Research Document - 2000/06

Straying Rates and Stock Structure of British Columbia Herring.

By D.M.Ware, C. Tovey, D. Hay, and B. McCarter

Abstract

Tag release and recovery information compiled between 1950-55 and 1980-92 for the five, major populations (stocks) of British Columbia herring were analyzed. The results indicate a high 'fidelity' rate of adult herring to the spawning area where they were tagged; 75% to 96% of tagged fish at-large for one year tended to be recovered near the spawning areas where they were released. However, these results also indicate that a significant amount of straying occurs (4-25%). A comparison between the two tagging periods indicates that the mean proportion of herring straying between populations was significantly lower in the early 1950s (7%) than it was in the 1980s (22%). The proportion of herring straying to other populations decreased exponentially with the distance between populations. Forty-one percent of the herring that strayed moved 200 km in one year, 12% moved 600 km and a few exceptional individuals (7%) dispersed a distance of 800 km. This important finding indicates that the populations are linked by an 'isolation by distance' model of gene flow. Tag recoveries during both time periods confirm that the southernmost herring populations in BC exchange individuals (and genes) with the most northern populations. Although there are other complicating factors, our analysis suggests that the straying rate is density-dependent -- it appears to increase linearly as the population approaches the carrying capacity of the spawn habitat. Evidence supporting this density-dependent dispersal response is apparent in 4 of the 5 major populations. Our analysis explains several stock assessment anomalies, and has significant implications for herring stock structure and management. The authors conclude that the high observed fidelity rate provides the biological basis for managing BC herring stocks, because it means that most of the adult herring that spawn in one of the major populations return to the same region to spawn the following year. Consequently, the current stock assessment areas (populations) form the basic management units. However, the significant observed straying rates also indicate that the five, major herring populations are linked by variable degrees of gene flow, and therefore form a single, large metapopulation.

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