ICES CM 2001/P:07 Not to be cited without prior reference to the authors Six years of German participation in the North Sea Acoustic Survey of Herring: Results and reflections on survey design Soenke Jansen, Christopher Zimmermann and Cornelius Hammer Abstract:

From 1995 onward Germany has been participating in the North Sea Acoustic Survey of Herring, surveying the south-eastern North Sea in June/July. A pre-survey-stratification by ICES statistical rectangles has been used. Plots of abundance by rectangle show that this part of the survey has covered the main concentration of herring in this area, as well as main concentrations of sprat for the whole of the North Sea. Recently, the data have been recalculated in a standardised way to produce an overview of the results and to prepare the development of an index for juvenile herring and sprat. The biomass of herring varied between 40 and 260 kilotons, that of sprat between 1 and 196 kilotons in the area covered. As the area is characterised by a very high proportion of juvenile herring in the catches, the contribution to the North Sea spawning stock biomass (SSB) of herring is very small. The year-to-year variability of abundance and biomass of both herring and sprat is high and no consistent trend in abundance could be detected over the period of investigation. Neither was there a significant trend in the length-at-age data. The geographical pre-survey stratification by ICES statistical rectangles is compared to post-survey stratification by similarity of cumulative length frequency distributions. The merits and drawbacks of both methods are discussed. Authors Adress: Soenke Jansen, Christopher Zimmermann and Cornelius Hammer: Federal Research Centre for Fisheries, Institute of Sea Fisheries Hamburg, Palmaille 9, 22767 Hamburg, F.R.Germany [tel: +49 40 38905 166, fax: +40 40 38905 263, e-mail: jansen.ish@bfa-fisch.de]

Introduction: Following a proposal by Parrish (1955), the ICES co-ordinated "Hydroacoustic Survey of Herring" in the North Sea and adjacent waters was established in 1979 and is conducted annually since 1983 in mid-summer in the ICES division IV as well as parts of VI and III by the United Kingdom (Scotland), the Netherlands, Norway, Denmark and occasionally Sweden and the Republic of Ireland. The purpose of the survey is to provide information on the overall abundance and distribution of the stock and specifically to gain an estimate of the spawning stock biomass (SSB) of North Sea autumn spawning herring for the use in stock assessment. From 1995 onwards, Germany has been participating in the survey, covering the south-easterly North Sea from about 2°W to the coast of Jutland and roughly from 54°N to 57°N. In none of the past 6 years the whole area was sampled and sampling tended to concentrate towards the geographical centre of this area of investigation. The development of an evaluation software to automatize the calculations of abundance and biomass from the acoustic and the haul data necessitated an explicit formulation of calculating procedures used in the this part of the survey. The data of the past years of the German part of the survey have in consequence been recalculated in a standardized manner. From these calculations it became apparent, that the pre-survey system of stratification into ICES statistical rectangles that had been used so far might introduce a high variability into the abundance estimate of the single stratum resulting from a relatively small number of biological samples inside each stratum. As a post-survey stratification by similarity of length-frequency distributions (LFDs) is used in other part-surveys and seems biologically more justifiable, the results from this alternative method of stratification were post-hoc compared to those from the system used up to now. In this paper, the results from this specific sub-survey are presented and the two systems of stratification presently used in the hydroacoustic survey of herring are compared using the year 1996 as an example. The merits and drawbacks of both stratification systems are discussed. Methods: Sampling: The "Hydroacoustic Survey of Herring" is conducted in the summer, mostly in June and July. In the part of the survey that has been covered by the German fishery research institute the research vessels "Walter Herwig" and "Solea" (1996 and 1999) have been employed to record echograms on parallel east-west tracks using a "Simrad EK 500" echo-sounder at 38kHz in the daylight hours between 06-22h local time (UTC+2). A maximum latitudinal distance of 15 nautical miles (n.mi.) between the tracks was aimed for. Thus, each surveyed ICES statistical rectangle of 0.5 by 1.0 (lat x lon) degrees was covered by at least two tracks. The echographs were scrutinized by eye and those echo traces that were decided to originate from schools of clupeids were integrated using a “Bergen Integrator” to arrive at 1-n.mi. track distance values of the nautical area backscattering coefficient sA [m2/n.mi.2]. An average of 2 hauls (1 to 5) of 30 minutes duration were conducted per stratum to sample the fish concentrations that produced the relevant traces in the echograms. Only those hauls that yielded more than 100 clupeids per hour trawled were considered valid - on average 0.9 hauls per stratum - and used for further calculations. From these hauls, the relative numerical species composition of clupeids and length-frequency distributions (LFDs) of about 200-300 fish (if available) per clupeid species were gained. 10 fish per species per half-centimetre length class per stratum were sampled (if available) to provide otoliths for age-reading and spawning-group differentiation, weight in grams and maturity-status; these fish could originate from more than one haul inside the stratum.

