Research Document - 2002/064
Status of Cultus Lake Sockeye Salmon
(Oncorhynchus nerka)
By Schubert, N.D., Beacham, T.D., Cass, A.J., Cone, T.E., Fanos, B.P., Foy, M., Gable, J.H., Grout, J.A., Hume, J.M.B., Johnson, M., Morton, K.F., Shortreed, K.S.,
Staley M.J. and Withler, R.E.
Abstract
This report documents an assessment of the status of Cultus Lake sockeye salmon (Oncorhynchus nerka). Sockeye escapements have declined precipitously on all cycles in recent years, a decline that is coincident with an earlier timing of migration into the river that is part of the broader phenomenon affecting all late run Fraser River sockeye populations. In association with early migration, there also has been a decline in spawning success that has resulted in the failure to observe a single successful spawner in some years. These observations led to the Pacific Science Advice Review Committee's request for a status report on this stock.
The Cultus sockeye population is among the most intensively studied salmon stocks in British Columbia. Studies of spawner abundance, lake characteristics and juvenile production began with the work of the Pacific Biological Station in the 1920's and have continued until the present with the work of the International Pacific Salmon Fisheries Commission and the Department of Fisheries and Oceans. This report summarizes or provides detailed data regarding: watershed geomorphology; lake limnology and fish ecology; sockeye life history; enhancement history; predator and exotic species control; spawner counts since 1925; sockeye fry assessments (lake hydroacoustic and trawl survey); smolt counts since 1926; fishery management processes and objectives; fishery catches and total returns since 1952; and marine distribution and migratory timing. These data are used to evaluate trends in escapements, juvenile abundance, catch and total return, and to calculate freshwater and total survival indexes and exploitation rates. Based on the available data and the analytic results, we provide an evaluation of the stock's productive capacity and current status, and use a simulation model based on Bayesian stock-recruitment analyses to evaluate future stock trajectories under different scenarios of prespawn mortality and exploitation.
Cultus is a potentially large stock (current escapements are a small fraction of the level that would utilize a substantial part of the stock's productive capability) that is less productive than the sockeye stocks with which it comigrates. The escapement of Cultus sockeye adults declined by 51% over the last three generations, a continuation of a trend that began following the construction of the Weaver Creek spawning channel in the late 1960's. The rate of decline is consistent with an Endangered classification as defined by the IUCN. There are two causal factors: exploitation rates that have exceeded the optimum rate associated with maximum sustainable yield in most years between 1952 and 1995; and extremely high prespawn mortalities that have occurred since the onset of the early migration in 1995. The result is a current effective spawner population that is less than 4% of the long term average on each of the four cycles. Our model simulations suggest that if the current conditions of high prespawn mortality continue, even in the absence of any fishing mortality, the prognosis for the stock is critical: the probability of extinction is conservatively estimated at one in three. If exploitation continues at moderate levels, the modelled rate of decline over three generations is >80% and the probability of extinction is >50%, conditions consistent with a Critically Endangered classification as defined by the IUCN.
We recommend the development of a risk assessment framework that evaluates risks of different fisheries and recovery options in terms of their cultural, ecological, economic and social values. While the framework is being developed, current mitigation efforts should continue and fisheries should be managed under a precautionary approach that recognizes the uncertainty associated with the early migration phenomena, and its potential severity, by minimizing exploitation rates to reduce the near-term probability of extinction and slow the rate of decline in spawner abundance. We also recommend the development of a comprehensive recovery plan that integrates options to improve freshwater survival with harvest controls and other measures, as well as the Department's support for the ongoing and new studies required to provide information important to our understanding of stock status and to the development of the risk assessment framework and recovery plan.
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