International Council for the Exploration of the Sea ICES CM 2000/Mini:06

Using discards estimates for assessing the impact of on biodiversity

by Marie-Joëlle Rochet, Verena M. Trenkel, Jean-Charles Poulard and Isabelle Péronnet

Abstract

Estimating discards is an important step in assessing the impact of fishing on biodiversity, and an unavoidable precursor for effective conservation. Whereas ICES working groups already put some effort into incorporating discard information into stock assessments of target species, less is done for non-target species. This paper focuses on how this assessment can be achieved, illustrated with an example from the Celtic Sea. Community level indices such as diversity indices, species composition and size spectra of the total catch by the fishing fleet are compared to those obtained from surveys. For selected non-target species, an assessment of the impact of incidental catches on population dynamics is attempted. Survey-based abundance indices are combined with discard estimates and biological information to detect the effects of fishing on individual populations. Due to the poor quality of the available information, the assessments are not reliable. Hence outlines for suitable discard sampling strategies and biological survey designs which would allow better assessments are proposed. The complexity of multispecies multifleet fisheries involving many countries calls for coordinated efforts to carry out the proposed assessments. ICES might be an appropriate international forum to discuss the indices to be monitored and to propose operational sampling schemes.

Keywords

Discards; biodiversity; impact of fishing; non-target species; sampling strategies; population dynamics; community indices.

Marie-Joëlle Rochet and Verena Trenkel: Laboratoire MAERHA, IFREMER, Rue de l'Ile d'Yeu, B.P. 21105, 44311 NANTES Cedex 03, FRANCE [tel: +33 2 40 37 41 21, fax: +33 2 40 37 40 75, e-mail: [email protected]]. Jean-Charles Poulard: Laboratoire ECOHAL, IFREMER, Rue de l'Ile d'Yeu, B.P. 21105, 44311 NANTES Cedex 03, FRANCE [tel: +33 2 40 37 40 00, fax: +33 2 40 37 40 75]. Isabelle Péronnet: Laboratoire Ressources Halieutiques, IFREMER, 8,rue François Toullec, 56000 LORIENT, FRANCE [tel: +33 2 97 87 38 17, fax: +33 2 97 87 38 01, e-mail: [email protected]]

1 1 Introduction

Two points of view can be adopted for assessing the impact of fishing on biodiversity: assess to which extent fishing affects the community structure, or assess the impact on selected indicator species. In both cases information on discards in addition to landings is required. In this paper methods and indices for assessing such impacts are proposed. They are illustrated for the French fleet operating in the Celtic Sea, for which discards were sampled in 1997. First, community structure indices (diversity measures, , size spectra) estimated from survey- based information are compared to those for the catch (discards + landings). Then, all available information (literature and survey-based) is used in an attempt to estimate the specific impact of fishing on some non-target species. Target species are not considered since they are covered by working groups. Based on this example, information needed to assess the impact of fishing on biodiversity is listed.

2 Materials and methods

2.1 Celtic Sea example

2.1.1 information The Celtic Sea supports an international groundfish fishery, targeting mainly gadoids, monkfish, Nephrops and flatfish, all of which are assessed by ICES Working Groups (Anonymous, 1999b; Anonymous, 2000b). The French trawlers fleet accounts for about one half of total international landings from this area (estimated from Stock Assessment Working Group Reports). As some of the species are subject to high amounts of discarding, discard sampling surveys are undertaken every 4-5 years. Discards by the French fleet were sampled during 26 trips (0.8% of trips) in 1997. This fleet can be grouped into three métiers: demersal trawlers, benthic trawlers and Nephrops trawlers. The métiers are defined by several factors: fishing zone, gear and the target species. Discards are expected to vary in species composition and quantity due to these factors. Hence stratified sampling with the métiers as strata and the following levels was used: i) fishing trip, ii) haul, iii) fraction of the total amount of discards sampled. Two sampling procedures were used. Either an onboard observer carried out sampling and measuring, or the sample was stored by the fishermen for later analysis in the laboratory. In parallel, the landings corresponding to the selected fishing trips were sampled using standard procedures. Discards were raised for each métier, using simple stratified estimators. The French fleet operating in the Celtic Sea is estimated to have discarded 30,000 tons in 1997 (Table 1). Catch is the sum of landings and discards. All discarded animals are assumed to be dead.

