Steelhead Growth in a Small Central California Watershed: Upstream and Estuarine Rearing Patterns

Abstract

AbstractWe monitored growth and life history pathways of juvenile steelhead Oncorhynchus mykiss and compared growth rates between the upper watershed and estuary in Scott Creek, a typical California coastal stream. Growth in the upper watershed was approximately linear from May to December for age‐0 fish. For passive integrated transponder (PIT) tagged, age‐1+ fish, growth transitioned to a cyclic pattern, peaking at 0.2% per day during February‐April, when maximum flows and temperatures of 7‐12°C occurred. Growth of PIT‐tagged fish then slowed during August‐September (0.01% per day), when temperatures were 14‐18°C and flows were low. During each spring, smolts (mean fork length [FL] ± SE = 98.0 ± 1.2 mm) and fry migrated to the estuary; some fish remained there during summer‐fall as low flows and waves resulted in seasonal sandbar formation, which created a warm lagoon and restricted access to the ocean. Growth in the estuary‐lagoon was much higher (0.2‐0.8% per day at 15‐24°C). Our data suggest the existence of three juvenile life history pathways: Upper‐watershed rearing, estuary‐lagoon rearing, and combined upper‐watershed and estuary‐lagoon rearing. We present a model based upon the above data that reports size at age for each juvenile life history type. The majority of fish reaching typical steelhead ocean entry sizes (∼150‐250 mm FL; age 0.8‐3.0) were estuary‐lagoon reared, which indicates a disproportionate contribution of this habitat type to survival of Scott Creek steelhead. In contrast, steelhead from higher latitudes rear in tributaries during summer, taking several years to attain ocean entry size.