Maternal Control over Offspring Life History in a Partially Anadromous Species, Oncorhynchus mykiss

Abstract

AbstractEnvironmental variability among freshwater habitats may influence migratory decisions in partially anadromous salmonids through (1) proximate effects on individual fish development and condition and (2) longer‐term genetic adaptations of populations to the various environments. However, the relative importance of the two factors has been difficult to sort out. We used the gonadosomatic index as an indicator of maturation—and therefore residency—in age‐1 and age‐2 parr collected from eight partially anadromous Oncorhynchus mykiss (Rainbow Trout/steelhead) populations occupying a diversity of freshwater habitats. Substantial environmental variability among the eight Hood Canal (Washington) streams had little effect on life history pathways in male and female offspring of anadromous females (hereafter, ANAD parr), with the exception of significantly higher maturation rates in male ANAD parr from one population (Little Quilcene River). Between 0% and 8% of the female ANAD parr had initiated maturation, whereas 33% (Duckabush River) or 53% (Hamma Hamma River) maturation was observed among the female offspring of resident females (hereafter, RES parr) in the two populations with substantial resident components. The results indicate strong maternal control over offspring life history pathways in systems where resident and anadromous forms are sympatric. The RES parr that were collected above and below barriers to anadromy showed similar likelihoods of maturation. The expression of residency and anadromy in Hood Canal populations appears to reflect genetic adaptations to the diverse freshwater habitats (including the effects of O. mykiss sequestered above anadromy barriers) and, to some degree, the phenotypically plastic responses of male parr to the environmental variability among streams.