Table 11: Estimates of discards and landings for the French trawlers operating in the Celtic Sea in 1997. Métier Discards (t) Variance C.V. Landings (t) Discards / Catch Benthic 5 469 872 608 0.17 16 930 0.24 Demersal 12 083 21 425 508 0.38 35 191 0.26 Nephrops 13 566 29 117 218 0.40 11 089 0.55 Total 31 119 51 415 335 0.23 63 210 0.33

2 2.1.2 Surveys Information on the Celtic sea community is provided by the records of the French "EVHOE" surveys for autumns 1997 to 1999. During each cruise, 57 to 69 30 minutes-tows were performed with a GOV36/47 bottom trawl in ten strata defined by depth and geographic region. 60 to 77 tows were also performed in the Bay of Biscay. All were identified, weighted by species and individually measured. Numbers at length and weights caught per haul were raised to whole Celtic sea by the ratio of total area to trawled area for each stratum. Similar surveys were performed with another vessel in 1990 and 1992; conversion coefficients for the catch of both vessels are available from a calibration study (Pelletier, 1998).

2.2 Community inindicesdices

Fishing might affect species diversity of a community by selectively removing some species, by reducing total biomass or by selectively removing a given size range. To assess the possible impacts of the fleet on the community and for comparing the métiers, we estimated indices of these quantities for the Celtic Sea using the survey data, and for the total catch of each métier and the fleet as a whole.

2.2.1 Biodiversity Both species richness and the Simpson index were used as community biodiversity indices. Species richness S is highly dependent on sampling effort. "Sampling effort" was approximated by the number of hauls: there were 462 commercial hauls but the cumulated hauls from the three surveys only amount to 186. To predict the theoretical species richness of a 462 hauls survey, the number of species present in subsamples was counted while gradually increasing the number of hauls included in the subsamples. A linear model of species richness as a function of the log number of simulated hauls yields the predicted number of species for 462 hauls (Lande, 1996). In contrast to species richness, the Simpson diversity index is not sensitive to sampling effort; it takes account of S = 2 the distribution of abundance among species: IS 1/ å pi (pi is the relative abundance of species i=1 = − − i). The equitability ES (IS 1)/(S 1) tends to 0 when one species dominates and to 1 when all species are evenly represented (Barbault, 1992). In addition, species compositions were also compared.

2.2.2 Biomass Biomass of the exploitable community was estimated by the biomass caught in the 1997 survey, raised to total Celtic Sea surface. Total caught biomass was obtained from landings statistics and discard estimates.

2.2.3 Size specspectratra Similarly, raised total numbers at length caught by each métier and the size spectrum of the 1997 survey were estimated.

2.3 Impact of fishing on nonnon---targettarget populations

A complementary approach consists in assessing the impact of fishing on individual populations. In a first appraisal, fishing will be suspected to have an impact on a non-target population if i) mortality due to fishing accounts for a significant part of total mortality and ii) the population size decreases. The information available from the discard sampling programme is the estimated number of fish caught by the French fleet operating in the Celtic sea Cˆ . We use abundance estimates from the 1997 survey to compute the following parameters:

3 • The total number of fish in the population Nˆ and the fishing mortality rate F = Cˆ / Nˆ (and its variance according to Kendall and Stuart, 1977) due to this fleet. ˆ • Adult mortality rate Z from a catch-curve analysis of numbers at length Nl (Sparre and Siebren, 1992), using growth parameters found in the literature. These estimates allow to compute F/Z, answering point i. To examine point ii, additional calculations are needed. We tried two approaches.

2.3.1 A simple snapshot assessment ˆ For a given species, the number of fish N98 found in the Celtic Sea in 1998 at the time of the survey was equal to the number of fish in the 1997 survey, minus those that died in between, plus ˆ = ˆ + ˆ − the recruits: N98 N97 R98 D97 . Hence, if an estimate of recruitment is available, the total ˆ = ˆ − ˆ + ˆ ˆ number of deaths in 1997 is D97 N97 N98 R98 , of which C were caught. The comparison of ˆ ˆ N98 and N97 indicates whether the population decreased or increased during 1997, and the ˆ ˆ comparison of C with D97 gives an indication of the contribution of fishing to total mortality. To estimate recruitment, the length-frequency distributions of the survey catch in 1998 were split into juveniles and older components, according to their modes. If juveniles were more abundant than older fish, the new year-class was considered to be fully recruited by the time of the survey, and its abundance was taken as a recruitment estimate. The advantage of this approach is that it implies few assumptions. The disadvantage is that it is only valid for 1997.

