Pink salmon (Oncorhynchus gorbuscha) marine survival rates reflect early marine carbon source dependency

Marine survival rate (the number of adult salmon returning divided by the number of salmon fry released) of pink salmon runs propagated by Prince William Sound, Alaska (PWS) salmon hatcheries is highly variable resulting in large year-to-year run size variation, which ranged from ∼20 to ∼50 million during 1998-2004. Marine survival rate was hypothesized to be determined during their early marine life stage, a time period corresponding to the first growing season after entering the marine environment while they are still in coastal waters. Based on the predictable relationships of 13C/12C ratios in food webs and the existence of regional 13C/12C gradients in organic carbon, 13C/12C ratios of early marine pink salmon were measured to test whether marine survival rate was related to food web processes. Year-to-year variation in marine survival rate was inversely correlated to 13C/12C ratios of early marine pink salmon, but with differences among hatcheries. The weakest relationship was for pink salmon from the hatchery without historic co-variation of marine survival rate with other PWS hatcheries or wild stocks. Year-to-year variation in 13C/12C ratio of early marine stage pink salmon in combination with regional spatial gradients of 13C/12C ratio measured in zooplankton suggested that marine survival was driven by carbon subsidies of oceanic origin (i.e., oceanic zooplankton). The 2001 pink salmon cohort had 13C/12C ratios that were very similar to those found for PWS carbon, i.e., when oceanic subsidies were inferred to be nil, and had the lowest marine survival rate (2.6%). Conversely, the 2002 cohort had the highest marine survival (9.7%) and the lowest mean 13C/12C ratio. These isotope patterns are consistent with hypotheses that oceanic zooplankton subsidies benefit salmon as food subsidies, or as alternate prey for salmon predators. Oceanic subsidies are manifestations of significant exchange of material between PWS and the Gulf of Alaska. Given that previously observed inter-decadal cycles of oceanic zooplankton abundance were climatically driven, exchange between PWS and the Gulf of Alaska may be an important process for effecting synoptic changes in marine populations of higher trophic levels, and thus an important consideration for climate-change models and scenarios.