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Seminar Abstract

Pete Lawson - December 3, 2002


Closing the loop: Environmental variability throughout the life-cycle of Oregon coastal natural coho salmon

P.W. Lawson, E.A. Logerwell, N. J. Mantua, R.C. Francis, and V. N. Agostini

Oregon Coastal Natural (OCN) coho salmon, spawning and rearing in the river basins of the coast range of Oregon, have shown marked fluctuations in abundance over the past 30 to 50 years and are currently at critically low numbers. Logerwell et al. (in press) have identified four environmental factors correlated with smolt to adult survival in the Oregon Production Index area. These factors are winter sea surface temperature (SST) in the smolt year, spring transition date, spring upwelling, and winter SST in the adult year.

The purpose of this study was to investigate climate effects on OCN coho in freshwater using smolts and smolts per spawner as indices of productivity and survival respectively. The results were then combined with the marine factors identified by Logerwell et al. to produce a life-cycle assessment of climate effects on OCN coho salmon production.Stream flow indices were developed for four time periods deemed important to successive stages of coho salmon development (first winter; eggs, first summer; parr, second winter; smolts, and second spring; out migration). In addition we created an index of the fall transition based on the date when winter storms first caused a rise in stream flows. Annual mean air temperature was used as a proxy for water temperature. Air temperature, fall transition, second winter flows and second spring flows were all significant predictors of smolt production. Fall transition and second winter flows predicted smolts per spawner, but correlations were weaker. The four marine factors identified by Logerwell et al. correlate with annual mean air temperature such that all five variables tend to be negative or positive for coho production in synchrony. In addition, spring transition and spring upwelling correlate with second winter and second spring flows so that good (poor) freshwater conditions are associated with poor (good) ocean conditions. Smolt numbers are relatively independent of spawners in the range of observed values. Environmental factors influence both freshwater and marine stages of the OCN coho salmon life cycle. These factors may interact to amplify variability in adult recruitment of naturally produced fish.

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