2.3.2 Impact of fishing on population growth rate Consider the simple population model with two stages: recruits and adults (Figure 1).

− 1 mλ Figure 11: Life cycle graph for a two stages population −−1Z model. R: recruits, A: adults. Annual survival of recruits is e λ R α−α A P1, adult mortality rate is Z, annual fecundity is m, and P λ age at maturity is α. 1

λα +1 − −Zλα − α = Following Caswell (1989), the characteristic equation of this life cycle is e P1 m 0 , whose real root λ is the population growth rate. λ is the answer to point ii: if λ = 1, the population is asymptotically stable, it declines if λ < 1 and grows exponentially otherwise. If the population stage structure can be assumed stable, λ can be estimated from successive abundance estimates: λˆ = ˆ ˆ λ t Nt+1 / Nt , and the contribution of fishing mortality F to (also known as the elasticity of F): ∂λ λ −Z = F = F e α λ eF − can be estimated knowing only Z, F, and (Heppel et al., 2000). λ ∂F λ αe Z − λ()1+ α λ λ λ For the Celtic sea example, was estimated as the geometric mean of 97 and 98. Longer term λ λ trends were also examined using data from the 1990 and 1992 surveys: 90−92 and 92−97 , the latter being estimated both with and without the conversion coefficients for change of research vessel. Although more assumptions are necessary, this approach has the advantage of considering population dynamics over a longer time period.

2.3.3 Selected species We selected nine species which both accounted for a significant part of discards by the French trawlers (a necessary condition for discards to be accurately estimated), and were caught in

4 sufficient numbers and in a sufficient number of hauls during the surveys (Table 2). Among these species, three are targets of other fisheries and are routinely assessed by ICES Working Groups: blue whiting (MICRPOU), mackerel (SCOMSCO) and horse mackerel (TRACTRU). The results of VPA assessments for these species were used as a reference for the present assessment.

Table 2:2 Selected non-target species for an assessment of the impact of French fishing activities in the Celtic sea. Area refers to the assumed geographical area of the population (educated guess based on survey α catches). Published information on their life history traits: age at maturity ( ) and maximal age (Amax) in years, ∞ and the parameters of the von Bertalanffy growth model: L , K and t0. ∞ Code Area Species Name α (y) Source Amax (y) Source L K t0 Source ASPICUC Celtic + Aspitrigla Red gurnard 1.4-2 F 20 F 43.1 0.31 –1.05 D Biscay cuculus CAPRAPE Celtic + Capros aper Boar fish 19 0.5 0 guess Biscay EUTRGUN Celtic Eutrigla Grey 3.5 F 16 F 40.5 0.38 –0.99 D gurnardus gurnard MICRPOU Celtic + Micromesisti Blue whiting 3.2 W, Q, F 16-20 Q, F 39.9 0.15 –3.53 D Biscay us poutassou 3-5 SCOMSCO Western Scomber Mackerel 2 W, Q, F 6-20 Q, F 36.2 0.49 –0.76 D Atlantic scombrus 3.5 SCYLCAI Celtic + Scyliorhinus Smallspotted 5 F 22 F 90.0 0.09- R Biscay canicula catshark 0.13 TRACTRU Celtic + Trachurus Horse 3.5 W, Q 15 Q 44.9 0.2 –0.59 T Bisca trachurus mackerel 4.5 TRISESM Celtic Trisopterus Norway 2 F 3-8 Q, F 21.1- 0.44- –0.8- F esmarkii pout 24.8 0.72 –0.9 TRISMIN Celtic + Trisopterus Poor cod 1-4 F 5 Q 33.5 0.18- –0.91 A Biscay minutus 0.48 Sources: A = Albert (1991); D = Dorel et al. ; F = Froese and Pauly (1999); T = Trouvery (1978); Q = Quéro (1984); W = Working Groups (Anonymous, 1999a; Anonymous, 2000a).

