ICES CM 2001/V:03

Towards a theory for discarding behaviour

Verena M. Trenkel and Marie-Joëlle Rochet

Laboratoire Maerha, Ifremer, Rue de l'Ile d'Yeu, BP 20115, 44311 Nantes Cedex 3, France [tel: +33 2 40 37 40 53, fax: +33 2 40 37 40 75, e-mail: [email protected]; [email protected]].

Abstract

Discards have been recognised as an important source of unaccounted mortality. Most research in this field has focused on estimating discarded quantities for a particular fishery; a necessary first step which requires large sampling efforts due to the inherent variability of catches. Currently no unifying theory exists which would try to explain discarding behaviour but a number of assumptions are implicit in many studies. We propose to explicitly state and test these underlying hypotheses in order to start building a theoretical basis that might help us to understand and predict discarding behaviour. The common implicit assumptions are: H1: Discards are proportional to catches H2: Discards are proportional to fishing effort H3: Discards are determined by year class strength H4: Legal landing size determines discards selection

We examine theses hypotheses for the case of the French Celtic sea groundfish trawl fisheries.

Keywords: discards, fishermen behaviour, groundfish, trawl fisheries, Celtic Sea.

Introduction

Discarding is a natural by-product of intentional selective fishing carried out by rather non-selective harvesting methods in a context of economical and technical constraints. From a stock management point of view discarding poses the problem of resource waste, whereas from an ecoystem point of view it enhances the perturbations incurred on the system. Given the importance of discards compared to landed catches in many trawl fisheries, a number of studies have been carried out to estimate discard biomass and numbers. The conclusion of many of these studies is that discard biomass (or numbers) vary tremendously in space and time and between fisheries (Murawski 1996; Rochet et al. 2002). In 1985 already, Hilborn (1985) pointed out that published information about discards was sparse and unstructured, due to the lack of a theory of discarding. Hilborn and Walters (1992) later proposed several explanations for discarding. In particular they postulated that market demand, hold capacity and regulations should influence discarding practices. Since then some attempts have been made to study the factors that explain discards such as landings' size and composition, vessel and fishing characteristics, year class strength, minimal landing size, quotas, etc. (Murawski 1996; Rochet et al. 2002; Stratoudakis et al. 1998). An important motivation for this search for predictors is that discards sampling is expensive. Hence being able to predict discards without having to sample them would be an enormous advantage. Unfortunately, in the various studies no overall determining factors could be identified that seemed important for all species and in different circumstances.

1 A theory for discarding behaviour is still lacking. Instead of trying to correlate discards with any available factor, it is time to look at the underlying processes directly. This means, investigating discards at various levels, but focusing mainly on the trip and haul levels where behavioural patterns are operating, rather than on integrated levels like fleets or total annual discards. As building blocks towards such a theory we propose to look at some of the implicit assumptions concerning discarding behaviour that are commonly made. Our approach is empirical as we hope to accumulate evidence supporting or refuting each assumption. We formulate the assumptions as null hypotheses which can then be tested against competing hypotheses within a statistical framework. The first three hypotheses deal with discarded quantities, whereas the fourth goes more deeply into the process and concerns discards' selection criteria.

H1. Discards are proportional to catch, hence to landings H2. Discards are proportional to fishing effort H3. Year class strength determines discarded quantities H4. Legal size determines discarding practices

This paper is focused on discarding behaviour for species that are partially discarded, because the reasons for those that are always discarded should be easier to identify. Depending on the species this can either be legislation (protection) or unfitness for human consumption or industrial use. The list of species that is always or, the opposite extreme, never discarded might depend on the particular fisheries. Hence for species that are never discarded in one fishery but sometimes in another, it can be of interest to elucidate the differences. However, in this paper we concentrate on the species that are partially discarded within a given fishery and leave the inter-fisheries comparison aside. We use data from the French fleet operating in the Celtic sea as our case study. Three métiers are practised by this fleet, métiers in the sense of using a given gear to target a set of species in a given area. These métiers target three species groups: demersal fish, gadoids and Nephrops. Discarding behaviour is expected to vary between métiers as they target different species and operate in different areas. A sampling program of this fleet was carried out in 1997 and the main results were reported in Rochet et al. (2002).

