Authors : John M. Erhardt and Dennis L. Scarnecchia
Five growth models were evaluated for their effectiveness in characterizing growth of an isolated population of bull trout, Salvelinus confluentus, undergoing a long term cessation of harvest (1994–2005) in the North Fork Clearwater River, Idaho, above Dworshak Dam. To explore the dynamics of growth in migratory bull trout, one of the models, the von Bertalanffy model, was also fitted based on age estimates derived from annuli and growth increments on pelvic fin ray sections for three juvenile outmigration age groups. Among five growth models evaluated, four models fitted to the age-length data described the fish growth for ages 3 to 11 comparably well. Only a model that exhibited positive growth acceleration as age progressed fitted the data less effectively. Growth was significantly (P < 0.001) related to the age of outmigration from natal streams (1–3 years). Migratory bull trout were found to reach maturation at ages 4–6; the common age-4 maturation was younger than most comparable studies. All mature fish studied reared for 2–3 years in downriver or reservoir habitat. As the adult population size has continued to increase since harvest closure in 1994, further research should be directed towards understanding the trade-offs between these increasing numbers of adult fish and growth, survival, migration, and maturation schedules.