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Calculation procedures: The 1-n.mi. track distance sA -values were averaged for each stratum and this average multiplied with the area of the stratum to produce an estimate of the total backscattering cross-section. This area was divided by the mean backscattering cross section of a single clupeid in this stratum to arrive at the total number of clupeids per stratum which in turn was split between the species according to their relative numerical proportion in the pooled catches (weighted by catch rate) of the stratum. The mean backscattering cross section of a single clupeid was calculated by multiplicating of the mean squared length of the clupeids <L2> with a constant value that takes account of the agreed target-strength to length relationship of clupeids (TS = -71.2 + 20 log10(L)). The <L2> of clupeids was calculated per stratum using the LFDs which were weighted by catch rate and pooled if more than one valid haul was made in the stratum. One length/weight-key was calculated for the whole area of the part-survey, as the length-weight relationship of North Sea herring seems to be very constant over large areas (Hammer et al., 1996). Keys for age and maturity at length were calculated for each stratum separately. These keys were used in combination with the specieswise pooled LFD of the stratum to distribute the calculated total number of fish per stratum onto the age and maturity combinations and to calculate the biomass for these. For those strata where no valid hauls could be made, the pooled length frequency-information and relative numerical species composition of the neighbouring strata with valid hauls was used together with the sA of the stratum without hauls. 'Over-the-corner' neighbours were downweighted by a factor of 0.5 for this calculation. The resulting total number of fish of each species was distributed onto the age and maturity combinations according to the mean distribution of these in the neighbouring rectangles which was calculated using the same downweighting. Stratification concepts: A pre-survey stratification into ICES statistical rectangles was used in the years 1995-2000 for the German part of the survey. The basis for selecting this type of stratification simply was that numbers and biomass per age and maturity combination on this geographical basis are the data that is called for in the data collection procedures of the international survey. For the comparison of stratification concepts in the second part of the paper, a different approach was used and both stratification concepts are compared for the year 1996. In the alternative concept, strata were selected post-survey depending on the similarity of the LFDs of single hauls. Here too, one length/weight-relationship was used to calculate the biomass from the absolute numbers of fish per length-class. Results: Survey coverage The coverage of the area of investigation varied from year to year. An estimate of herring and sprat abundance and biomass was calculated for 11 to 28 ICES statistical rectangles (fig. 1a). Of these, in between 5 and 20 were calculated using biological information from hauls in the same rectangle (fig. 1b). On average, 78% of the estimated herring biomass originated from these rectangles, the others were interpolated using haul information from neighbouring rectangles. While the centre of the area of investigation has been covered in all years of the survey, the fringes often have not been sampled by echo-acoustics and to an even a smaller extent by (valid) hauls.

