Effects of 70 Years of Freshwater Residency on Survival, Growth, Early Maturation, and Smolting in a Stock of Anadromous Rainbow Trout from Southeast Alaska
Frank P. Thrower and John E. Joyce
Abstract.—Progeny of wild, freshwater sequestered (resident) rainbow trout Oncorhynchus mykiss, descendants of a stocking of steelhead (anadromous rainbow trout) in 1926, and progeny of the wild, ancestral steelhead lineage and their reciprocal crosses were compared for two brood years in a hatchery environment to determine the effects of 70 years of freshwater residency on growth, survival, early maturity, and smolting proportion. Resulting smolts were tagged, released, and recovered as maturing adults to evaluate marine survival. For the 1996 brood, 75 families were maintained in separate freshwater raceways for 10 months. Approximately 100 fish from each family were tagged with passive integrated transponder tags, pooled by type, and cultured until age 2. An additional group was tagged with coded-wire tags and reared in the same manner. For the 1997 brood, 80 families were coded-wire-tagged, separated by breeding type, and cultured at different densities. Size-at-age and survival were reduced significantly in progeny of resident females when compared with progeny from anadromous females during the first 2 months after first feeding. No significant differences were observed in subsequent growth or survival through age 2. A higher proportion of smolting at age 2 and a lower proportion of early male maturity was observed in families from anadromous parents. Smolts produced by anadromous parents had four to five times higher marine survival than those from resident parents. While smolting proportions and smolt survival were lower for the progeny of freshwater resident fish, the results indicate that significant numbers of smolts and adults can still be produced by populations landlocked for up to 70 years and 20 generations. The results have substantial implications for the use of natural freshwater environments for the preservation of endangered anadromous stocks of rainbow trout, the rehabilitation of anadromous stocks, and the actual effective breeding size of anadromous rainbow trout populations.