3 Results

3.1 Impact of the French trawlers fleet on the Celtic sea community

The fleet caught only a fraction of the available species, the smallest diversity being caught by Nephrops trawlers which took less than half of the species present (Table 3). But in terms of evenness of the species distribution, the fishermen took a greater diversity than the one available: the species composition of the Celtic sea, as seen through the surveys, is dominated by very few species, whereas the composition of the catch by the fishing fleet is more even and dominated by other species (Table 3, Figure 2). The benthic trawlers had the most diverse catch among the three métiers. The fleet removed only 6% of total available biomass in 1997. The catch consisted mainly of larger fish; the fleet removed the majority of the population for the largest individuals (Figure 3). The demersal métier took the largest part at all sizes, whereas the Nephrops trawlers caught relatively smaller fish.

5 Table 33: Diversity indices and biomass estimates for the French trawlers catch and the Celtic sea community.

Métier Species Richness S Simpson Is Equitability Es Biomass (t) Catch Discards Landings Catch Catch Catch Discards %* Benthic 69 57 40 8.4 0.11 22 399 5 469 1 Demersal 62 54 37 5.8 0.08 47 274 12 083 3 Nephrops 52 37 36 4.3 0.06 24 655 13 566 2 Fleet 85 71 47 8.5 0.09 94 328 31 118 6 EVHOE 149 (estimated for 462 hauls)¥ 3.9 0.02 1 609 483 1997 128 (observed in 186 hauls)¥ * % catch biomass relative to total biomass. ¥ S estimated from 1997, 98 and 99 surveys.

MERL-MNGCeltic sea Benthic

LEPIWHF

LEPIWHM

LOPHPISLOPHBUD

CLUP-HAR MELAAEG EUTRGUN

ASPICUC CAPR-APE RAJANAE ARGE-SPH ARGE-SIL SCYLCAI other TRIGLUC ARGESPP other TRIS-MIN SCOMSCO MICR-POU TRIS-ESM TRAC-TRU CAPRAPE TRACTRU SARD-PIL

TRISMIN

ARNOSPP

MERLMCC DemersalMELAAEG Nephrops

LEPIWHFGLYPCYN MERLMNG LIMALIM GADUMOR MERLMCC EUTRGUN MELAAEG LEPIWHM NEPHNOF LEPIWHF ASPICUC MERLMNG GADUMOR other other TRISMIN MICRPOU TRISMIN

TRISESM

TRACTRU NEPHNOM

MICRKIT

PLEUPLA

Figure 22: Species composition (numbers) of the demersal community of the Celtic sea, and of the catch by the three métiers of the French fishing fleet. Species codes are as follows: Code Species name Code Species name Code Species name ARGESIL Argentina silus LEPIWHM Lepidorhombus wiffiagonis / PLEUPLA Pleuronectes platessa ARGESPH Argentina sphyraena LIMALIM Limanda limanda RAJANAE Raja naevus ARGESPP Argentina species LOPHBUD Lophius budegassa SARDPIL Sardina pilchardius ARNOSPP Arnoglossus species LOPHPIS Lophius piscatorius SCOMSCO Scomber scombrus ASPICUC Aspitrigla cuculus MELAAEG Melanogrammus aeglefinus SCYLCAI Scyliorhinus canicula CAPRAPE Capros aper MERLMCC Merlucius merlucius TRACTRU Trachurus trachurus CLUPHAR Clupea harengus MERLMNG Merlangius merlangus TRIGLUC Trigla Lucerna EUTRGUN Eutrigla gurnadus MICRKIT Microstomus kitt TRISESM Trisopterus esmarki GADUMOR Gadus morhua MICRPOU Micromesistius poutassou TRISMIN Trisopterus minutus GLYPCYN Glyptocephalus cynoglossus NEPHNOF - LEPIWHF Lepidorhombus wiffiagonis - NEPHNOM Nephrops norvegicus /

6 log(Number)

B D N 5101520

12345 log(Length) Figure 33: Size spectra of the fish community of the Celtic sea estimated from the 1997 survey data (bold line), of the catch by the French fleet (thin line), and by each métier (dashed lines: B benthic, D demersal, N Nephrops trawlers).