3

Abundance and biomass Apart from a total of 37 sardines (Sardina pilchardus) in the years 1996 and 2000, 4 twaite shad (Alosa fallax) in 1998 and 2000 and a single anchovy (Engraulis encrasicolus) in 1998, all clupeids caught during the survey were either herring (Clupea harengus) or sprat (Sprattus sprattus). The total abundance and biomass of clupeids have been very variable over the 6 years of investigation. For herring, total abundances of 2'539 (1998) to 11'610 (1995) million fish have been estimated while the biomass varied between a minimum of 39.9 kilotons in 1999 and a maximum of 263.0 kilotons in 1995 (fig. 2). The biomass of sprat was even more variable and was estimated between 1.1 and 195.6 kilotons with abundances in between 137 million and 18'156 million individuals (fig. 3). For neither of the two species a continuous trend could be detected in this relatively short dataset. Distribution Figure 4 shows the distributions of the abundance of herring over the area of investigation for the surveys 1995 to 2000. In most years the highest herring abundances are found in the eastern/central part of the area of investigation between 55°0' and 56°30' N and between 6° and 8° E. Generally lower concentrations were encountered towards the fringes. Sprat showed a different distribution of abundances (fig. 5) with a tendency for higher concentrations in the south of the area of investigation. This pattern was especially conspicuous in recent years and induced the southward extension of the area of investigation that was started this year. Length composition and age composition The length composition of herring in the catches and consequently in the calculated populations of the survey-area was quite variable (fig. 6, left). In most years the distribution was bimodal with the majority of herring smaller than 13cm. 1998 was the notable exception, when larger fish outnumbered the smaller fish. The modal length of age-class 1 (0 winter rings) varied between 7,75 and 9,75cm. The length-composition of the sprat population did not show a separation of modes as seen in herring. Here, the age-classes overlapped to a large extent (fig. 6, right). The age composition of the calculated herring population of the survey area was dominated by the age-classes 1 and 2 (0- and 1-ringers). More than 95% of all herring belonged to these two age-classes in any year. In terms of biomass, the age-class 2 alone accounted for more than half the weight in all years (fig. 7). The age-composition of sprat was equally dominated by these two age-classes (fig. 8). In most years, the age-class 2 dominated the age-composition but in the last 2 years the age-class 1 was first in terms of numbers as well as biomass. Length at age There is no significant trend in the length-at-age data (figure 9). In 1996 the mean lengths have been below average for all age-classes and in 1999 herring of the age-class 3 (2wr) were significantly smaller than in the other years. The age-class 4 was not present throughout the whole period of observation.

