Catch-per-unit effort (CPUE) has frequently been used as an index of abundance and more specifically to calibrate virtual population analysis (VPA). In multi-species fisheries, CPUE calculated from fishing trips targeting the species seems to be more effective for calibration than the classical ratio total landings/total effort. Target species are determined from an analysis of the composition of catches (landings) of each fishing trip, each trip being categorised as to whether it targets the given species or not. It is obvious that existing but not reported discards would affect these results. Classification of trips can be achieved on a single-species basis, each species being considered successively and each trip possibly being relevant for more than one species. Using a more general approach, classification can be achieved by metiers, each of these being determined by one or a group of target species and each trip being categorised into one and only one metier. This method of categorising trips is based on thresholds of target species contribution to the catch and on an overall explanatory level. For the main species of the French demersal fishery off the west coast of Scotland and in the Celtic Sea, CPUEs are calculated using different methods to define the trips used to calculate fishing effort and associated landings. Besides important differences in actual values, CPUEs may also differ in their trends depending on the delinition of directed effort. Tuned VPA, carried out for some species, shows that large variations in population estimates, fishing mortality or short-term predictions could occur when using directed effort, while the catchability model fits the data better. 0 Ifremer/Elsevier, Paris Multi-species fisheries / directed effort / target species / metier / tuned virtual population analysis / stock assessment / NE Atlantic Resume -Definition d'un effort de p&he dirige dans une p&herie pluri-specitique, et son impact sur I'evaluation des stocks. Les captures par unite d'effort de p&he (CPUE) sont frequemment utilisees comme indice d'abondance et servent notamment a la calibration des analyses de cohortes (VPA). Plutot que le simple rapport entre la production et l'effort total developpe sur une zone de p&he, les CPUE calculees a partir de (< mar&es D ciblant l'espece consideree semblent plus pertinentes. A partir de l'analyse de la composition des debarquements de chaque G make N, il est possible de determiner des especes cibles et de classer les << makes >> selon que le pecheur recherche ou non ces especes. Cette classification des makes peut se faire soit en considerant chaque espece successivement, une maree pouvant &tre pertinente pour plusieurs d'entre elles, soit plus globalement, en reference a des metiers, chacun ttant defini par des seuils d'une ou plusieurs especes cibles, chaque mar&e &ant alors dediee a un metier et un seul. Cette etude pmsente la methode utilisee, baste sur des seuils d'especes cibles et un niveau global d'explication. Pour les pecheries francaises de l'ouest de 1'Ec...
Current scientific management objectives for ICES roundfish stocks are to ensure conservation of the biological resource and do not explicitly consider economic or social objectives. For example, there are currently no objectives to maximize the sustainable yield or to reduce variability in total allowable catches (TACs). This is despite the fact that the current system can result in wide annual fluctuations in TAC, limiting the ability of the fishing industry to plan for the future. Therefore, this study evaluated management strategies that stabilized catches by setting bounds on the interannual variability in TACs. An integrated modelling framework was used, which simulated both the real and observed systems and the interactions between system components. This allowed the evaluation of candidate management strategies with respect to the intrinsic properties of the systems, as well as our ability to observe, monitor, assess, and control them. Strategies were evaluated in terms of risk (measured as the probability of spawning-stock biomass falling below a biomass threshold for the stock) and cumulative yield. In general, bounds on interannual TAC change of 10% and 20% affected the ability to achieve management targets, although the outcome of applying TAC bounds could not have been pre-judged because results were highly dependent on the specific biology of the stock, current status, and the interaction with assessment and management. For example, for North Sea haddock, management became less responsive to fluctuations resulting from large recruitment events. Simulated target fishing mortality levels were rarely achieved, regardless of the TAC bound applied, and actual fishing mortality rates oscillated around the target. In the longer term, more restrictive bounds resulted in oscillations of greater amplitude and wavelength in yield and SSB. Bounds had less effect when a stock was close to the biomass corresponding to the target F. Risk for stocks that are declining or currently at low abundance may be greater, because if bounds restrict the extent to which TACs can be reduced each year, they could lead to collapse of the stock and the loss of all future revenue. However, for a recovered stock or one at high abundance, the loss of yield as a result of bounds would be smaller than that caused by the total collapse of the fishery. At low stock size or if the stock was declining, catches should be changed more rapidly than when the stock was increasing or at a high level, especially high stock sizes acting as an insurance against uncertainty. Therefore, rebuilding strategies, and strategies aimed at maintaining the stock above prescribed limits, should be considered separately.
This paper describes a simulation study that evaluated the performance of the scientific advisory process used by ICES to recommend total allowable catches (TACs) for roundfish stocks. A “management strategy evaluation” approach is used, involving development of an operating model to represent the underlying reality, and an observation model to generate pseudo data that are then used within a management procedure. The management procedure comprises an assessment that uses data to estimate parameters of interest and a decision rule to derive TAC recommendations for the following year. There are two important results: including realistic sources and levels of uncertainty can result in far from optimal management outcomes based on the current procedures; and current ICES biomass and fishing mortality reference points are not always consistent, and several are clearly inappropriate. This is because the types of projection used by ICES do not incorporate important lags between assessing stock status and implementing management measures, and they also ignore important sources of uncertainty about the actual dynamics, as well as our ability to collect data and implement management regulations (i.e. model, measurement, and implementation error, respectively). The simulation approach also showed that better management is not necessarily going to be achieved by improving the assessment, because even with a perfect assessment (where the simulated working group knew stock status perfectly), stocks may crash at fishing levels that standard stochastic projections would suggest were safe. It is proposed that, in future, operating models that represent the best available understanding of the actual system dynamics be used to evaluate models and rules considered for application. These operating models should capture the plausible range of characteristics of the underlying dynamics, but not necessarily model their full complexity. In general, they will be more complex than those used by assessment working groups, so developing management procedures that are robust to a broad range of uncertainty. However, the models and rules used as part of the management procedure should be simpler than those used at present.
French deepwater fish exploitation (500 m) began off the West coast of the British Isles in 1973. It further developed in the 1980s, and intensified in the 1990s. In 1997, the French fleet comprised 49 boats, and since 1995 has landed yearly about 19,500 t of deepwater fish species, of which, the grenadier Coryphaenoides rupestris (Macrouridae) has been the most abundant. Discard data were sampled on-board by observers on French high sea trawlers from 1995 to 1997. Eight species were identified as totally landed, 43 non-commercial species as totally discarded and only one, the grenadier, as both landed and discarded. The mean discarding rate by haul was 48.5% in weight, whereas the mean grenadier discarding rate was 21.2%; these rates increased with depth. Discard species composition and discard quantities varied with depth, and depended on the specific composition of the fish communities, species abundance and their length-frequency distribution. The latter, established for grenadier and Alepocephalus bairdii (Alepocephalidae), assumes a bathymetric ontogenic migration. Annual discards of the fishery accounted for about 17,500 t in 1996 and 1997, but the results are quite uncertain.
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