Paper Refutes Alleged Link between Over-spawning and Stock Collapse in Pacific Salmon

Vancouver, June 8, 2004 - The belief that excessive returns of salmon to their spawning grounds (over-escapement) will lead to a collapse of salmon production has been shown to be unfounded by a new technical paper prepared by the Pacific Fisheries Resource Conservation Council (PFRCC). To obtain a copy of the report, go to: www.fish.bc.ca.

The paper, entitled Does Over-Escapement Cause Salmon Stock Collapse?, concludes there is no evidence to link over-escapement and salmon stock collapse. That link has frequently been proposed to support an argument to take higher catches and avoid surplus numbers of salmon on the spawning grounds.

"This long over-due paper brings much needed science to the question of over-escapement and salmon stock collapse", said the Honourable John A. Fraser, chair of the PFRCC. "On the basis of the data available there is no evidence that higher escapements have resulted in stock collapse; fisheries officials will in future be able to make sounder, science-based decisions on the management of the stock".

Prepared for the Council by Dr. Carl Walters, Paul LeBlond and Brian Riddell, the paper examines the issue of over-escapement and its potential impact on subsequent production. It was initiated after a request from the Minister of Fisheries and Oceans Canada for a review and information on the notion of over-spawning as it relates to fisheries management strategies and recent large spawning escapements to some Fraser River sockeye salmon populations.

Based on observations, as opposed to theory, the paper involved an assessment of available data for 21 sockeye salmon stocks and two pink salmon stocks. While there is evidence of declining rates of production at higher escapement levels, there is no evidence to support anything like a "collapse" or "near-collapse" occurring. This observation is significant as it demonstrates that very productive stocks will not suffer when risk averse management and other precautionary measures are undertaken to protect weak stocks.

As previously suggested in a seminal paper by Dr. W. E. Ricker in 1954, the Council's work confirms there is a valid concern for the efficiency of salmon production at higher spawning population sizes. In other words, when very large numbers of salmon spawn the rate of offspring produced per female parent declines. Collapse does not occur, however, although the subsequent run size does not increase in proportion to the large number of spawners. This proportionality between overspawning and reduced productivity is the basis for management efforts to determine an intermediate spawning level that both supports harvesting and sustains the fish population.

Regarding the Adams River and other late-run sockeye stocks, pre-spawning mortality has increased, associated with earlier entrance of the fish to the Fraser River. This has led necessarily to reduced fishing which has resulted, in some years when pre-spawning mortality was not as great as expected, in very high spawning numbers. This high level of escapement combined with good data availability provided an exceptional opportunity to observe what happens when there are very large returns of salmon. In such situations there is increasing evidence of a carrying capacity limitation to sockeye production in the large Fraser Basin lakes, which results in a leveling off of salmon production for runs having a high escapement, but again this scenario shows no evidence of stock collapse beyond small reductions in the size of juveniles.

The paper acknowledges that fishers have been correct in expressing concern about lost economic value and spawning efficiency of these surplus spawners, but disproves their claim of stock collapse. The paper also notes other values associated with large returns. Overall ecosystem benefit that First Nations and others have raised is one such value but it has not been assessed in this report. Nor has the report examined whether or not the high escapements trigger dispersal and recolonization of habitats that would ultimately result in higher production.

For more information, contact:

Paul LeBlond
Council Member PFRCC
250-539-2310

Gordon Ennis
Managing Director PFRCC
604-775-6070

Related Reports:

Does Over-Escapement Cause Salmon Stock Collapse?