Research Document - 2003/107

Analyses of juvenile surveys for recruitment prediction in the Strait of Georgia

By Hay, D.E., Schweigert, J.F., Thompson, M.,
Haegele, C.W. and Midgley, P.

Abstract

We estimated annual variation in the relative abundance of juvenile herring from purse seine surveys conducted from 1991 to 1999 in the Strait of Georgia. The objective was to evaluate the predictive capability of the surveys to estimate the relative size of the recruiting year class before it enters the fishery at age 3. In some years, a substantial part of the fished population (20-50%) consists of herring that recruit in the same year. Therefore such predictive capability would be useful as ancillary information for determining total allowable catches for the fishery. Purse seine surveys were made throughout the Strait of Georgia in September and October. Sets were made at ten fixed transects, each with about five fixed sampling stations that varied in depth and distance from shore. Juvenile herring in their first year of life (about 5-6 months of age) were the most common species captured, followed by age-1+ herring. Juvenile herring abundance changed significantly among years, but there were also significant interannual differences in abundance among different regions of the Strait of Georgia. For each year of the survey, we compared the numbers and weight of age-0+ juvenile herring catches with the number of age-3 recruits, of the same cohort, estimated independently 3 years later, from age- structured analyses used for the annual assessments. There are several alternate ways to consider these comparisons, each differing in the estimate of the annual juvenile abundance. In general however, all comparisons show a positive but variable relationship, with the juvenile index accounting for less than 50% of the variability in the recruitment index. The most aberrant data point, in an otherwise convincing regression of seven points, is the exceptionally abundant 1999 cohort, that recruited as age 3 herring in 2002. This was the second largest cohort seen since the 1954 cohort, which was only marginally greater. The troubling aspect of these juvenile surveys, therefore, is that they were unable to anticipate this exceptionally large cohort. At best, we would have anticipated only moderately good recruitment. However, we anticipate that additional data will allow further refinement and understanding of the predictive utility of the approach, as well as factors contributing to the variation. With further data collection and analyses we suggest that the results of the survey could become a key indicator of potential recruitment in the Strait of Georgia. Such a prediction could be made nearly two years prior to the recruitment and we comment on the potential of this approach for future use by management. We conclude the paper with a brief discussion of biotic factors that might have contributed to the strong 1999 cohort.

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