II. Field investigations - distribution and transport

Rapp. P.-v. Réun. Cons. int. Explor. Mer, 191: 43-49. 1989

Transport mechanisms of larval plaice (.Pleuronectes plates sa L.) from the coastal zone into the Wadden Sea nursery area

M. J. N. Bergman, H. W. van der Veer, A. Stam, and D. Zuidema

Bergman, M. J. N., Veer, H. W. van der, Stam, A ., and Zuidema, D. 1989. Transport mechanisms of larval plaice (Pleuronectes platessa L.) from the coastal zone into the Wadden Sea nursery area. - Rapp. P.-v. Réun. Cons. int. Explor. Mer, 191: 43-49.

The transport mechanism of larval plaice from the coastal zone into the Wadden Sea area is investigated. From a study of larval distribution in the plankton inside the Wadden Sea area, it is concluded that larval plaice seem to be transported passively by the tidal currents. Consequently the magnitude of the larval supply through a tidal inlet depends on the amount of "new" North Sea water entering a tidal basin with each tide. Depending on the morphology of the various tidal basins which determines the settlement of larvae, the key factor in the process of larval transport is either the absolute amount of “new” North Sea water entering with the flood tide or the amount of North Sea water still present in the area after the subsequent ebb tide.

M. J. N. Bergman, H. W. van der Veer, A . Slam, and D. Zuidema: Netherlands Institute fo r Sea Research, P.O. Box 59, 1790 A B Den Burg, Texel, The Netherlands.

Introduction

The Wadden Sea is one of the major nursery areas for North Sea plaice, Pleuronectes platessa (Zijlstra, 1972). About 60% of the recruitment of juveniles to the parent stock in the North Sea is thought to originate from this area (Anon., 1985). Extensive studies in the Dutch and German parts of the Wadden Sea have revealed that for the major proportion of the plaice population food conditions are excellent, resulting in a maximum poss­ible growth at the prevailing water temperatures (Kui- pers, 1977; Zijlstra et al., 1982; van der Veer, 1986; Berghahn, 1987; Bergman et al., 1987, 1988). This means that as regards the productivity, and hence the carrying capacity of the area for plaice, the limiting factor seems to be the larval supply from the North Sea.

Spawning of plaice occurs from about December to March, during which time the adults concentrate in specific areas in the open sea. The main spawning areas are the Southern Bight and the German Bight (Simpson, 1959; Harding et al., 1978). The developing eggs and larvae are transported by the residual currents from these spawning grounds towards the coastal zone (Tal­bot, 1976, 1978). There is little information available on the subsequent transport mechanism from the coastal zone through the tidal inlets into the nursery. In general, two types of transport mechanism can be distinguished. One is a purely passive transport, the tidal currents

acting as the forcing function causing a passive swirling up and sinking of the individuals in the water column depending on the tidal current, just like suspended matter. The other type is an active one: a retention by the larvae from being swirled up or a swimming behaviour under the influence of environmental factors. Inside the Wadden Sea, the distribution of plaice larvae in the plankton seems to be determined by a com­bination of active and passive components (Rijnsdorp et a l., 1985).

In this paper the transport of plaice larvae is studied in more detail. First, the distribution of larvae in the Wadden Sea area is studied, and, based on the results, an attempt is then made to explain the transport from the open sea into the various tidal basins of the Dutch Wadden Sea and to determine the key factor in the process of larval transport.

Materials and methods

Figure 1 shows the area of study, the Dutch Wadden Sea, with the main tidal inlets and the corresponding tidal basins. The borders between the various tidal basins are formed by watersheds running between the islands and the mainland coast, and they are char­acterized by a negligible water exchange. The total surface area amounts to about 2800 km2.

43

North Sea

Germany

The Netherlands

Larval distribution inside the W adden Sea

The larval distribution of plaice in the Wadden Sea was studied at three stations in the Ems-Dollard Estuary in April 1982 (Fig. 2). At each station fishing was carried out at a fixed depth stratum at one-third of the total water depth. This was done as continuously as possible during a whole tidal cycle. The weather conditions during sampling, as well as some hydrographic data, are listed in Table 1.

