A detailed compilation and analysis of the available data for longspine thornyheads (Sebastolobus altivelis) found in west coast Canadian waters is presented. This analysis was prompted by concerns over the rapid development of a new bottom trawl fishery directed at this species since 1996. An analysis of the available length frequency data from the commercial fishery showed that these distributions have been quite stationary over the four years of the fishery. Relative abundance indices estimated from CPUE data using general linear modelling methods showed a 16% decline in biomass over the four year history of the fishery. Population modelling using a dynamic age-structured model fitted to the estimated relative biomass indices and the annual observations of length structure in the commercial fishery estimate that the population has declined between 10 and 30% over the four years of the fishery. These estimates are unreliable due to the lack of a validated growth function and uncertain estimates for natural mortality. This report recommends the development of an independent biomass survey for this species and further research on growth rates. This report also hypothesises that this species may have very wide stock boundaries due to its extended pelagic larval phase (18-20 months) and the consequent opportunity for wide dispersal due to prevailing ocean currents.
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