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Browsing Journal Articles by Series "Collective Volume of Scientific Papers"

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    A Bayesian assessment of the South Atlantic population of albacore which explicitly models changes in targeting
    (International Commission for the Conservation of Atlantic Tuna (ICCAT) Secretariat, 2004) Rademeyer, Rebecca A; Butterworth, Doug S; Penney, Andrew J
    The primary assessment method applied to South Atlantic albacore during the October 2000 ICCAT SCRS assessment session was an age-structured production model which assumed a fleet-aggregated selectivity pattern with a single change only (in 1969), and estimated model parameters using only CPUE data. A particular problem of this approach is that the Japan CPUE series is not considered comparable over the complete period for which it is available because of changed targeting practices, so that this series needs to be split into three separate segments when estimating model parameters. This paper refines that 2000 assessment by treating the Japanese, Taiwanese and Brazilian fisheries all as initially primarily albacore directed fisheries which have shifted over time to bigeye tuna targeted fisheries, taking albacore as a decreasing bycatch. The transitions over time are explicitly estimated, as are separate selectivities for the two components of each national fishery by also including catch-at-age information in the data fitted by the model. Compared to the high levels of uncertainty in past assessments, the refined model provides a reasonable to good fit to all available information, and suggests that the productivity of the resource is somewhat higher than estimated previously.
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    An illustrative example of a management procedure for Eastern North Atlantic Bluefin tuna
    (International Commission for the Conservation of Atlantic Tunas, 2016) Rademeyer, Rebecca A; Butterworth, Doug S
    This document provides an illustrative example of the development of Candidate Management Procedures (MPs) for the Eastern North Atlantic bluefin tuna resource. Its purpose is to draw attention to key components of this process, including the specification of a number of alternative Operating Models (OMs) which describe plausible dynamics for the resource, the choices of abundance indices for use for input to MPs and of the error structures associated with the generation of future data corresponding to those indices, and consideration of key performance statistics related to future catch levels and resource conservation to allow consideration of the different trade-offs between these for alternative MPs. The MPs examined use a combination of target and slope based approaches applied to simulated future abundance indices from Japanese longline operations and a larval survey in an area of the western Mediterranean. MP trials are carried out for four OMs which reflect alternative resource assessments and choices for relationships between recruitment and spawning biomass. The greatest challenge appears to come from a scenario with both high and low recruitment regimes when there is a change from the former to the latter. If catches are allowed to go high to benefit from the period of high recruitment, can the change in regime be identified sufficiently soon to allow for adequate catch limit reductions to ensure resource conservation during the later years of lower recruitments?
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    Structure and estimation framework for Atlantic Bluefin tuna operating models.
    (International Commission for the Conservation of Atlantic Tunas, 2016) Carruthers, Tom; Kimoto, Ai; Powers, Joseph E; Kell, Laurence; Butterworth, Doug S; Lauretta, Matthew V; Kitakado, Toshihide
    A preliminary spatial, multi-stock statistical catch-at-length assessment model is developed as a basis for defining operating models for Atlantic bluefin tuna. The modifiable multi-stock model (M3) aims to improve upon previous multi-stock models such as MAST (Taylor et al. 2011) in three core areas. The first iteration of the model: (1) makes use of indices of abundance specific to time-area strata (e.g. for a given ocean area and month of the year), (2) does not use conventional tagging data to inform exploitation rates, (3) is fitted to samples of length composition data and therefore avoids established problems related to ageing individuals based on a growth curve and length data only. In this paper we provide a full account of preliminary M3 model equations and discuss the results of simulation evaluations of model estimation performance. Limitations of the current approach and future areas for model development are also discussed.