All hauls were made with nets of polyamide plankton gauze (Monodur no. 2000, 2 mm aperture) with an opening of 0.7 m2 and a length of 5 m. The porosity (0.59) and the mesh area (12 m2) of the net (definitions according to Smith et al., 1968) proved to be large enough to prevent overflow of the net. In general, reduction of the water flow through the net amounted to about 10% (van der Veer and Sadée, 1984). The

EMS-DOLLARD

2 0 k m

Figure 2. The Em s-Dollard estuary with the sampling stations. 1: Borkum; 2: Oostereems; 3: Termunten.

Figure 1. The Dutch Wadden Sea together with the main tidal inlets and corresponding tidal basins. 1: Marsdiep; 2: Eierlandse Gat; 3: Vlie; 4: Borndiep; 5: Pinkegat; 6: Friesche Zeegat; 7: Eilanderbalg; 8: Lauwers; 9: Schild; 10: Ems-Dollard.

amount of water filtered was measured with a flowmeter mounted in the mouth of the net. Depending on the current velocity (up to 200 cm s-1), haul duration varied between 5 and 15 min, providing samples from between 100 and 500 m3 filtered water. In this way, about 40 hauls could be made during a tidal cycle.

After the net washdown, the sample was preserved in 4% formaldehyde seawater solution and sorted out in the laboratory. All flatfish larvae were counted and the numbers converted into densities per 1000 m3. No correction was made for net efficiency, since previous studies did not suggest any net avoidance of the larvae related to current strength or larval length (van der Veer, 1985). Current velocity (cm s ' 1), salinity (ppt), and sediment load (mg 1“ *) of the water were measured simultaneously.

Larval immigration into the W adden Sea

Since it is impossible to get an accurate indication of the total amount of larvae transported through all inlets in spring between February and May, and because a significant relationship exists between this total immi­gration and the ultimate abundance of demersal 0-group plaice in a tidal basin in autumn (Rijnsdorp etal., 1985), a demersal fish survey in autumn covering all tidal flat and sublittoral areas in the tidal basins has been used

Table 1. Characteristics of the sampling stations in the E m s- Dollard estuary in 1982.

Location DateWind stress

(m s*1)

D epth at low water

(m)Sampling depth (m)

Borkum 26-04 8-13 9 3Oostereems 27-04 4-8 9 3Termunten 28-04 10-13 6 2-3

44

as an index of larval supply through the various inlets in spring.

The demersal survey was carried out in August and early September 1987 and covered the whole Wadden Sea. Previous work (Zijlstra et al., 1982) showed that the distribution of 0-group plaice is restricted to the tidal flat areas and to a lesser extent to the sublittoral areas. The survey therefore remained restricted to these areas and the abundance of 0-group plaice in the deep tidal channels was assumed to be negligible.

At the tidal flats, demersal fishing was carried out with a rubber dinghy equipped with an outboard motor (25 hp) and a 1.9 m beam trawl with a single tickler chain and a mesh size of 5 x 5 mm, at a speed of approximately 35 m min-1, following the procedure of Riley and Corlett (1966). All sampling was carried out during daytime and was restricted to a period of 3 h around high water, during which tidal phase the distri­bution of juvenile plaice appeared to be more or less random on the tidal flats (Kuipers, 1977). The position of a haul was located with a Philips AP Navigator fitted in the rubber dinghy, and the exact length of a haul was assessed with a meter wheel fitted outside the trawl. Each haul covered a surface area of at least 250 m2. All catches were sorted out immediately and numbers caught were corrected for net efficiency, according to Kuipers (1975), and expressed in numbers per 1000 m2 (n. 1000 n r 2).