4

Comparison of stratification methods The graph of cumulative LFDs from the 1996 survey shows a distinctive split into 2 groups (fig. 10a), basically representing hauls which contained only the age-class 1 on the one hand (fig. 10b) and hauls which contained only the the age-classes 2 and over on the other (fig. 10 c). Thus, the alternative stratification process effectively reduces the number of strata from 20 (statistical rectangles) to 2 (areas with similar LDFs in the hauls). The distribution of the hauls belonging to the two groups (fig. 10d) results in a roughly 2 to 3 split of the area of investigation and a discontinuous shape for one of the two strata (fig. 10e). Table 1a-c shows the results of the comparison exercise for the 1996 Survey. The results from the stratification by rectangles (table 1a) and the stratification by LFDs (table 1b) are compared to a calculation without any stratification (table 1c) in which the addition of all valid standardised hauls was used as the basis for the calculations of abundance from the mean of all 1-mile sA-values. All calculations estimate a similar total abundance of herring in the area of investigation: between 5.5 and 5.8 billion fish. But the biomass estimate resulting from the two types of stratified calculations differ widely. Compared to the unstratified calculation that estimates a biomass of 84 kilotons, the stratification by statistical rectangles gives a lower estimate (68 kilotons) and the stratification by LFDs gives a higher estimate (98 kilotons). The difference in biomass estimates of the alternative stratification concept as opposed to the traditional stratification thus amounts to 44%. This is the consequence of a much higher estimate for the larger, older fish. While the ratio of ac-1 to ac-2 and over is about 3:1 in the old stratification it is 1.3:1 in the stratification by LFDs. Again, the unstratified calculation gives an intermediary result (1.5:1). Discussion: It appears that in the past 6 years, the German part of the international hydroacoustic survey has covered the main concentrations of herring in the south-eastern North Sea quite well. This is in spite of the fact that in none of these years the entire area was covered. However, in all years the calculated abundances of herring drop towards the fringes of the actual area of investigation and for this reason it is assumed that no large concentrations have been missed outside its limits. The coverage for sprat probably is not as good, as the highest concentrations of this species were recorded towards the southern limit of the area of investigation. Probably the survey would need to be extended towards the south to cover the main sprat concentrations. An extension in this direction was started in the year 2000 and is to be continued in the coming years. The fact that only two age-classes constitute the overwhelming majority of herring in the south-eastern North Sea has been the reason why the area was only sampled from 1995 onwards although the annual hydroacoustic survey had begun 12 years before that year. If an estimate of the biomass of the spawning stock of herring for the whole North Sea is the only parameter of interest, then the effort for surveying the south-eastern part of the North Sea might well be spent elsewhere as the contribution of this area to the SSB is minimal. Assuming all fish of age-class 3 and over to belong to the spawning stock, the biomass of these fish from the south-eastern North Sea on average contributed only 2% to the SSB. The justification for this part of the survey rather has to be seen in its contribution to an index of sprat for the North Sea and for the development of an index of juvenile herring in addition to the IBTS, which itself is produced from a single haul only per statistical rectangle without the use of any hydroacoustic information. For these two objectives the south-eastern North Sea contributes a large part of the information necessary.

5

Usually, the reasoning behind the use of stratification in a survey is to minimise the overall variance of the parameter under investigation. This is achieved by selecting strata that internally show a lower variance than the variance that the whole area of investigation would have if it was not stratified. In a way this is the case in the hydroacoustic survey of herring too. If the 1-mile sA-values were taken as the observed parameter whose variance was to be minimised, then the overall coefficient of variance (cv) for the south-eastern part of the North Sea survey in 1996 was found to be 6.0. Using the old stratification, the cv inside the strata ( =statistical rectangles) was in between 2.3 and 8.4 with a mean of 4.8. Using the stratification by similarity of LFDs it turned out to be 5.4 in stratum 1 and 5.7 in stratum 2. Thus, a reduction of variance is indeed achieved by both types of stratification. In fact the parameter surveyed is the number and the biomass of herring per age-class. Because of this, not only the sA-values are of importance but also the size-distributions of the backscattering objects, which determine the abundance of these objects at any given sA-value. This size distribution is investigated in the hauls and the stratification by LFDs is an attempt to minimise the error in the abundance estimate by trying to allot the sA-values to the “appropriate” size-distributions. Therefore, the stratification by LFDs does not so much reduce the variance in the abundance estimate but the variance in the ratio between ac-1 and ac-2 and over or the variance of the abundance estimate at a given sA-value. In order for this to be effective, an assumption has to be made: it is assumed that a haul within a stratum defined by one type of LFD has a higher chance to show an LFD of the stratum-defining type, than to show a LFD of another type. In the example of the south-eastern North Sea in 1996 we would assume a haul from rectangle 40F5 to have a higher chance of containing mostly herring of ac-1 than containing mostly herring of ac-2 and over (fig. 10). This is by no means certain, as the herring are not evenly distributed but occurring in distinct schools. From the distributions of LFDs in the hauls (fig 10a) we might conclude that about one school is caught per haul and that inside one school there are mostly herring of one size-group. In that case it might be an entirely stochastical process, which LFD we find in which haul. An indication for this is the relative rarity of intermediary LFDs containing both size-groups to a considerable amount. 0nly 3 of 23 hauls showed an intermediary form in 1995, 0 of 15 in 1996, 4 of 39 in 1997. These intermediary LFDs could either be the result of mixed schools or of the catch of more than one school. Whether or not the schools are so uniform in terms of size-composition and if so, how the different types of schools are spatially distributed would be a worthwhile object of investigation. To this end several intensive small-scale samplings in several limited regions distributed over the area of investigation would be necessary. In case there is no spatial pattern in the distribution of these schools, an unstratified calculation would be the best method. As there is no conclusive evidence for the random distribution yet, the choice is between the two types of stratification that are being used in the hydroacoustic survey of herring in the moment. Summarised, the advantages and disadvantages of the stratification schemes would look like this:

