Identifying movement behavior states of striped bass (Morone saxatilis) in a large regulating forebay using cluster analysis

Predation by Striped Bass ( Morone saxatilis ) 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. Consistent with recent fish behavior literature, this study used acoustic telemetry and clustering analysis to determine whether Striped Bass exhibit discrete movement behavior classes that differ in residency, habitat use, detectability, and movement activity. We then tested whether the occurrence of these classes varies with season, hydrologic conditions, fish age, and radial gate operations. Acoustic detections from 543 tagged Striped Bass were summarized into 10,608 month long behavior states across six years. Five behavioral classes were identified: Inside Residents–Undetected (assumed persistent forebay residency with few detections), Inside Sedentary (high detection within the forebay at a limited number of receivers), Inside Roamers (primarily forebay-associated with active internal movement), Commuters (high movement and frequent use of both forebay and outside habitats), and Outside Residents–Undetected (persistent outside residency with few detections). Behavioral class composition differed significantly by month, water year type, age class, radial gate openness, and a relative salmonid catch index. Movements were concentrated from February through May, with additional peaks in late summer and fall. The identified classes represent distinct combinations of space use and activity that determine when fish are present in the forebay, how often they cross key control points, and how observable they are to fixed receiver arrays. These differences are directly relevant to predator management because capture probability and exposure to capture gears may vary among behavioral classes. Telemetry-derived behavior classification provides a practical basis for targeting predator removal timing and locations, potentially improving the efficiency of actions intended to reduce predation pressure on native fishes in the Delta.