In the sublittoral areas fishing was carried out with a3.0 m beam trawl with a single tickler chain and a mesh size of 1 x 1 cm. The trawl was towed by RV “Navicula” at a speed of approximately 60 m min-1 during a period of 3 h around high water in daytime. The location and exact distance of a haul was assessed with a Philips AP Navigator. Each haul covered a surface area of at least 900 m2. All catches were sorted immediately and the numbers caught were corrected for net efficiency and converted into numbers per 1000 m2 (n. 1000 m-2). The net efficiency of the 3.0 m beam trawl was inves­tigated by fishing simultaneously with a 1.9 m and a3.0 m beam trawl 20 times. A comparison between the catches of the 3.0 m beam trawl and the corrected ones in the 1.9 m beam trawl resulted in the following relationship between efficiency of the 3.0 m beam trawl (EFF; %) and the size of the fish (L; cm):

EFF = 3 7 .4 -1 .3 1 L

Results

Larval distribution in the W adden Sea

Weather conditions were not optimal during sampling. Half of the time, wind stress amounted to between 8 and 13 m s-1. As a result, turbidity of the water mass will have fluctuated, depending on wind stress, and might have resulted in some masking of the transport

mechanisms. Furthermore, the Ems tidal basin is rather large, with a tidal volume of about 1000 million m3, and when sampling at a fixed station in a wide tidal channel, it is impossible to follow the same water mass during the whole tidal cycle. Because of lateral variations in current velocity, changing water depth, and ship posi­tion, fairly large fluctuations will be induced. First, mean densities at the various sampling stations will be presented to indicate the general direction of the transport. Subsequently, mean densities during flood and ebb tide will be compared and, finally, flood and ebb differences will be analysed and related to either current velocity or sediment load of the water. It must be kept in mind that current velocity and sediment load of the water are not independent. Depending on current strength, a varying proportion of the sediment com­position will be swirled up or will settle. So, during a tidal cycle this sediment load will have a fluctuating grain-size distribution.

The plaice densities during the tidal cycle at the three stations in the Ems estuary are presented in Figures 3, 4, and 5 together with the current velocity, salinity, and sediment load of the water. Large fluctuations in plaice

EMS-DOLLARD STATION 1

160-

120-

80-

40-

0-200 -

cn 120- £

p lo i c e lo rvoe

sus pe nd e d m a t t e r

sa l i n i t y

(r = -0 .55; N = 30)

200 -

cu r re n t v e lo c i ty160-

4 0 -f lo od ebb

o-112 13 14 15 16 17 18 19 208 9 10 II

local t im e in h ou rs ( g m t + 2 )

Fig. 3. Abundance of plaice larvae (n. 1000 m~3) in relation to suspended matter (mg I-1), salinity (ppt), and current velocity (cm s~ *) during a complete tidal cycle at Station 1 in the Em s- Dollard on 26 April 1982.

45

EMS - DOLLARD STATION 2

p lo ic e lo rvoe

4 0 0 -s u s p e n d e d m a t t e r

_ 300-

200 -

100-

s a l in i t y2 1 -

19-1

200 - cu r re n t v e lo c i ty

160-

a> 120-

8 0 -

4 0 -ebb f lo od ebb

0J15 16 17 18 19 20

locol t im e in h ou rs ( g m t + 2 )

Fig. 4. Abundance of plaice larvae (n. 1000 m*3) in relation to suspended m atter (mg I“ 1), salinity (ppt), and current velocity (cm s “ 1) during a complete tidal cycle at Station 2 in the Em s- Dollard on 27 April 1982.

abundance could be observed between the stations and during a tidal cycle. Mean densities of plaice during the tidal cycles are listed in Table 2. Highest densities were found in the inner parts of the estuary. The differences in mean abundance in the Ems estuary suggest an accumulation of the larvae in the inner parts. This accumulation is supported by the differences in flood and ebb tide (Table 2). With the exception of the outermost part of the estuary, there is a clear difference between flood and ebb tide. Table 3 lists the relation­ships between the abundance of plaice larvae and the

Table 2. Mean densities of plaice larvae during the whole tidal cycle and separated for flood and ebb tide at the various stations in the Ems-Dollard estuary.

