Research Document - 2008/072

Cultus Lake Sockeye Population Viability Analysis

By J. Korman and J. Grout

Abstract

A stochastic simulation model was developed to evaluate the efficacy of alternate captive broodstock programs, harvest rates, and freshwater habitat enhancement to recover the Cultus Lake sockeye population. The model simulates the abundance of emigrating smolts and returning adults based on a spawner-to-smolt stock-recruitment model and density-independent marine survival and pre-spawn mortality rates. The model accounts for removals of spawners for broodstock collection, and tracks the abundance of hatchery-produced smolts and returns as well as the production from hatchery-origin fish that spawn in the wild. The simulated numbers of both wild and naturally produced progeny from hatchery-origin spawners are compared to the recovery goals defined in the Cultus Lake Sockeye Recovery Strategy.

There was little information in the spawner-to-smolt data concerning depensatory mortality or density dependence. A depensatory model fit using prior information on carrying capacity based on Shortreed et al.’s (2001) euphotic volume model was consistent with error assumptions and eliminated over prediction of smolt numbers at low stock size that was apparent when using a standard Ricker stock-recruitment model. With termination of captive broodstock collection in 2007, the probability of meeting recovery objectives declined with increasing harvest rate and there were very large differences in performance between pessimistic and optimistic marine survival-pre spawn mortality (PSM) scenarios. Extinction probability increased substantially under higher harvest rates and was very sensitive to the assumed marine survival-PSM scenario.  Under a limited set of conditions, continuation of the hatchery program or habitat enhancement improved recovery statistics and reduced the risk of extinction. Performance measures were very sensitive to the assumption of depensation in freshwater survival rate. Population recovery was best under a combined policy, which included reduction in harvest rate, continuous habitat enhancement, and extension of a hatchery supplementation program with a doubling in smolt capacity through 2015. As for any population viability analysis, the results presented here should be viewed with healthy skepticism. However, the modeling exercise was useful for examining the relative benefits of alternate recovery options, and for highlighting priorities for data collection and research.

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