Multi-year effects of high densities of sockeye salmon spawners on juvenile salmon growth and survival: a case study from the Exxon Valdez oil spill

Abstract

Reduced fishing after the 1989 Exxon Valdez oil spill in Prince William Sound, Alaska, contributed to exceptionally large numbers of sockeye salmon spawners up to 860 km from the spill site. We measured annual scale growth of adult sockeye salmon from four affected populations, 1970–1997, in order to test the hypothesis that large spawner densities can have multi-year effects on juvenile size and subsequent adult abundances as a result of intraspecific competition among juveniles in the nursery lake. Sockeye salmon scale growth in fresh water was significantly reduced by the large 1989 spawner densities in the Kenai River system, Red Lake, Akalura Lake, but not Chignik Lake. Scale growth in the three affected systems recovered to previous levels 2–4 years after the oil spill, but subsequent moderately high spawner densities led to exceptionally low growth. Juvenile salmon growth was negatively related to parent spawners, spawners from the next brood year (second season in lake), and spawners from the previous brood year. Multi-variate time series analyses indicated adult sockeye salmon abundance increased with greater numbers of parent spawners, but decreased as a result of either large numbers of spawners in the previous year or small juvenile salmon size. These results indicate sockeye salmon spawners can affect juvenile growth and adult production of adjacent year classes. Implications for stock-recruitment modeling and spawner density management are discussed.