3.2 Impact on selected nonnon---targettarget species

Estimated fishing mortality rates caused by the study fleet varied from 0.01 to 1 depending on ˆ species (Table 4). The catch-curve method resulted in high estimates of Z ( Z1, table 4), therefore ˆ the subsequent analyses were performed both with this estimate and with a lower guess ( Z2 ) that seemed reasonable, given the published longevity of the species. Clearly, the estimated ratio of fishing to total mortality varies according to the assumptions on total mortality. Fishing mortality is high mainly for grey gurnard, for the species exploited by other fleets, and, to a lesser extent, for red gurnard. However, the gurnard populations increased from 1997 to 1998 (Table 4). Four populations were found to have decreased: blue whiting, horse mackerel, Norway pout and poor cod. For most species it was not possible to estimate the 1998 recruitment from the length distribution, because the first mode of the distribution was lower than the subsequent ones: the snapshot assessment was only possible for the two Trisopterus species. For both, catch by the fleet played a minor part in the total number of deaths. Over a longer period, all populations but one (horse mackerel) were increasing, based on surveys (Table 5). Whereas boar fish and grey gurnard populations declined between 1990 and 1992, all populations increased from 1992 to 1997, with and without correcting abundance estimates for vessel change (not shown). The contributions of F to population growth rate were generally low, and highly dependent on the assumptions made on population parameters (Table 5).

In conclusion, fishing did not seem to affect any of the non-target populations examined. The few populations for which fishing mortality significantly contributed to total mortality were not decreasing neither in the short nor in the long term.

7 ˆ Table 44: Population parameters of selected non-target species. Z1 is total mortality estimated by the catch- ˆ curve method, Z2 is a lower estimate obtained either by VPA (for MICRPOU, SCOMSCO and TRACTRU) or a reasonable guess given the maximum observed age (Table 2). Ct refers to total catch for species being targeted by other fleets.

Species ˆ ˆ ˆ ˆ F ˆ ˆ Nˆ − Nˆ ˆ ˆ ˆ ˆ ˆ Z1 Z2 C N97 F Z1 F Z2 98 97 R98 D97 C Ct C D97 6 (x10 ) (x108) (x109) (x109) ASPICUC 1.71 0.40 13 0.68 0.20 0.115 0.493 6.7E+6 NA CAPRAPE 2.23 1.00 22 22 0.01 0.004 0.010 1.0E+9 NA EUTRGUN 2.01 0.50 17 0.17 1.03 0.513 2.064 4.0E+7 NA MICRPOU 1.94 0.52 2.8 860 3.E-5 2.E-4* 0.615¥ -2.1E+10 1.3 22 6.2E-3 1.2E-4 SCOMSCO 0.38 2.0 260 8.E-5 0.605¥ 1.1E+10 16 5.1 6.2E-4 4.0E-4 SCYLCAI 0.65 0.30 2.0 0.4 0.05 0.078 0.168 2.4E+7 NA TRACTRU 0.29 0.32 7.5 260 3.E-4 0.001* 0.750¥ -2.8E+9 3.6 6.3 2.8E-3 1.2E-3 TRISESM 1.38 1.00 6.8 13 0.01 0.004 0.005 -5.4E+8 0.6 1.1 6.0E-3 TRISMIN 0.90 1.00 15 22 0.01 0.008 0.007 -7.4E+8 0.32 1.1 1.5E-2 *: F and Z estimated from the present study ¥: F and Z estimated by ICES working groups

Table 55: Population growth rates of selected non-target species, and the elasticity of fishing mortality due to α α the French fleet, under various combinations of total mortality and age at maturity estimates. 1, 2 lower and higher bounds of published age at maturity. Species ˆ α α λ 1 (ˆ α ) (ˆ α ) 2 (ˆ α ) (ˆ α ) eF Z1, 1 eF Z2, 1 eF Z1, 2 eF Z2, 2

ASPICUC 1.01 1 -1.9E-02 -9.8E-02 2 -1.3E-02 -7.8E-02 CAPRAPE 1.29 1 -4.3E-04 -1.7E-03 2 -2.9E-04 -1.2E-03 EUTRGUN 2.40 2 -2.0E-02 -1.0E-01 3 -1.5E-02 -8.0E-02 MICRPOU 1.08 3 -7.5E-02 5 -5.4E-02 SCOMSCO 1.16 2 -7.4E-02 3.5 -5.6E-02 SCYLCAI 1.37 4 -5.6E-03 -9.7E-03 8 -3.2E-03 -5.9E-03 TRACTRU 0.93 3.5 -1.1E-01 4.5 -9.8E-02 TRISESM 1.33 1 -5.6E-04 -8.6E-04 4 -2.4E-04 -3.8E-04 TRISMIN 0.87 1 -2.2E-03 -1.9E-03 4 -1.1E-03 -9.1E-04

