UC Santa Cruz

Association with physical structure or conspecifics can impact the survival and growth of individuals. The necessity and strength of these associations can change with environmental conditions and ontogeny, acting in concert or in opposition to influence an individual’s behavior and ultimate success. To test the hypotheses that affinity for physical structure and aggregating behavior changes along a dynamic river gradient and with development of a migratory fish, I conducted a field experiment on the San Joaquin River, California with juvenile Chinook salmon (Onchorynchus tschawyscha). I created orthogonal combinations of turbidity, fish size, and structural cover by manipulating the presence of artificial structure within net pens at three locations along the river continuum. I recorded the affinity of juvenile Chinook salmon to physical structure and propensity to aggregate with conspecifics with video cameras. I used generalized linear models and model selection to evaluate the effects of turbidity, size (fork length), density, and structure level on proximity of individuals to structure and conspecifics. First, I found there was no difference in the total number of salmon in structure or no-structure levels. Second, I found the number of aggregating observations increases with increasing density and fork length. When structure was present, aggregating behavior was low across turbidities. In contrast, when structure was absent, aggregating behavior increased in low turbidities. Ultimately, I found both ontogeny and a changing environmental gradient influenced the number of aggregating observations. These results indicate how environmental conditions (turbidity and availability of physical structure), size, and density interact to determine fish behavior and increase understanding of the complicated interactions surrounding fish habitat use.