Identifying Movement Behavior States of Striped Bass in a Large Regulating Forebay Using Cluster Analysis

Predation by Striped Bass in Clifton Court Forebay, a water regulating reservoir in California’s Sacramento–San Joaquin Delta, is considered a major contributor to losses of juvenile salmonids. Understanding how individual Striped Bass use the forebay and surrounding channels is important for designing more effective predator management actions aimed at reducing loss. This study used acoustic telemetry and unsupervised clustering to test whether Striped Bass exhibit discrete movement behavior classes that differ in residency, habitat use, detectability, and movement activity. Then tested whether the occurrence of these classes vary with season, hydrologic conditions, fish age, and radial gate operations. Acoustic detections from 543 tagged Striped Bass were summarized into 2,568 six-month behavior states across eight water years. Four behavioral classes were identified: Commuters (high movement and frequent use of both forebay and outside habitats), Inside Residents–Undetected (persistent forebay residency with few detections), Inside Roamers (primarily forebay-associated with active internal movement), and Outside Residents–Undetected (persistent outside residency with few detections). Behavioral class composition differed significantly by season, water year type, age class, and radial gate openness. Movements were concentrated from February through May, with additional peaks in late summer and fall.