Location

Density (n. 1000 m 3)

Whole cycle Flood tide Ebb tide

Borkum 33.8 28.6 39.0Oostereems 94.3 135.3 53.2Termunten 83.4 128.6 38.1

EMS-DOLLARD STATION 3

p la ice la rvae3 0 0 -

o 2 0 ° -oo- 100-

c

0J1000-

s u s pe nd e d m a t te r8 0 0 -

600-

£ 400-

200 -

0J2 0 -

18-CLQ. sa l in i t y

16-

14-

200 - cu rre n t v e lo c i ty

160-

5 120-

i 80-

40-f lo od ebbEBB

15 16 17 18 19 20

local t im e in hou rs ( g m t + 2 }

Figure. 5. Abundance of plaice larvae (n. 1000 m 3) in relation to suspended m atter (mg I“ 1), salinity (pp t), and current vel­ocity (cm s_ ') during a complete tidal cycle at Station 3 in the Ems-Dollard on 28 April 1982.

sediment load of the water. At the two outermost sta­tions the relationships, during both flood and ebb tide were significant, which means that the transport of larvae during these tidal phases can be explained by assuming a passive transport, as in the case of sediment. At the innermost station, the relationship is found only during flood tide. Furthermore, the relationships during flood tide become less pronounced from the outer part of the estuary towards the inner part.

Table 3. Relationship between sediment load of the water and larval plaice abundance at the stations in the Ems-Dollard estuary. (Spearman rank correlation R s.) Num ber of obser­vations in parentheses.

Location

Spearman rank correlation

Flood tide Ebb tide

Borkum 0.62 (22)** 0.78 (16)**Oostereems 0.55 (28)** 0.75 (25)**Termunten 0.33 (23)* -0 .1 9 (16)

* p < 0.05; ** p c O .O l.

46

Table 4. Mean densities (n. 1000 m 2) and absolute numbers of 0-group plaice in each tidal basin on the tidal flats (T) and in the sublittoral area (S).

Tidal basin

Mean density (n. 1000 m 2) Total abundance (x lO 6)

T S T + S T S T + S

Marsdiep 38.1 12.1 18.07 4.76 5.07 9.83

Eierlandse Gat 60.5 22.0 51.38 7.02 0.79 7.81Vlie 38.4 50.7 43.91 12.52 13.44 25.95Borndiep 39.5 32.1 37.74 7.98 2.02 10.00

Pinkegat 78.8 18.5 70.15 4.49 0.28 4.77Friesche Zeegat 47.9 37.2 45.53 4.60 1.00 5.60

Eilanderbalg 112.0 5.1 95.00 3.58 0.03 3.61

Lauwers 96.3 12.4 80.74 10.59 0.31 10.90Schild - - - - - -Ems-Dollard 30.7 13.5 25.21 7.46 1.54 9.00

Total 63.00 24.48 87.48

Larval immigration into the W adden Sea

At each of the 200 locations fished, between four and five hauls were made, resulting in an overall total of about 1000 hauls. For each tidal basin mean densities were calculated for the intertidal zone and the sublittoral zone (Table 4). Mean densities on the tidal flats seemed to be fairly similar, between 38 and 112 individuals per 1000 m2. In the sublittoral area the variation is between 5 and 50 per 1000 m2. Multiplying these values by the surface areas of the intertidal zone and sublittoral area (Table 5) resulted in an estimate of total abundance per tidal basin. In both the intertidal zone and in the sublittoral area the variation in total abundance is more extreme than that observed in the mean densities. This is a result of the differences in surface areas between the various basins. A total abundance of 87.5 x 106 0-group plaice was estimated in the intertidal zone and sublittoral area of the Dutch Wadden Sea at high water in August-September 1987.

Discussion

Larval distribution in the W adden Sea

In fish species where spawning areas and nurseries are spatially separated, such as in North Sea plaice, trans­port mechanisms of the developing eggs and larvae play an important role. The larval plaice distribution, as found during the survey in the Ems-Dollard, showed increasing densities towards the inner part of the estu­ary. This picture was especially clear during flood tide, and the significant correlations of the plaice abundance during this tidal phase with the sediment load of the water, suggests that the transport mechanism would have been a passive one, resulting in an accumulation of the larvae similar to what occurs in suspended matter and has been found for barnacle larvae (de Wolf, 1973). Most other studies about larval transport mechanisms suggest active behaviour components (for a review, see Boehlert and Mundy, 1988); however, because of the experimental designs of most studies the importance of passive transport cannot be analysed.

