A Pilot Study on the Application of Hydroacoustic Surveys to Assess the Abundance of Delaying Sockeye in Southern Georgia Strait

During their return migration from the Pacific Ocean, some stocks of Fraser River sockeye spend several weeks holding in southern Georgia Strait prior to commencing their migration up the Fraser River. The run size of these delaying stocks is challenging to accurately assess in-season because in-river hydroacoustics at Mission cannot provide estimates of daily abundances. Instead, more variable catch-per-unit-effort (CPUE) data from marine test fisheries must be relied upon exclusively. In years where a high proportion of sockeye are expected to delay, a troll test fishery has also been operated in southern Georgia Strait to inform run size assessments of delaying stocks. However, CPUE indices from this troll test fishery are highly variable and the resulting CPUE-based estimates of the abundance of delaying stocks are often uncertain, impeding fisheries management decisions.

We propose a pilot program using fisheries hydroacoustic surveys to estimate the abundance of delaying sockeye stocks in southern Georgia Strait. The project would be carried out over 5 weeks from the middle of August to September 2018, the same period that the Gulf Troll test fishery has traditionally operated and when Late-run sockeye abundance is expected to peak in Georgia Strait. A vessel will be chartered to tow a split-beam transducer along pre-determined transect lines within the Strait. The split-beam transducer will collect high resolution acoustic data on fish abundance along the transects, providing a weekly index of abundance within the Strait.