Research Document - 2000/156

Feasibility of a bottom trawl survey for three slope groundfish species in Canadian waters.

By P.J. Starr and C. Schwarz

Abstract

The feasibility for conducting research surveys using commercial bottom trawl gear targeted at three slope groundfish species located in the Pacific coast of Canada is presented. The objectives of such a survey are to generate comparable indices of population size over time which can be used as inputs into population assessment models for each of the three species. This feasibility analysis is based on the calculation of the variability in catch per hour for each species from commercial catch and effort data to determine the amount of stratification by depth and the number of tows required in each stratum to achieve a target level of precision. Of the three target species, longspine thornyheads appeared to be the least variable in commercial CPUEs and hence required the fewest tows to monitor the population. Shortspine thornyheads were of intermediate variability and Pacific Ocean perch were highly variable. Pacific ocean perch have a completely separate spatial distribution from the two thornyhead species and can be monitored independently of the other two species. The two thornyhead species are spatially commingled, with shortspines having a more shallow distribution compared to the longspines. The final size and aerial extent of the survey will be dependent on the target precision level at which each species is required to be monitored. These decisions are largely management based or require additional stock boundary research. Several alternative options for number of strata and suggested levels of precision are presented, ranging from under 150 tows to nearly 400 tows for the entire survey, with estimated relative costs varying from ~$275,000 to nearly $800,000. An additional issue is that the performance of the nets while towing must be monitored electronically to ensure comparability both within and between surveys as it is likely that net efficiency will improve over time.

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