Effects of a single event of close inbreeding on growth and survival in steelhead

Abstract

For two populations of Alaskan steelhead (Oncorhynchus mykiss) of common ancestry we evaluated effects of inbreeding in second-generation descendants of wild fish by comparing progeny of full-sibling matings to those of non-inbred controls to determine if a single event of close inbreeding has significant effects on survival and growth in captivity or the wild. In captivity, both survival and size were highly variable between inbred and control types within each line and among the five broods during five periods of freshwater culture. However, no consistent patterns of inbreeding enhancement or depression between types within lines across years were evident. In contrast, in the wild marine environment, 34 of 34 pairwise comparisons between inbred and control types in body size of returning adults after 2 or 3 years at liberty in the ocean were consistent with inbreeding depression with significant inbreeding depression varying from 2.9% for female length to 20.0% for female weight. Survival of marked juveniles (smolts) to adults in the wild marine environment was consistently and significantly lower in inbred types for both lines, for an average inbreeding depression of 78.8%. The results underscore the potential problems that can arise from using protective culture technologies, including captive broodstocks, to supplement endangered populations, and they highlight the genetic hazards that can be faced by small wild populations. This study demonstrates that high natural mortality or selection increases the amount of inbreeding depression detected in survival. Inbreeding effects on survival and growth in captivity can be poor indicators of survival and growth in a wild marine environment.