Aggregated yield and fishing effort in multispecies fisheries: an empirical analysis

Abstract

Many tropical fisheries are inherently of a multispecies nature, with any given type of fishing gear harvesting a wide range of species. Species-aggregated relationships between fishing effort and yield or catch per unit of effort (CPUE) provide important information for the management of such fisheries, as well as insights into ecosystemlevel responses to fisheries exploitation. We used a model selection approach to study species-aggregated, yield-effort relationships in spatially replicated, multispecies inland fisheries. Of three alternative models considered, the data strongly supported a sigmoid functional form (Akaike weight 0.95) over the alternative, asymptotic exponential or quadratic (Schaefer) models (Akaike weights 0.03 and 0.02, respectively). The sigmoid form implies the occurrence of an inflexion point in the rising part of the yield-effort curve and no decline in aggregated yield even at the highest effort levels observed. Aggregated CPUE declines steeply near the origin of the effort scale before stabilizing or rising to a local maximum and then declining inversely with further increases in effort. This highly nonlinear response suggests that extreme caution is required when interpreting aggregated CPUE as an indicator of fishing impacts on exploited communities. Further research is required to identify the mechanisms generating the observed relationship.Résumé : Plusieurs pêches tropicales sont multispécifiques de nature et tout type d'engin de pêche y récolte une gamme étendue d'espèces. Les relations pour l'ensemble des espèces entre l'effort de pêche et le rendement ou les captures par unité d'effort (CPUE) fournissent des informations importantes pour la gestion de telles pêches, ainsi que des perspectives sur les réactions écosystémiques à l'exploitation par la pêche. Nous utilisons une méthodologie de sélection de modèles afin d'étudier les relations rendement-effort pour l'ensemble des espèces dans des pêches inté-rieures multispécifiques qui se répètent à l'échelle spatiale. Des trois modèles considérés, les données appuient fortement le choix d'une forme fonctionnelle sigmoïde (pondération d'Akaike de 0,95) plutôt que des modèles de rechange asymptotique exponentiel ou quadratique de Schaefer (poids respectifs d'Akaike de 0,03 et de 0,02). La forme sigmoïde implique l'existence d'un point d'inflexion dans la partie croissante de la courbe rendement-effort et l'absence de déclin du rendement de l'ensemble des espèces même aux niveaux les plus élevés d'effort observés. La CPUE de l'ensemble des espèces décline abruptement près de l'origine de l'échelle des efforts avant de se stabiliser ou de remonter pour atteindre un maximum local, pour ensuite diminuer en fonction inverse des augmentations subséquentes de l'effort. Cette réaction qui est loin d'être linéaire indique qu'il faut être très prudent lorsqu'on interprète la CPUE de l'ensemble des espèces comme indicateur des impacts de la pêche sur les communautés exploitées. Des recherches supplémentaires pourraient identifier le...

Rapid shifts in catch composition in the artisanal Red Sea reef fisheries of Eritrea

“…The CPUE decline could also be due to "hyperdepletion" (Hilborn and Walters, 1992;Lorenzen et al, 2006), i.e. the initial phase in the evolution of catches in which CPUE shows a steep decline as the highly vulnerable subset of the population is depleted (Hilborn and Walters, 1992;Myers and Worm, 2003).…”

“…In addition, our estimates of existing fishing pressure, as suggested by official statistics, were in fact conservative, because they did not include the supposedly large amounts of unreported catches taken to Yemen (Moussalli and Haile, 2001). Therefore, the changes in CPUE could indeed represent substantial biomass changes, not least the risk of localized depletion (Walters, 2003;Lorenzen et al, 2006).…”

Resource sustainability in small-scale fisheries in the Lower Amazon floodplains

“…Multispecies fisheries do not react to fishing pressure as regular unimodal stock-production models, which typically show increasing yields with increasing effort up to a point at which maximum yields are realized and after which increasing effort brings declining yields (Schaefer, 1954). In multispecies fisheries, catch levels rise with increasing effort, then stabilize and do not decline even at very high effort levels (Laë, 1997;Lorenzen et al, 2006). Catch levels remain constant at high effort levels through the "fishing down" process-a process in which collapsed fish populations (usually large predators) are replaced with other (usually smaller) species (Welcomme, 1999).…”

Reconstructing the observation process to correct for changing detection probability of a critically endangered species

“…There is increasing empirical evidence to suggest that these responses lead to nonlinear relationships between CPUE and abundance (e.g. Lorenzen et al 2006). If the bias and uncertainty of relative abundance estimates change over time, this can have a serious impact on the validity of monitoring programmes (Pollock et al 2002), and hence on our ability to plan conservation interventions and assess their effectiveness.…”