The life history of diadromous fishes is characterized by major habitat transitions. However the timing of these transitions can vary, sometimes for unknown reasons. Chinook salmon use estuaries as nursery habitat during their first spring and summer of life, which is also considered a critical period in their life history. During summer, some individuals use adjacent sandy beach surf zones before moving to deeper waters. The size and time at which juveniles move from the estuary to the surf zone can vary, potentially influenced by temperature, consumption rates and/or energetic content of prey. We hypothesized that when many juveniles are present in the estuary, competition for food becomes high, leading to some of the smaller juveniles leaving for adjacent surf zones. We tested this hypothesis by collecting juveniles in two Oregon coastal estuaries (Coos and Alsea Bay) and adjacent surf zones (Coos and Alsea surf) from 2008-2010. Using a multiple regression analysis, we determined if surf zone juveniles densities were correlated with estuarine density and consumption rates, which were obtained from a bioenergetics model. We determined diet and growth rates using stomach content and otolith analysis, and obtained temperature and prey energetic content in situ or from literature.
Results/Conclusions
Juvenile densities were greater in estuaries than in surf zones, and usually peaked in mid-July. We observed lower densities at Alsea south when compared to Coos south, despite higher numbers at Alsea Bay than at Coos Bay, potentially because of the larger size fish had at Alsea when compared to Coos Bay. At the beginning of summer, juveniles were usually smaller in surf zones than in estuaries, but were of similar size by the end of the season. In both habitats, juveniles mostly fed on amphipods, other types of crustaceans and insects. Consumption rates ranged from 0.4 to 1.0 g/individual/day, and were positively correlated with juvenile size and temperature, and negatively with prey energetic content. Surf zone densities were significantly correlated with an interaction between estuarine densities and consumption rates 1 to 6 days prior. Therefore our results provide evidence that the size and timing of juvenile migration from estuaries to surf zones is influenced by competition for food in estuaries. Because consumption rates are influenced by water temperature and the energetic content of prey, climate change, which is predicted to affect temperature and the amount of nutrients available through upwelling, may enhance our results.