Estimating the Abundance of Juvenile Coho Salmon in the Strait of Georgia by Means of Surface Trawls

Abstract
A fixed survey design with a randomized depth component and a large rope trawl that fished surface waters at a speed of approximately 5 knots was used to estimate the abundance of juvenile coho salmon Oncorhynchus kisutch in the Strait of Georgia. The estimates of 4.2 million juveniles in September 1996, 3.0 million in September 1997, and 3.0 million in September 1998 were minimal because the catchability of the net was probably lower than that used in the analysis. In 1997, by using hatchery‐marking percentages, we estimated that 3.4 million wild coho salmon smolts entered the Strait of Georgia from Canadian rivers. The estimates of juvenile abundance made in September 1997 were considerably larger than the estimated total returns in 1998, indicating that the marine mortality in fall and winter is an important component of the total marine mortality determining the final strength of the brood year. The use of surveys for estimating juvenile coho salmon abundance is a contribution to the understanding of the processes that regulate salmon abundance naturally and can potentially provide management information well in advance of any fishery.