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

Abstract

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.

Wild chinook salmon survive better than hatchery salmon in a period of poor production

“…The mechanisms linking temperature to the decreasing marine survivals remain to be identified, but the existing temperatures could be considered stressful for chinook salmon (Hinke et al 2005;Beauchamp 2009). The declines in the marine survival of chinook salmon are similar to the declines observed for coho salmon in the Strait of Georgia indicating that large-scale ecosystem changes probably are occurring possibly as a consequence of the increasing trend in temperature (Beamish et al 2008. Thus, we propose that the general warming of the surface waters is an indicator that juvenile Pacific salmon will continue to be under increasing stress as the temperatures approach critical values (Beauchamp 2009).…”

“…7 Most sets were at the surface, but sets were made with the head rope at 14 and 29 m in the Gulf Islands and deeper sets were included in the surveys in the Strait of Georgia. Abundance was determined using the procedures in Beamish et al (2000) and modified in Beamish et al (2008). The procedures for the stratum volumes are in Thomson and Foreman (1998).…”

“…If the catchability was less than 1.0, actual abundance would be underestimated. Catches of juvenile chinook salmon in the standard trawl surveys in the Strait of Georgia (Beamish et al 2008 were used to show that juvenile chinook salmon originating from the Cowichan River were rarely found in the Strait of Georgia in July and September.…”

“…2) and by the changing composition of the major species of Pacific salmon. In recent years, the marine survival of coho (O. kisutch) and chinook (O. tshawytscha) salmon dropped to the lowest levels in recorded history (Beamish et al 1995(Beamish et al , 2008Beamish and Neville 1999) while the abundances of pink (O. gorbuscha) and chum (Beamish and Neville 1999;Beamish et al 2007). Sockeye salmon (O. nerka) returns to the Fraser River drainage declined since about 1994 and in 2009 were the lowest ever recorded.…”

“…4). This decline in survival is likely related to ecosystem changes within the Strait of Georgia (Hinke et al 2005;Beamish et al 2008Beamish et al , 2010Beauchamp 2009). …”