Table 5. Surface area of tidal flats, sublittoral and tidal channels for the tidal basis of the Dutch Wadden Sea. Contribution to total area in parentheses. After Dijkema (pers. comm.).

Tidal basin

Surface area (km2)

Total Tidal Flats Sublittoral Channel

Marsdiep 696 125 (18%) 419(60%) 152 (22%)

Eierlandse Gat 163 116(71) 36 (22) 11 (7)

Vlie 692 326 (47) 265 (38) 101 (15)

Borndiep 293 202 (69) 63 (22) 28 (10)

Pinkegat 70 57 (81) 11(16) 2 (3 )

Friesche Zeegat 143 96 (67) 27 (19) 20 (14)

Eilanderbalg 40 32 (80) 6(15) 2 (5)

Lauwers 148 110 (74) 25 (17) 13 (9)

Schild 27 23 (85) 4 (15) 0 (0)

Ems-Dollard 511 243 (48) 114 (22) 154 (30)

47

At high water an increasing retention of larvae from the plankton could be observed from 0% near Borkum to 71% at the innermost station. Creutzberg et al. (1978) and Rijnsdorp etal. (1985) assume that the larvae settle if they arrive at suitable feeding grounds on the tidal flats at flood tide, or that they remain in the plankton during the subsequent ebb tide. Such a mechanism would explain the observed differences in retention rate between the outer and inner stations. At Borkum very few tidal flat areas are submerged by the flood water, so hardly any settlement will occur. As a consequence the retention rate will be low, resulting in about the same densities during ebb tide. Towards the inner part of the estuary, the mean water depth decreases and the surface area of the tidal flats increases. Therefore, the likelihood that larvae reach these suitable feeding grounds with the flood increases, resulting in higher retention rates towards the inner part of the estuary.

Larvae which have not settled at high water are washed back with the subsequent ebb tide. The signifi­cant correlations during ebb tide with suspended matter suggest that also this transport is a passive one, at least at the two outermost stations. At the innermost station, the relationship is absent, probably because the ebb water is too great a mixture of water originating from the tidal flats and from the gully.

Overall, the following picture takes shape: plaice larvae are transported passively in the estuary at flood tide and sink when current velocities drop around high water. If they encounter suitable feeding areas (tidal flats), they will settle and change to a demersal way of living. Otherwise, they are swirled up by the tidal cur­rents during the next ebb tide and transported back again. Due to the differences between flood and ebb tide in the estuary (Postma, 1961, 1982), the final result will be an accumulation of the larvae in the area after a number of tidal cycles. Like suspended material, the consequence will be an increased retention of the larvae in the inner part of the estuary where the tidal flats are more extensive.

Larval immigration into the W adden Sea

Based on the larval transport inside the Wadden Sea, it seems reasonable to assume that plaice larvae will be transported passively from the coastal zone through the various inlets into the Wadden Sea. This means that the magnitude of larval supply through an inlet will depend on the amount of “new” North Sea water entering with each flood tide. The strong tidal currents in the deep tidal inlets will prevent the larvae from sinking to the bottom at high water and in the tidal inlets retention of larvae will be low due to the absence of suitable settling areas, i.e. tidal flat systems. Consequently, most of the larvae will be washed back during the next ebb tide. The amount of larvae entering the area will therefore definitively be related with the amount of “new” North

Table 6. Mean tidal prism (106m3) and exchange coefficient (102m 3 • s“ 1) of the various tidal inlets of the Dutch Wadden Sea (after Philippart, 1988).

Tidal basinMean tidal

prismExchangecoefficient

Marsdiep 1077 39.3Eierlandse Gat 160 8.5Vlie 1117 50.1Borndiep 478 22.4Pinkegat 100 8.4Friesche Zeegat 200 10.9Eilanderbalg 70 7.0Lauwers 1601 8.8Schild 70 11.4Ems-Dollard 1000 40.3

Sea water still present in the area at the end of the subsequent ebb tide.

