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