Determining Whether Biotic and/or Abiotic Factors Affect Oversummer Growth by Quantifying Juvenile Steelhead Trout Growth in a Mediterranean-climate Stream

Streams in Mediterranean-climate regions experience peak flow in the winters and low flow in the summers. Summer low flow conditions are associated with habitat contraction due to lack of precipitation and dropping water levels.  An extreme form of ecosystem contraction occurs in intermittent streams, in which shallow riffle habitat is eliminated resulting in the formation of isolated pools. The John West Fork (JWF) is a first-order, intermittent stream in the Point Reyes National Seashore, Marin County, California. Fish growth in intermittent streams can be influenced by a number of factors such as reduced invertebrate drift, low dissolved oxygen concentrations, high temperatures, and increased fish densities. Our study aims to (1) quantify growth of individual juvenile steelhead trout (Oncorhynchus mykiss) during the summer low flow period and to (2) determine whether biotic (e.g., fish density) and/or abiotic factors (e.g., temperature) explain oversummer growth.  At the outset of each summer (2010 and 2011), we captured, weighed, and measured fish. We tagged individuals that were large enough to tag (>60mm) with distinctly coded PIT tags. At the end of the summer,  we held a second capture event, where we weighed and measured all fish. Taking the difference in weight of recaptured fish for the two capture events allowed us to estimate oversummer growth. Preliminary results suggest that growth appears to correlate with fish density. For both years, we observe a negative correlation between the relative growth rate and fish density (2010:P < 0.001, R^2 = 0.2464; 2011:P < 0.001, R^2 = 0.3175).