a) stratification by statistical rectangles

advantages: • simple to calculate • output is a deliverable of the survey • relatively good spatial resolution of the results • traditional procedure secures continuity of results in case of bias

disadvantages: • no biological justification for the strata whatsoever • many strata have to be interpolated, using the biological information of the neighbouring strata

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b) stratification by similarity of LFDs

advantages: • as there are no strata without hauls, no interpolation is necessary • similar LFDs reduce the variance of the abundance estimate at a given sA

disadvantages: • biological data of remote areas may be used to calculate numbers of fish from very local

peaks sA-values • the prerequisite of a higher likelihood for the LFD from a haul to belong to the stratum-defining

group was never tested in the south-eastern North Sea In the absence of any more convincing reasons for a change of procedures, it seems advisable to stick to the concept of stratification that had been used so far. References: Hammer, C., Reid, D.G. & Greenstreet, S.P.R. (1996) The use of weight length relationships for the

analysis of acoustic data from herring surveys: a source of variability? ICES C.M. 1996/H:6

Simmonds, E.J., Toresen, R., Corten, A., Pedersen, D.G., Fernandes, P.G. & Hammer, C. (1996) 1995 ICES coordinated acoustic survey of ICES Division IVa, IVb VIa and VIIb. ICES C.M. 1996/H:8.

Simmonds E.J., Bailey M., Toresen R., Couperus B., Pedersen J., Reid D.G., Fernandes P.G. and Hammer, C. (1997) 1996 ICES - Coordinated acoustic survey of ICES Divisions IIIa, IVa, IVb and VIa. ICES 1997/H:11.

Simmonds, E.J., Toresen, R., Torstensen, E., Pedersen, J., Götze, E., Fernandes, P.G. & Couperus, A.S. (1999) 1998 ICES coordinated acoustic survey of ICES Division IVa, IVb VIa and VIIb. ICES C.M. 1999/J:36.

Simmonds, E.J., Toresen, E., Torstensen, E.., Zimmermann, C., Götze, E., Reid, D.G. & Couperus, A.S. (2000) 1999 ICES Coordinated Acoustic Survey of ICES Divisions IIIa, IVa, IVb and VIa (north). ICES C.M. 2000/K:39

Simmonds, E.J., Svendsen, V., Torstensen, E., Fernandes, P.G., Reid, D.G., Couperus, A.S., Stæhr, K.J., Zimmermann, C. & Götze, E. (2001): The 2000 ICES coordinated acoustic survey of ICES Divisions IIIa, IVa, IVb, and VIa (north). ICES C.M. 2001/P:28

Parrish, B. B. (1955). A proposal for the introduction of organised echo-search in North Sea herring investigations. Rapports et procès-verbaux des réunions du Conseil Permanent International pour l'Exploration de la Mer, 139:27-31

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Figures and tables:

Figure 1a: Survey coverage: rectangles for which an estimate of abundance and biomass was given.

Figure 1b: Survey coverage: rectangles for which the biological information for the calculation of

biomass and abundance originated from the same rectangle.