Hypotheses for discarding behaviour

H1 Discards are proportional to catch

Total discards are most often estimated using a ratio estimator based on landings or discard rates (discards/landings), the underlying assumption being that discards are proportional to landings. This is equivalent to saying that discards are proportional to catches and that a certain fixed proportion of catch is discarded. As a consequence, the bigger the landings, the higher the expected discards. This assumption is convenient for raising discards to the fleet level provided landing statistics are available. It is also sometimes used to estimate discards from landings in years when discards are not sampled. The assumption can be investigated on various levels, from hauls to fleets. Several authors have investigated the hypothesis, generally in the context of finding the best discards estimator. Some evidence rejecting the hypothesis has been published (Allen et al. 2001; Evans et al. 1994; Stratoudakis et al. 1999).

2 H1a Discards increase non linearly with catch

Alternatively, the proportion of animals discarded might increase with trip duration as storage capacity is becoming limiting. Saturation might also arise on the fleet level when the total allowable catch has been almost caught. This hypothesis has been put forward by Hilborn and Walters (1992). To investigate the hypothesis, the ideal sampling scheme includes haul by haul sampling of discards and landings. If landings by haul are not available, some insight can be gained by regressing trip discards against trip catch or haul number. However, inter-haul catch variability might prevent clear conclusions. Looking at total landings as a function of hold capacity can also provide evidence in support of the hypothesis.

H2 Discards are proportional to effort

Because data on fleet effort are often available, many authors have used discards per unit effort (DPUE) for raising sample discard estimates to fleet estimates (e.g. Dupouy et al. 1998; Trujillo & Pereda 1997). Not all authors checked the underlying hypothesis that discards are proportional to fishing effort. This assumption is based on the idea that catch would be proportional to effort: if discards are proportional to catch they are also proportional to effort. In addition to the latter assumption, already examined above (see H1), catch must also be proportional to effort. This might not always be true, either at the haul level (Gillis 1999; Somerton et al. 2002) or on a larger time or spatial scale due to behavioural reactions of both fish and fishermen (Hilborn & Walters 1992).

H2a Discarding behaviour changes with haul duration

Instead of being linearly related to effort as measured by haul duration, discarding could increase non-linearly due to fish being damaged by long hauls. The alternative hypothesis is therefore that discarded biomass is a non-linear function of haul duration. It might be expected to be true for fragile species such as whiting which could get squashed by the accumulating biomass in long hauls. Hauls with large biomasses could have the same effect.

H2b Discarding behaviour changes with trip length

Instead of being determined by the individual haul duration, overall trip length could influence discarding. Under this competing hypothesis, discards would be related to trip duration. Several factors could lead to this scenario: conservation problems on vessels without freezing facilities or fish species with mainly a market for fresh fish. For example hake and rays might be concerned by this problem as they are more perishable than other species.

H3 Year class strength determines discards

A commonly accepted thesis is that mainly recruits are discarded and hence discarded biomass is determined by year class strength. In order to investigate this hypothesis it is necessary to dispose of time series of both discards by species and estimates of recruits which are either obtained from scientific surveys or from VPA stock assessments. These time series are not available for many fisheries. Rochet et al. (2002) used an alternative method employing standardised estimates per species as replicates instead of solely a time series for a

3 given species. The drawback is that this method will only detect a relationship if it existed for most of the investigated species.

H3a Discards are related to the length distribution in the catch

Instead of the number of recruits, their size might determine discarded quantities. This implies that if smaller animals are caught, more would be rejected. The hypothesis necessarily applies to a particular species and can be investigated at the trip as well as at the fleet level. The assumption can be reformulated to say that discards of a given species are related to the size of the smallest fish in the catch, which can be tested using multiple regression for log- transformed discards biomass (Rochet et al. 2002).

H4 Legal landing size determines selection

Minimum landing size is a common measure for managing fisheries. Compliance with the regulation requires that all fish below the minimum size are discarded. Minimum landing size as a management tool can only be effective if most discarded fish survive or if the minimum landing size is consistent with gear selectivity. It is a common assumption that fish are only discarded if they are below minimum landing size.

H4a Length distribution of catch determines selection

Instead of legal landing size that determines size selection for a given species, it could be the length distribution in a given haul or trip or year. This means that if many small fish are caught, smaller fish would be retained compared to a scenario with more large fish.

