Evaluation of the Depensatory Fishing Hypothesis as an Explanation for Population Cycles in Fraser River Sockeye Salmon (Oncorhynchus nerka)

Abstract

Causes of cyclic fluctuations in the abundance of Fraser River sockeye (Oncorhynchus nerka) have been the focus of considerable debate for much of this century. Most hypotheses to explain cyclic behaviour in Fraser River sockeye centre on identifying agents of depensatory mortality, and several recent papers argue that sockeye cycles are maintained primarily by depensatory fishing patterns. We assess the evidence for depensatory fishing as an explanation for these cycles. Using simulations, we demonstrate that empirical evidence for depensatory fishing is likely an artifact caused by bias in estimating stock composition of catches in mixed-stock fisheries. Moreover, recent trends in harvest rates among comigrating stocks with asynchronous cycles are inconsistent with the depensatory fishing hypothesis. It also seems very unlikely that aboriginal and early commercial fisheries were intensive enough to maintain cycles that persisted prior to 1860. Only during the unregulated, early commercial fishery prior to the collapse of the upriver runs following the 1913–14 disaster at Hell's Gate is there any convincing evidence of depensatory fishing.