This amount of “new” North Sea water can be esti­mated for the various inlets from the exchange coef­ficients, expressed in m3s_1, which represent the amount of “new” North Sea water that has entered the Wadden Sea after a tidal period (Philippart, 1988). They are listed in Table 6, together with the mean tidal prisms of the various inlets, i.e. the total volume entering with flood tide. This amount of “new” North Sea water varies between 315 000 m3 and 1 813 500 m3 depending on the inlet, or between 16 and 73% of the corresponding mean tidal prism. As a consequence, differences in exchange coefficient between inlets with the same tidal prism will result in differences in mean larval abundance in the plankton, as indeed has been found for the Marsdiep and Vliestroom basin by Rijnsdorp et al. (1985). This view is supported by the relationship between the total amount of demersal plaice found per tidal basin in August - as an index of total larval supply in spring (Rijnsdorp et al., 1985), and the exchange coefficients of the corresponding inlets. A significant, though not perfect, relationship exists between the two (Fig. 6; Spearman’s rank correlation r = 0.73; n = 9; p < 0.01). There are other factors that may be of importance. First, differences in larval abundance in the open sea along the coast will affect the absolute larval supply through the various inlets. No information is available as to whether all inlets are supplied by one or more larval patches, or to what extent larval abundance varies along the coast. Secondly, some tidal inlets are characterized by a large tidal flat area directly associated with the main inlet, especially the smaller tidal basins. In such areas the retention of larvae enter­ing with the flood tide will be higher than in other larger basins, where most larvae are washed back with the ebb. In these areas larval supply will not be related to the amount of “new” North Sea water still present in the area after one tidal cycle, but with the amount of “new” North Sea water entering during the flood. Therefore, relating larval supply with the exchange

48

30 -

<ß

O •

D

1 2 -

6-

0 - |----------------- r-----------------1----------------- .----------------- 1-----------------1----------------- 1-----------------1-----------------1—

0 15 3 0 4 5 2 6 0exchange coefficient (10 m - s’ )

Figure 6. Relation between the exchange coefficient of fresh North Sea water of a tidal inlet (102m 3s ') and the absolute abundance of 0-group plaice (millions) in the corresponding tidal basin.

coefficients will result in an underestimation of larval immigration into these areas. A final factor might be the processes occurring after settling in spring. Although predation on the 0-group plaice population has been observed (van der Veer, 1986), these mortality factors seem to be density-dependent, which means that an overall pattern in larval abundance between the various inlets persists throughout the first year of life after settling. This is further supported by the relationship found by Rijnsdorp et al. (1985) between larval abun­dance in spring and demersal density in autumn.

The next step in solving this problem should be a comparison of larval abundance outside and inside the Wadden Sea in the vicinity of the various tidal inlets during larval immigration in spring.

Acknowledgements

Thanks are extended to the crew of RV “Navicula” and to H. Ridderinkhof and J. J. Zijlstra for their critical reading of the manuscript.

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Group, IJmuiden, 21-25 November 1983. ICES CM 1985/ G: 2.

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Riley, J. D ., and Corlett, J. 1966. The numbers of 0-group plaice in Port Erin Bay 1964-1966. Ann. Rep. Mar. Biol. Stn. Port Erin, 78: 51-56.

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Talbot, J. W. 1976. The dispersal of the plaice eggs and larvae in the southern Bight of the North Sea. J. Cons. int. Explor. Mer, 37: 221-248.

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Veer, H. W. van der. 1985. Impact of coelenterate predation on larval plaice Pleuronectes platessa and flounder Platichthys flesus stock in the western Wadden Sea. Mar. Ecol. Progr. Ser., 25: 229-238.

Veer, H. W. van der. 1986. Immigration, settlement and density-dependent mortality of a larval and early post-larval 0-group plaice (Pleuronectesplatessa) population in the west­ern Wadden Sea. Mar. Ecol. Progr. Ser., 29: 233-256.

Veer, H. W. van der, and Sadée, C. F. M. 1984. Seasonal occurrence of the ctenophore Pleurobrachia pileus in the western Wadden Sea. Mar. Biol., 79: 219-227.

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Zijlstra, J. J ., Dapper, R ., and Witte, J. IJ. 1982. Settlement, growth and mortality of post-larval plaice (Pleuronectes pla­tessa L.) in the western Wadden Sea. Neth. I. Sea Res., 15: 250-272.

49