Research Document - 2004/127
Forecasted status of Cultus and Sakinaw sockeye salmon in 2004
By Wood, C.C., C.K. Parken
Abstract
The sockeye salmon populations originating in Cultus and Sakinaw lakes (hereafter called Cultus sockeye and Sakinaw sockeye) were both designated “endangered wildlife species” by the Committee on Status of Endangered Wildlife in Canada (COSEWIC) following an emergency assessment in November 2002 that was confirmed in May 2003. In 2003, the number of mature individuals (spawners) has continued to decline to 1,485 in Cultus and 3 or fewer in Sakinaw. Even so there are reasons to be optimistic about recovery: Pre-spawn mortality in Cultus sockeye, considered by COSEWIC to be one of the principal threats, was less severe during the past two years, and as a result, the number of spawners in 2002 and 2003 has increased relative to the brood (parental) years in 1998 and 1999 when pre-spawn mortality was much higher. For Sakinaw sockeye, two years of intensive supplementation with hatchery-reared fry is expected to significantly increase adult returns in 2004 and 2005. However, it must be emphasized that little or no supplementation has occurred in subsequent years when few (2002) and no (2003) broodstock were available; thus, prospects for survival and recovery depend almost entirely on the number of spawners achieved in 2004 and 2005.
Adult returns in 2004 were forecasted with a new conditional Bayesian forecast model that considers both the smolt abundance observed in 2002 and the age-3 (“jack”) returns from the same brood year observed in 2003. The prior distributions of age-4 adult returns used in the Bayesian model were generated by multiplying estimated smolt abundance in 2002 by “smolt-to-adult survival”, considered as a log-normally distributed random variable whose parameters were estimated from recent data (1990-1998) for Chilko sockeye. The Bayesian model is conditional on the assumption that the age-3 proportion (by brood year) is known exactly. Sensitivity of forecasts to errors in the age-3 index assumption was evaluated for both Cultus and Sakinaw populations. An alternative, higher estimate of the age-3 index for Sakinaw sockeye (0.025) was also estimated from age composition data available from a single brood year (1971). The conditional Bayesian forecasts performed better in 3 out of 4 performance measures than the corresponding (prior distribution) forecasts based on smolt abundance alone in retrospective analyses using historical data for Cultus sockeye. As expected, the median posterior estimates were unbiased (over-forecasting in 50% of retrospective comparisons) compared with the prior distribution (which over-forecast in 67% of comparisons).
The recommended median forecast for total returns (before any fishing or pre-spawn mortality) of Cultus sockeye in 2004 is 281 individuals based on the Bayesian model using Chilko smolt survival parameters. The odds are 9 in 10 (90% probability) that total returns will exceed 150, and 3 in 4 (75% probability) that the return will exceed 203 individuals. The recommended median forecast for total returns of Sakinaw sockeye in 2004 is 390 individuals with odds of 9 in 10 (90% probability) that returns will exceed 215, and 3 in 4 (75% probability) that returns will exceed 286 individuals. The Sakinaw forecast is considered highly uncertain, and potentially very optimistic, because the underlying assumptions of smolt-to-adult survival and age-3 proportions are based on speculation that Sakinaw sockeye smolts will survive and mature precociously (“jack”) like Cultus sockeye. Alternatively it seems plausible that Sakinaw sockeye smolts are experiencing unusually poor marine survival which could account for the current status of the population. The Sakinaw forecast procedure could not be evaluated in retrospective analyses because no historical data are available.
In conclusion, the recommended median forecasts of adult returns to both populations in 2004 are below the criteria for population viability used by COSEWIC (IUCN criteria) and the respective SARA recovery teams; in this sense, both populations are likely to remain at imminent threat of extinction even with no incidental fishing mortality in 2004. However, fishing restrictions could significantly improve the status of both populations and hasten recovery. Probabilities for achieving the minimum viable population size threshold of 500 spawners in 2004 were computed under alternative assumptions about exploitation rate and pre-spawn mortality, and for Sakinaw sockeye, for opening dates in a plausible but hypothetical fishery.
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