The timing of anadromous bull trout migrations in estuarine and marine waters of Puget Sound, Washington

The timing of anadromous migrations varies greatly among species, reflecting foraging opportunities, predation risk, and physical factors in freshwater and marine habitats. We studied the timing of bull trout, Salvelinus. confluentus, migrations downstream into Puget Sound, Washington, and the return migration using data from a combination of traps and beach seining to capture juveniles and adults, and acoustic telemetry to quantify movements in river, estuary, and marine waters. The period of marine residence overlapped among fish from different river basins and was generally brief; most bull trout entered marine waters from April to June and migrated back into rivers by July and August as temperatures were increasing, despite the increasing availability of prey fishes. The timing of return migration upriver occurred earlier in rivers with warmer temperatures than in cooler rivers, such that the fish entered at similar temperatures but different dates. However, the trapping and acoustic tag detections indicated that some exceptions occur, as individuals were in marine waters at times of the year when most of their populations were upriver. These timing patterns and other complex migration pathways and life history patterns indicated by the combination of sampling techniques, revealed the diversity in migratory behavior in this species, and the connections to the region’s abiotic and biotic conditions.