of marine species (25 species, 40.3 %) tototal species number estimated in this study for the Elbeestuary for the period 2009–2010 was somewhat lower thanreported earlier by Thiel and Potter (2001) with 31 species(53.4 %) for the time period 1989–1993. The contributionmade by freshwater species (25 species, 40.3 %) was similarcompared to the contribution of marine species, but washigher than estimated by Thiel and Potter (2001) (17 species,29.3 %). Possible reasons are lower salinities at downstreamstations during the period 2009–2010 and a higher samplingeffort at upstream sampling sites in the present study. Anotherreason could also be the different values of river run-offbetween the time periods compared. The years 1989–1993showed lower river run-off, especially between autumn andspring, than the years 2009 and 2010. The river run-off inthese years was very high in March and April 2009 andMarch, April, August, and October 2010. This entry of fresh-water probably restricted the upstream migration of marinespecies.

A high proportion of marine species is also typical for theZeeschelde estuary, the inner Severn estuary, the Thamesestuary and other holarctic estuaries where marine speciescomprise more than 50 % of all fish species (Potter et al.1997; Araújo et al. 1998; Maes et al. 1998a; Thiel 2011).

However, the contribution of marine species to the total num-ber of individuals was only 0.4 % in the Elbe estuary in 2009and 2010, and 9.1 % from 1989 to 1993 (Thiel and Potter2001). This is in contrast to other holarctic estuaries, where thenumber of individuals of marine fish species contributes be-tween 70 and 90 % to total abundance of fish species (e.g.Hamerlynck and Hostens 1994; McLusky and Elliott 2004).

Compared to investigations by Thiel and Potter (2001), notonly a lower percentage of marine species individuals, butalso a higher proportion of freshwater species individuals(1.4 % vs. 0.5 %) was found.

The higher contribution of individuals of marine fish spe-cies found by Thiel and Potter (2001) could be based onhigher abundances of juveniles of marine fish species duringthe first time period. Another reason could be that highersalinities at the downstream stations and a very low river runoff occurred in this time period. Additionally, the observeddifferences could result from difference in year class strengthsof selected fish species in the bordering North Sea, but no datain the necessary quality are available to prove this. However,our study also suggests that the Elbe estuary plays an impor-tant role as a nursery ground, not only for anadromous, butalso for marine species, which could be an effect of watertemperature-dependent growth rates and hence of survivalrates (e.g. Perry et al. 2005; Dulvy et al. 2008; Doney et al.2012).

Anadromous species comprised 98.1 % of the individualsduring the present study, but only eight anadromous species(12.8 % of all fish species) were caught. The high proportionsof the anadromous smelt O. eperlanus in the Elbe estuarycould be explained by high reproduction success of this spe-cies due to relative intact spawning grounds in the Elbeestuary upstream the Hamburg harbour. Additionally, highdensities of Eurytemora affinis which serve as larval food inthe nursery grounds ensure optimal growth conditions, and therather high water quality with appropriate oxygen concentra-tions provides conditions for optimal growth and reducedmortality (e.g. Lillelund 1961; Sepúlveda et al. 1993;Sepúlveda 1994; Thiel 2001).

In the present study, only three estuarine fish species(Syngnathus rostellatus, Pomatoschistus microps, andCoregonus maraena) with very low numbers of individuals(<0.1) were detected. The very low abundance of this life cyclecategory is typical for several holarctic estuaries like the Severnestuary or the Thames estuary, where changing tidal watermovements, turbidity, and variable salinity influence the devel-opment of eggs and larvae (Thiel and Potter 2001).

Boreal and lusitanian species dominated the fish fauna atsampling sites close to the North Sea, these life cycle catego-ries mainly contain marine species. However, there have beenchanges of marine fish species distribution in marine waters,in the last 50 years especially, caused by climate change (e.g.Perry et al. 2005; Doney et al. 2012).

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In the upstream direction of the Elbe estuary, the percent-age of freshwater species increased, whereby freshwater fishspecies belonged to the European guild. The second importantguild comprising five species was the Euro-Siberian guild,followed by the Holarctic and Nordic guilds. One of the twoPale-Artic species was caught at all stations. Regarding theindividual numbers, boreal species dominated at all stations,followed by lusitanian and freshwater species. However,according to Sala et al. (2000), the decline in biodiversity offreshwater species is likely to exceed that of species in marineecosystems until the year 2100. This will eventually lead tofuture changes of the present proportions of the geographicaldistribution categories in the Elbe estuary and other Europeanestuaries.

Spatial and temporal differences in species compositionin relation to environmental factors

In comparison to upstream stations, a more diverse fish spe-cies composition was found at downstream sampling sites.These differences were probably caused by salinity differ-ences between downstream and upstream stations as well asby greater salinity changes at downstream stations during thetidal cycle (Fig. 7). Tidally triggered changes of the fishassemblage similar to those found in the present study werealso detected by Thiel and Potter (2001).

Significant longitudinal changes in the fish fauna commu-nity were revealed by the declining percentage of marinespecies in the upstream direction which was probably causedby the tendency of marine stragglers to stay in the lowerestuary. The upstream decline in total abundance is partiallyassociated with the fact that mean abundances of marineestuarine-opportunists, like C. harengus, P. flesus, andS. sprattus, decreased in the upstream direction. Moreover,other species like L. liparis and Pleuronectes platessa werenot found further upstream than station 3, and some specieslike G. morhua and P. lozanoi only appeared at two of thethree most downstream stations. Similar observations weremade in other estuaries where diverse marine stragglers andmarine estuarine-opportunists showed higher abundances atstations closer to the sea (e.g. Marshall and Elliott 1998; Thieland Potter 2001; McLusky and Elliott 2004).