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Herring abundance and biomass in the German part of the acoustic survey 1995-2000

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Figure 2: Herring abundance and biomass 1995-200

Sprat abundance and biomass in the German part of the acoustic survey 1995-2000

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Figure 3: Sprat abundance and biomass 1995-2000. *) low coverage in 1999 (see fig. 1a and 5)

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Figure 4: Distribution of herring abundances by statistical rectangle in the survey 1995 to 2000

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Figure 5: Distribution of sprat abundances by statistical rectangle in the survey 1995 to 2000

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Herring Sprat

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Figure 6: Length distributions of herring and sprat in the calculated populations (in 109 Ind.)

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Herring abundance

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Figure 7: Proportions by age-class of herring abundance and biomass

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Herring length-at-age in the German part of the acoustic survey 1995-2000

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Figure 10: Alternative stratification procedure. a) cumulative length-frequency distributions (LFDs) of all valid hauls of 1996. b) and c) separation into stratum-defining groups with their summed LFDs. d) survey track and distribution of the hauls belonging to the above groups. e) resulting strata.

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Table 1a-c: Herring abundance and biomass estimates for the 1996 survey from different types of stratification and an unstratified calculation.

table 1a) by rectangles Sum stratum 42F4 42F5 42F6 42F7 41F4 41F5 41F6 41F7 40F4 40F5 40F6 40F7 39F4 39F5 39F6 39F7 38F4 38F5 38F6 38F7 area [n.m.2] 987 987 987 987 1000 1000 1000 1000 1013 1013 1013 1013 1026 1026 1026 1026 1039 1039 1039 1039 20260 track [n.m.] 52 68 70 43 31 75 69 56 59 66 66 16 22 69 82 68 16 73 78 66 1145 mean sA [m2/n.m.2] 0.0 1.0 15.2 32.5 1.9 57.2 8.3 90.2 7.7 59.8 35.5 25.0 0.0 18.1 101.7 91.4 25.9 1.6 44.6 38.4 cv of mean sA -- 8.2 5.5 2.9 5.6 3.1 4.8 6.5 7.7 4.8 8.1 4.0 -- 3.0 3.9 2.8 2.4 6.0 4.4 3.0 Nsprat [mio] 0 0 0 0 0 0 0 0 0 3 0 1 0 0 9 0 0 0 0 118 137 Nherring [mio] 0 3 71 879 6 176 30 306 24 314 8 283 0 355 461 1571 526 8 208 310 5537 AC-1 0.0 0.32 60.5 878.6 0.6 18.6 15.1 0.1 17.0 158.2 4.8 212.3 0.0 354.9 20.8 1568.6 525.5 3.8 18.3 307.2 4.165 AC-2 0.0 2.38 10.2 0.0 4.6 139.9 14.6 302.8 6.4 148.4 2.9 70.7 0.0 0.0 440.4 2.7 0.0 4.0 189.2 2.6 1.342 AC >2 0.0 0.29 0.1 0.0 0.6 17.2 0.7 2.6 0.8 7.1 0.0 0.4 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 30 Wherring [t] 0 104 682 1’395 200 9’859 633 13’973 362 6’456 127 3’583 0 1’007 13’410 5’444 2’712 159 6’035 2’064 68’205 shaded: all data except from the mean sA-value were interpolated from neighbouring rectangles as there was no valid haul inside the rectangle

table 1b) by LFDs Sum table 1c) unstratified stratum group 1 group 2 area [n.m.2] 7788 12472 20260 area [n.m.2] 20260 track [n.m.] 652 493 1145 track [n.m.] 1145 mean sA [m2/n.m.2] 25.0 52.0 mean sA [m2/n.m.2] 36.6 cv of mean sA 5.4 5.7 cv of mean sA 6.0 Nsprat [mio] 83 37 119 Nsprat [mio] 114 Nherring [mio] 3073 2719 5792 Nherring [mio] 5373 AC-1 3072 185 3257 AC-1 3236 AC-2 0 2520 2521 AC-2 2126 AC >2 0 14 14 AC >2 12 Wherring [t] 10’997 86’949 97’946 Wherring [t] 84’343

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