H4b Species composition of catch determines selection

Some species are more sought after than others as they fetch higher prices or for some other reason (e.g., less perishable might be sought at the beginning of a trip). Thus the retention or discarding of non-target species could depend on the amount of desirable fish in a haul if overall storage capacity is limiting (see also H2b). Investigating this hypothesis requires discards and landings information by haul. Hypotheses test would then involve investigating whether the amount landed of target species had significant explanatory power for the proportion retained of a non-target species.

H4c Variation of selection pattern is due to fixed fishermen behaviour

It might be conceivable that there exists a systematic fisherman effect on selection patterns whereby a given crew (individual) always rejects smaller (or larger) individuals compared to other fishermen. The corresponding hypothesis is that if a crew rejects fish larger than the average fish for a given species, it does so for all other species and at all occasions. This crew or individual rejection behaviour might then be explainable by some independent variables related to the vessel or the crew composition. Studying this hypothesis requires identification of the crew (individual) sorting the catch as well as repeated observations for several trips distributed across the year in order to avoid season effects.

4 H4d Market demand determines selection

Fluctuating prices for seasonally high valued species might be another criterion for discarding or not, including size-dependent prices. To test this hypotheses would require monitoring both prices and discards during a full season. Instead of legal minimum landing size it could be local customs that determined length selection and thus discarding practices. Many prejudices circulate concerning national preferences. However, it is not clear how to study this hypothesis. It would be difficult to distinguish the case where certain sizes or species are not landed because nobody would buy them from the case where nobody has ever tried to land them but assumes they would be unsaleable. Future research might help to shed some light on this question.

Case study

Materials

The French fleet operating in the Celtic Sea discarded an estimated 30,000 t in 1997 while landing around 63,00 t (Rochet et al. 2002). These estimates are based on a sample of 462 hauls during 26 trips. Depending on the métier, the estimated percentage of the catch discarded ranged from around 25% for benthic and gadoid trawlers to 55% for Nephrops trawlers. For the present study, data from the 1997 sampling exercise as well as some data for 1982 and 1991 were used. Hypotheses for discarding behaviour have been tested by métier when possible. Some hypotheses could not be tested as only discards where sampled by haul, but landings were only available by trip. The assumption that trip discards are proportional to catch was compared to the alternative hypothesis that the discarded proportion increased with catch by comparing a linear regression passing through the origin with a polynomial regression also passing through the origin, with separate slopes and curvatures per métier. Spearman's rank correlation ρ was used for testing hypotheses based on positive relationships (Conover 1971).

H1 Discards proportional to catch

The assumption of discards being proportional to catch is rejected in favour of a non- linear relationship, with negative curvatures supporting the idea of a saturation process (Figure 1, Table 1). Due to the lack of landings data by haul, this alternative saturation hypothesis could not be tested on the trip level. Hence we looked at the relationship between hold capacity and total landings per trip as an indication for a saturation (Figure 2). A positive relationship between the two quantities appeared providing evidence that hold capacity might play a role in determining total landings and thus discards (Table 1).

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Figure 1. Discarded biomass (t) and landings (t) per trip by métier for the French fleet operating in the Celtic Sea during 1997. (a-c) Total biomass caught and landed by benthic (a), gadoid (b) and Nephrops trawlers (c). (d) Nephrops biomass caught and landed by Nephrops trawlers. Points would lie along the continuous line if the whole catch was landed. Dashed line: a robust local smoother.

Figure 2. Trip landings (t) versus vessel hold capacity (tons) for French fleet operating in the Celtic Sea during 1997. B: benthic trawlers, G: gadoid trawlers, N: Nephrops trawlers. Line shows a robust local smoother.

6 Table 1. Results for hypotheses tests for discarding process of French Celtic Sea fleet targeting Nephrops, gadoids and benthic species in 1997.

Hypothesis Test result Conclusion H1 Discards per trip are proportional F20,23 = 8.82, p = 0.0006 Proportionality rejected: to catch/landings saturation more probable H1a Saturation process due to storage limitations H2 Discards are proportional to benthic ρ = 0.32, rejected effort p = 0.0001 benthic : positive relationship gadoid ρ = -0.03, (no proportionality) p = 0.78 gadoids : none Nephrops ρ = -0.21, Nephrops : negative p = 0.002 H2a Discarding behaviour changes No data with haul duration