3.3 Quality of the assessments

Whereas the species richness of the catch is exactly known, the simulation of the number of species as a function of the number of survey hauls suggests that, with the current sampling effort, the

8 number of species caught is far below the total potential number. The extrapolated number of 149 species is therefore not very reliable. In addition, the composition of EVHOE catch is dominated by a few species which are not purely demersal, due to the large vertical opening of the GOV trawl. This makes the direct comparison of species compositions questionable. The total biomass in the Celtic sea is probably underestimated, because many species have low or null catchabilities with the GOV trawl; the small mesh may also imply that larger fish will escape the gear. This can also be seen in the comparison of size spectra, which indicates that the fishing fleet seems to remove an unreasonably high proportion of large fish. For most of the studied species, the order of magnitude of discards is known (CV<1/2). Since discards account for most of the catch, the CV of the catch can be assumed to be equal to the CV of discards (Table 6). Population numbers however, are not known as accurately as only three species have a CV lower than 50%. In addition, these estimates were obtained under the assumption that catchability is 1 for all species in all trawled areas, which is probably not reasonable. As a consequence, the fishing mortality estimates are not accurate (Table 6). Clearly, total mortality estimates Z are even more questionable, though no variance estimates were available. Estimated numbers at length have attached CVs larger than those reported in Table 6. Due to lack of availability, growth curve parameters had to be taken from other populations. This can introduce bias since growth might vary substantially between populations. These bias and inaccuracies are fully illustrated by the discrepancy between the estimates obtained for mackerel, horse mackerel and blue whiting, using the present methodology, and by ICES Working Groups (Table 4); in addition, we used information only from the Celtic Sea and the Bay of Biscay; this is not relevant for these populations, which have a much larger distribution. Population growth rate estimates were not accurate (Table 6). However, if data of several cruises were available, this parameter could probably be estimated with the highest precision. On the other hand, estimating the contribution of F to λ requires information on adult mortality and age at maturity that is currently not available. In conclusion, the assessments of § 3.2 are not reliable. Moreover, knowledge of the catch by all fleets, not only the French trawlers operating in the Celtic sea, would be required for a complete assessment. Table 6:6 Coefficients of variation (%) of the estimates of abundance, discards, fishing mortality and population growth rates, and proportions of discards in the catch (%), for the nine non-target selected species.

Species CV( Nˆ ) CV(Discards) Discards/Catch CV(F) CV(λ) ASPICUC 54 19 99 57 69 CAPRAPE 131 47 100 140 169 EUTRGUN 36 37 99 52 96 MICRPOU 43 44 100 62 63 SCOMSCO 77 63 100 100 102 SCYLCAI 37 34 79 50 69 TRACTRU 110 30 100 114 166 TRISESM 52 69 100 86 108 TRISMIN 63 49 100 80 74

9 4 Data requirements forfor carrying out the proposed impact assessments

To perform trustworthy assessments of the impact of fishing on a community, and on non-target species along the lines presented in this paper, good estimates of total catch (landings and discards) are necessary. Reliable estimates of population size require appropriate survey designs and efforts. Additional biological information is also needed.

4.1 Biological information

As exemplified by the discrepancy between the present assessments and those of the Working Groups taking account of all available information for horse mackerel, mackerel and blue whiting (Table 4), knowledge of population boundaries is a precursor for carrying out species specific assessments. For the rough assessments we propose, the minimal biological information needed for each population is age at maturity and length-at-age information for fitting growth curves. For most unexploited species of the Celtic sea, age at maturity has simply never been measured and growth parameters were never estimated. This information could be collected during the annual surveys.

