Using a Spatially Structured Model to Assess the Tasmanian Fishery for Banded Morwong (Cheilodactylus spectabilis)

Using a Spatially Structured Model to Assess the Tasmanian Fishery for Banded Morwong (Cheilodactylus spectabilis)

Malcolm Haddon, Philippe Ziegler, Jeremy Lyle, and Paul Burch

Using a Spatially Structured Model to Assess the Tasmanian Fishery for Banded Morwong (Cheilodactylus spectabilis)This is part of Fisheries Assessment and Management in Data-Limited Situations
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Description

Fisheries can be regarded as data poor if insufficient information is available to produce a defensible stock assessment. Data limited fisheries can be (1) new fisheries with no time-series of information; (2) low-value fisheries for which little data are collected; (3) bycatch fisheries, which are often ignored; and (4) spatially structured fisheries where data collected may not be representative of the whole stock. This last category is especially problematic because when little data are available, stock assessments are generally limited to simple models. Unfortunately, the impact of fishing and our ability to assess a fishery can be greatly affected when the dynamics of a stock or stocks are influenced by some spatial structure. In the instance studied here, there are similarities to studies of fished areas adjacent to marine protected areas closed to fishing. A simulation study of a spatially structured fishery was undertaken, using the Tasmanian banded morwong fishery to mimic such a data-limited situation. It was found that the characteristics of the catch (age structure and sex ratio), which were available through a relatively focused research program, could only be fitted using a model that included a spatially structured population, with fished and unfished components (based on depth) and movement in between. Based on such a fishery assessment, the stock was seen to be declining. This outcome contrasted greatly with the outcome of considering only the catch and effort data typically available for such inshore reef fisheries in Australia. The catch and catch rate information, when modeled as coming from a spatially homogeneous single population, gave the appearance of relative stability and sustainability. Fisheries for species likely to have some spatial structuring, such as widely distributed reef dwelling species, may possibly be far more complex and difficult to manage than the classic theory of homogeneous fish populations would suggest. Some form of explicit spatial management would be a more appropriate approach to ensuring long-term sustainability in such fisheries.

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