H2b Discarding behaviour changes No data with trip length H3 Year class strength determines slope = 0.3, p = 0.04 evidence discards (10 species for 3 years) H3a Determined by length p< 0.05 for 18 species evidence distribution depends on métier and species H4 Legal landing size determines rejected selection differences between métiers and species H4a Length distribution of catch not shown evidence determines selection differences between métiers and species H4b Species composition of catch No data determines selection H4c Variation of selection pattern is ANOVA p = 0.21 rejected due to fixed fishermen behaviour H4d Market demand determines not shown rejected selection

H2 Discards proportional to effort

Trawl fishing effort on the haul level is measured by the haul duration in hours while effort on the trip level might be expressed as the trip duration which includes travelling time. For the Celtic sea most sampled fishing trips were of similar length (14 days), thus not providing enough contrast to investigate the impact of trip duration on discarding. In contrast, discards as a function of haul duration were looked at by métier (Figure 3). Discards increased with effort for benthic trawlers, but not linearly (hence not proportionally), but decreased for Nephrops trawlers; there was no relationship between discards and effort for gadoid trawlers (Table 1).

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Figure 3. Discarded biomass (t) versus haul duration by métier for the French fleet operating in the Celtic Sea during 1997. (a) benthic, (b) gadoid (c) Nephrops trawlers (d) whole fleet. Lines show a robust local smooth.

H3 Year class strength determines discards

For the ten species we had both discard estimates and estimated numbers of recruits from ICES working group reports for three years (Rochet et al. 2002). Due to the shortness of the time series, we pooled across species and standardised both discards and recruits by dividing by their respective mean values. A significantly positive relationship was found between scaled discards and scaled recruits (Table 1). This provides evidence in support of the hypothesis of year class strength influencing discards. The competing hypothesis was that the length distribution in the catch could be determining. Indeed, the length of the smallest animal of a given species in the catch was a significant predictor for the amount discarded for all tested species except Nephrops (Table 1). Full results are reported in (Rochet et al. 2002, Table 9).

H4 Legal landing size determines selection

The length at which 50% of the fish were discarded (DL50) was estimated fitting separate logistic curves for each of nine commercial species (by sex for megrim and Nephrops). The estimated DL50 was then compared to the minimum landing size (MLS) in vigour at the time of the study (Figure 4, Table 1). Linear models of DL50 per trip for 19 species were selected by stepwise addition of variables chosen among quartiles of the length distribution of catch by trip, total catch weight of the species, prices of the small and large categories of the species, trip duration, métier, gear, season, fishing area. Selected models

8 differed between species, but minimum or first quartile of the length distribution were the most often selected variables, showing evidence in favour of the competing hypothesis H4a. In contrast, prices were never selected, suggesting that hypothesis H4d should be rejected for this fishery. It is also possible to test whether a crew effect on the selection pattern existed. An analysis of variance on the DL50 estimates using species, métier and crew as fixed effects showed no crew effect on length selection after removing the effect of métier (Table 2). Unfortunately only one trip per crew had been sampled so that the replicates for a given crew come from the hauls.

Nephrops (male)

Nephrops (female)

megrim (male)

megrim (female)

whiting

lemon sole

whitch flounder

plaice

hake

cod

0 5 10 15 20 25 30 35 40 DL50

Figure 4. Mean length (cm) at which 50% of the fish were discarded by métier : Nephrops trawlers (grey), gadoid trawlers (white) and benthic trawlers (black). The vertical lines indicate the minimum landing size.

Table 2. Analysis of variance for mean length at 50% discarding, DL50.

Factor df SS mean SS F p-value Species 20 27210.9 1360.5 45.5 0 Métier 2 107.7 53.8 1.8 0.17 Crew 3 98.6 32.9 1.1 0.34 Species x Métier 39 3002.2 77.0 2.6 0 Species x Crew 56 1954.3 34.9 1.2 0.21 Residuals 260 7768.5 29.9

Discussion

The Celtic Sea case study provides some empirical evidence against H1, H2 and H4 and some evidence in support of H3. Many of the competing hypotheses seem more likely than the original set of hypotheses, which are part of the assumed body of valid assumptions fisheries science relies on. This indicates to us that the process of discarding is still not very well understood and lacking any theoretical progess, more empirical studies need to be carried

9 out to elucidate the process. We propose that discard studies of other fisheries look at the hypotheses investigated in this paper in order to increase the empirical basis for a future theory on discarding behaviour. These studies should be supplemented by questionnaire surveys or interviews of fishermen.

Acknowledgement

We would like to thank Isabelle Péronnet and her collegues for collecting the Celtic Sea discards data.

References

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