COS 1-5
Life history portfolios within salmon populations: When do different outmigration phenotypes contribute?
The maintenance of life history diversity within and among populations is thought to be critical for the persistence of salmon stocks. Preserving and restoring diversity in life history traits is thus central to many recovery efforts, but it is necessary to first understand the way in which environmental factors affect their expression and success. Rotary-screw trap (RST) sampling in the California Central Valley has indicated that juvenile salmon outmigrate at different sizes, ages and times of the year. Here, we used otolith (“earstone”) strontium isotopes in adult Chinook salmon returning to the Stanislaus River, a regulated stream at the southern end of the species range, to determine the influence of river conditions on juvenile migration behavior, habitat use and survivorship. Paired otolith and scale samples were used to reconstruct size-resolved outmigration patterns in “successful” adult spawners. For each returning adult, the size that it had emigrated from its natal tributary was reconstructed by coupling otolith strontium isotope and radius measurements. Juveniles were classified by size, and the proportions of fry (<55mm), parr (>55-75mm) and smolts (>75mm) sampled by RSTs were compared with those reconstructed in the adults from the same cohort, and used to estimate survival of the different phenotypes.
Results/Conclusions
Differences in precipitation patterns and local water operations resulted in contrasting flow regimes during the seven outmigration years examined (1999-2000, 2003-04 and 2008-10). RST sampling indicated that in wetter years, the majority of juveniles emigrated as fry, while in drier years a greater proportion emigrated as parr and smolts. Metrics of juvenile outmigration behavior (abundance, size distributions and phenology) varied largely as a function of hydroclimatic regime, while relative survival rates appeared to be driven by local environmental conditions as well as size- and time-selective mortality. While fry survival is generally assumed to be negligible in this system, our data indicate that they can represent more than 20% of the adult spawning population, particularly in wet years. This result might partly explain the long-recognized pattern of larger adult returns following wetter outmigration years. Patterns in juvenile outmigration behavior and survival are discussed in the context of salmon life history diversity, fisheries management and the portfolio effect.