In contrast, the number of freshwater species increased inan upstream direction with only six species at the most down-stream station and 17 species nearest to the Hamburg harbour.This difference is certainly based on the limited osmoregula-tory ability of freshwater species and a key factor for theextent of entering the Elbe estuary in downstream direction.The most abundant freshwater species at downstream stationswere the Percidae G. cernua and S. lucioperca which have ahigher salinity tolerance than e.g. species of the familyCyprinidae (Lehtonen et al. 1996; Vetemaa and Saat 1996;Kafemann et al. 1998; Brown et al. 2001).

Although the seasonal changes of the fish fauna composi-tion in the Elbe estuary showed interannual differences, theirprincipal seasonal patterns were similar to other holarcticestuaries like the Severn estuary, the Humber estuary and theScheldt estuary (Marshall and Elliott 1998; Potter et al. 2001;Maes et al. 2005).

The ichthyofauna composition in the Elbe estuary un-dergoes progressive changes due to longitudinal migrationsof various marine and freshwater species. Furthermore, differ-ences in salinity tolerance and habitat preferences of differentspecies (Elliott and Hemingway 2002) contributed to struc-turing the fish species assemblage throughout the Elbeestuary.

Looking at the two different years of investigation and theeleven sites sampled, it becomes apparent that interannual andspatial differences of the fish fauna structure exist in the Elbeestuary. These differences may reflect the variability in therecruitment success of certain species of marine estuarineopportunists. L. liparis, for example, was represented in2009 in higher quantities than in 2010 and showed regionaldensity differences between the 2 years of investigation. Thisvariability in the recruitment success of L. liparis into the Elbeestuary shows similarities to other marine fish species in otherestuaries (e.g. Loneragan et al. 1989; Valesini et al. 1997;Nicolas et al. 2011).

The progressive change of the ichthyofauna of the Elbeestuary in an upstream direction is similar to findings, e.g., inthe Thames estuary and the Río de la Plata estuary (Araújoet al. 1998, 1999; Jaureguizar et al. 2004), whereas cleardifferences exist compared to southern Australian estuaries.Those differences are based on the fact that the last mentionedestuaries differ in their morphology (Loneragan et al. 1989;Potter and Hyndes 1999) from a number of, e.g., holarcticestuaries like the Elbe estuary, which leads, e.g., to differentsalinity conditions of salinity gradient. Differences of otherenvironmental factors like water temperature, oxygen concen-tration, and water depth are also possible reasons for spatialand temporal differences of the fish fauna in the Elbe estuaryas well as other holarctic estuaries. Seasonal changes of watertemperature have, for instance, the ability to affect the fishfauna structure during the course of the season. During theseason, different fish species appear at different times inhigher numbers, since they have different reproduction pe-riods. For example, the adults of the anadromous twaite shadoccur in higher numbers between April and June in the Elbeestuary (Magath and Thiel 2013), whereas many of freshwaterfish species perform an upstream run during winter and spring(Claridge et al. 1986; Thiel and Potter 2001; Thiel 2011).According to our study, the seasonal course of fish abundancein the Elbe estuary was strongly affected by the juveniles ofthe fish species. Their abundance is controlled by, e.g., sea-sonal water temperatures (Thiel et al. 1995). Moreover, inter-annual changes of water temperature were of importance.

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Very low water temperatures in winter can, for instance, causea higher mortality of juvenile fish (Costa et al. 2002).

According to our study, oxygen concentration levelsaffected the fish species distribution in the Elbe estuary.According to Poxton and Allouse (1982) a lot of marine fishspecies are stressed at oxygen concentrations of≤4.5 mg/l.However, Pomfret et al. (1991) pointed out that dissolvedoxygen levels less than 7.5 mg/l coupled with temperaturesgreater than 15 °C (which mainly occur during summer) act asa barrier to fish movement. Thus, low levels of oxygen con-centrations will affect the species composition through thetolerance limits of the different species, although the syner-gistic effects of other parameters, e.g. high temperature, willinfluence those tolerance limits (Almeida et al. (1993); Pihl(1994); Maes et al. 1998b and Marshall and Elliott 1998).

The effect of water depth on the fish fauna structure hasbeen shown in some other estuaries e.g. the Humber estuary inthe U.K. Here, the three-spined stickleback has a high abun-dance in shallow waters (Marshall and Elliott 1998).Comparable results were obtained during our study in theElbe estuary, where this species was found in higher abun-dances in shallow water areas.

Hence, we have shown that the main changes of the fishfauna structure in the Elbe estuary occurred dependently onvariations of environmental factors with regard to spatial andtemporal gradients as follows: salinity: along the estuary;water temperature: seasonally; oxygen concentration: season-ally; water depths: across the estuary.

Acknowledgments We would like to express our gratitude to Mr. W.Zeeck and Mr. C. Zeeck for their support during this work. Many thanksalso to Mr. B. Trübner for his help and support during the sampling. Thiswork was partly funded by the Federal Ministry of Education and Re-search (BMBF) within the research project Klimzug-Nord (grant number01LR0805D).

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