4.2 Discard sampling strategies

Suitable discard sampling plans depend on the quantities of interest. The quantities considered are the total discarded biomass, discard numbers by age for commercial and some non-commercial species, discard numbers by length groups and species richness. Good estimates of total discarded biomass can be obtained by counting the number of baskets thrown over board. Currently the corresponding entry of fishermen log books is optional and never filled. Making it mandatory would allow to estimate total discards without any additional sampling effort. The estimation of discarded numbers at age (or length) can be split into estimating the total discarded biomass (see above) and estimating the proportion of each species (or length group). Given a multi-stage sampling design with trips, hauls and a fraction from each haul being examined, the sampling effort at each stage is obtained by the following considerations. Firstly, the sampling fraction, i.e. the number of baskets at the haul level is determined by the proportion of the species of interest in the discards. The smaller the proportion or the larger its variability, the larger the sampling fraction has to be (Cochran, 1977). Secondly, for obtaining good estimates of total discards, sampling effort has to be concentrated on the levels of largest variability. In the Celtic Sea example the variability between trips was found to be far greater than at the between and within haul level. Hence the expected variability of discarded biomass across fishing trips will determine the number of trips to be included in the sample (Cochran, 1977). Thirdly, a minimum number of hauls per trip has to be sampled in order to allow variance calculation at this level. Sampling five hauls per trip should be sufficient for this purpose. Sampling plans for estimating numbers by length group can be developed along the same lines. For the Celtic Sea table 7 gives a sampling plan for estimating discard numbers by métier for some commercial species and the nine non-commercial species examined in this study. A larger fraction of each haul has to be sampled if we are interested in obtaining reliable estimates for the discard numbers of the non-commercial species than for the commercial species.

10 Table 7 : Annual sampling plan for estimating discarded numbers of A) whiting, cod, monkfish and Nephrops; B) the nine non-commercial species by the three métiers of French fishermen operating in the Celtic Sea.

Métier Number of trips Number of hauls / trip Number of 10 kg baskets / haul A) Commercial species Benthic 3 5 4 Demersal 10 5 5 Nephrops 15 5 4 B) Non target species Benthic 3 5 10 Demersal 10 5 16 Nephrops 15 5 8

4.3 Survey designs

Estimation of community species richness requires a large sample size (>400). Cumulated survey information over several years, as done in this study, can remedy the problem. Combination across surveys carried out by different nations might be another possibility. Improving the assessments of non-target species relies on obtaining more accurate abundance indices, and hence requires an increase in survey sampling effort. International co-ordination of surveys is needed to ensure complete coverage of each population. To improve the abundance estimates of older (larger) fish and the estimation of total mortality via catch curve analysis, a sampling design ensuring the catch of more large fish is necessary. The use of different gears with a higher catchability for large fish for some tows might achieve this.

4.4 Role of ICES

The role of ICES in improving the assessments of fishing impacts on biodiversity could first be to discuss the relevance of community-level indices. The impact of fishing on a community level has proven difficult to demonstrate, because community indices are highly integrated and change under various confounded processes (Allen et al., 1999; Greenstreet et al., 1999; Rogers et al., 1999). The present study does not make an exception, and the results are not easy to interpret. There is a need to discuss which among diversity indices, size spectra and other indices are relevant for assessing the impact of fishing. On the other hand, as populations and fisheries are multinational, there is a need to co-ordinate international studies on populations potentially impacted by international fisheries. The design of discard sampling schemes and of surveys should be co-ordinated, to obtain comparable and complementary abundance indices, as well as synchronised discard estimates. Biological information cannot be collected on all species caught. Some species should be focused on to i) obtain all relevant information about chosen populations, potentially exploited or discarded by several fleets; ii) compare the answer to fishing of populations of chosen species between various fisheries. For example, the present results (if they are correct) seem consistent with observations from the , where grey gurnard, poor cod and other non-target populations increased over the 1970-1993 period, despite a high fishing pressure (Heessen and Daan, 1996). How can this be generalised? Which would be relevant indicator species? ICES might also be a forum to discuss recommendations to preserve biodiversity. For example, we found that the diversity caught by the French fleet is not similar to the community diversity. The use of selective gears has often been recommended, but from an ecosystem point of view, this can

11 seem controversial: the diversity of the community may be less affected by a fishery catching a species composition similar to the one of the community, than by a more selective one.

Acknowledgements

We are indebted to all the people involved in the EVHOE surveys, especially Robert Bellail and Jean-Claude Mahé, for making the data available to us. We thank Benoît Mesnil for useful comments on a previous version of this manuscript.

References

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