Changes in the Population Ecology of Hatchery and Wild Coho Salmon in the Strait of Georgia

An analysis of the results of a 10‐year study of the population ecology of juvenile hatchery and wild coho salmon Oncorhynchus kisutch in the Strait of Georgia produced new information about the interannual and interseasonal fluctuations in abundance and marine survival. A decline in the percentage of hatchery coho salmon was related to declines in hatchery fish abundance and marine survival; wild coho salmon abundance was more stable. The declining marine survival of hatchery coho salmon appeared to be related to a fixed average date of release from hatcheries and a possibility of earlier prey production. The relatively stable abundance of wild coho salmon may relate to a natural trend toward earlier ocean entry dates. Oscillations in hatchery coho salmon percentage and abundance were related to oscillations in abundance of juvenile pink salmon O. gorbuscha. The impact of oscillating density affected marine survival of hatchery coho salmon more than that of wild coho salmon. Marine survival and abundance of hatchery and wild coho salmon in July were positively related to average fork length, indicating that growth within the first few months after ocean entry affected marine survival. However, absolute size was not important, as wild coho salmon were consistently smaller than hatchery coho salmon. Wild coho salmon responded to conditions in the marine ecosystem differently than hatchery coho salmon, as relationships among growth, survival, and abundance were apparent for wild coho salmon earlier in the year than for hatchery fish. The length increase between July and September was inversely related to marine survival, suggesting that fish that were larger in July grew less and survived better because they were storing more lipids than smaller coho salmon. The study indicated that a more experimental management strategy is needed for both hatchery and wild coho salmon.