Growth of Juvenile Chinook Salmon: A Model Comparison Between Estuaries and Adjacent Sandy Beach Surf Zones

During summer, juvenile Chinook salmon are present in estuaries, adjacent sandy beach surf-zones and the coastal ocean. Although early marine residence is considered an important period, there is little information on the role of surf zones as a habitat for juvenile Chinook salmon. As part of a larger project using distribution, diet, and growth of juvenile Chinook salmon to evaluate the role of surf-zones, we modeled and compared potential growth rates of juveniles in two Oregon estuaries and adjacent surf-zones. Five years of data collection indicate that juveniles are present in surf zones during July and August; therefore potential growth was modeled at a daily time step from July 1 to August 31, 2008 – 2010. Our null hypothesis was that surf zones do not provide a habitat in which juvenile Chinook salmon grow. We used locally obtained data on water temperature, diet and prey energetic content to predict growth rates in each of these habitats across years, assuming maximum consumption rate (approximately 2.3 g/d). On average, potential growth rates were 15% higher in surf zones than in estuaries (t-test, p<0.04). Across years, growth rates were statistically similar (<5% difference, p>0.05). Mean growth rates between surf zones and between estuaries were similar (<5% difference, p>0.05). Higher growth rates in surf zones were associated with lower temperatures and higher prey energetic content (p<0.02). Water temperatures ranged from 10.6 to 16.3˚C (mean = 12.5˚C) in estuaries and from 7 to 12.5˚C (mean = 10.1˚C) in surf zones. Mean energy densities of prey in surf zone (4370 Joules/g) were greater than for estuarine prey (3656 Joules/g) due to a higher proportion of energy-rich species, including amphipods and isopods, in surf zones. In both habitats, potential growth rates declined over time although the rate of decline varied annually within each habitat. Results indicate that for juvenile Chinook salmon, sandy beach surf zones can provide a habitat with comparable to greater growth than estuaries and therefore could be used more in years when estuarine growth is sub-optimal. In the future, we will combine catch, diet, and environmental data with empirical and model estimates of growth to determine if conditions in estuaries influence the timing of juvenile Chinook salmon migration in coastal habitats. We will then use the model to examine the potential impact of water temperature increases that are predicted to occur in the next 50 years on growth rates of juveniles in nearshore environments.