Browsing by Subject "Age-structured production model"

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    Illegal and unreported fishing on abalone—Quantifying the extent using a fully integrated assessment model
    (Elsevier, 2011) Plagányi, Éva E; Butterworth, Doug S; Burgener, Markus
    Illegal, unregulated and unreported (IUU) fishing is a major problem in many of the world's fisheries. The stocks most severely impacted centre on those characterised by high economic value, such as abalone, as well as long lived and slow growing species such as Patagonian toothfish (Dissostichus eleginoides). Effective management of these stocks as well as assessment of the impacts of IUU fishing on the resources is impeded by the technical difficulties associated with determining the magnitude of the IUU catches. The South African abalone Haliotis midae fishery rates as an extreme example of extraordinarily high levels of illegal and unreported (IU) catch. To assess the level and trends in IU catches, we used a combination of approaches that included collation of confiscation records from law enforcement, development of a novel index (the confiscations per unit policing effort—CPUPE), estimation of illegal catches using a spatial and age-structured assessment model, and cross-checking of model outputs through comparison with trade data on abalone imports in destination countries. The model-predicted 2008 IU estimate was 860 tonnes, more than 10 times the total allowable catch (TAC) for that year, and implied that, on average, 14% of all IU catches are confiscated. Associated management responses included the listing of H. midae on Appendix III of CITES (the Convention on International Trade in Endangered Species of Wild Fauna and Flora), and a temporary closure of the commercial fishery. We summarise both technical and management lessons to be learnt from this integrated approach to assess and verify the magnitude of IU fishing.
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    Preliminary assessment of the Falklands Patagonian toothfish (Dissostichus eleginoides) population: Use of recruitment indices and the estimation of unreported catches
    (Elsevier, 2005) Payne, Adam P; Agnew, David J; Brandão, Anabela
    Longline fishing for Patagonian toothfish (Dissostichus eleginoides) has taken place in Falkland Island waters for over 10 years. This species was previously only caught as bycatch in the Loligo gahi and finfish trawl fishery. This paper presents a preliminary assessment of the Falkland Islands population of Dissostichus eleginoides using an age-structured production model (ASPM). Two models were investigated; one using a Beverton–Holt stock recruitment relationship and another using trawler CPUE based estimates of toothfish abundance to estimate yearly recruitment. A stock recruitment relationship was not included within the model that used abundance estimates, as recruitment to the population was estimated directly from the index of relative recruit abundance calculated from trawler CPUE. The model was fitted to longline standardised CPUE and to the catch-at-length data. The models produced estimates that provided similar declines in the toothfish population although the start and end biomass estimates varied slightly. The models provided estimates of between 13,000 and 26,000 tonnes of current spawning stock biomass and showed current biomass to be between 38 and 46% of virgin biomass. The fit to CPUE was poor between 1994 and 1996, which we hypothesised could have been due to unreported catches or changes in q or M. This was a time when there was considerable IUU fishing in the southwest Atlantic. When the model was allowed to estimate a level of extra catch the fit was improved and 5000 tonnes of extra catch was estimated. Two further models were briefly examined to investigate the possibility of changes in M and q but both required large changes in those parameters in order to fit. Estimates of MSY varied widely, from 912 to almost 3000 tonnes.
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    Statistical catch-at-age analysis vs. ADAPT-VPA:the case of Gulf of Maine cod
    (Oxford University Press (OUP), 2008) Butterworth, Doug S; Rademeyer, Rebecca A
    In 2003, given an estimate of a spawning-stock biomass (Bsp) of 27% of the maximum sustainable yield (MSY) level based on an adaptive framework-virtual population analysis (ADAPT-VPA) assessment using data only after 1981, the Gulf of Maine cod (Gadus morhua) stock was deemed “overfished” under the US Magnuson–Stevens Act. However, an alternative statistical catch-at-age assessment (SCAA) at the time, using survey data from 1964, indicated Bsp above . This is investigated, together with other (sometimes conflicting) suggestions made during a number of recent assessment reviews of this stock. The primary reason for the different result is that the ADAPT-VPA assessment imposed asymptotically flat selectivity-at-age when there was strong statistical evidence for dome-shaped selectivity. Once adjusted for this, either assessment method robustly estimates Bsp relatively close to rather than below the “overfished” threshold of 0.5 . SCAA allows the longer series of survey data available to be incorporated, providing a better basis to estimate MSY-related targets and doubling the related precision in some cases. As such targets are important when implementing the Magnuson–Stevens Act, SCAA seems preferable to ADAPT-VPA for assessing this stock. Some broader inferences to be drawn from this comparative process include the need for: (i) careful treatment of the plus-group, especially if selectivity may be dome-shaped; (ii) flexible parameterizations of selectivity-at-age in SCAA to avoid false perceptions of the precision of results; and (iii) care in the use of the Beverton–Holt stock–recruitment function, as it gives inappropriately low estimates of if there is an overall negative trend in the estimates of recruitment plotted against Bsp.