Because selective harvest targets potential breeders with particular characteristics, it can change a population in ways that affect its viability. Chinook salmon exhibit a wide range of sizes and ages at adulthood, and these fish are exposed to size-selective harvest during much of their lives. Fishing for chinook—as well as other salmon species—often eliminates the largest, oldest, and fastest growing individuals. In addition to its immediate demographic effects, then, fishing may have longer-term consequences that reduce productivity and adaptive potential. Salmon caught in several fisheries have shown declining trends in body size. However, the effects of size-selective fishing are not limited to changes in body size; fishing may yield a cascade of responses in other traits. How important is fishing likely to be as a factor affecting trends in size, and what are the consequences of change in size for age, growth, and reproductive characteristics? To address these questions, I apply genetic data from chinook salmon to multivariate selection models that predict evolutionary responses to size selective harvest. I estimate the genetic and phenotypic components of variation for several life-history traits, and use these data and models to explore life-history responses to simulated harvest regimes. The results of these analyses indicate that (1) the effects of fishing on size depend on the correlations among life-history characters, (2) fishing can produce detectable changes in correlated life-history traits, and (3) these changes are likely to differ between cultured and wild fish and to vary with ocean productivity. These findings point to opportunities for size and age to evolve in age-structured species under size-selective fishing, potentially eroding viability of exploited salmon populations.
Jeff Hard is with the Conservation Biology Division’s Population Biology Program at NOAA Fisheries’ Northwest Fisheries Science Center, where he has been since 1992. After receiving a BS in Marine Biology from Oregon State University, he completed an MS in Fisheries Science from the University of Alaska, where he studied the ecology of chinook salmon in barriered lakes of southeastern Alaska with Bill Smoker and Tony Gharrett. He pursued long-standing interests in evolution and insects (vastly more practical as subjects in multi-generation selection experiments, in any case) as a PhD student in Ecology & Evolutionary Biology at the University of Oregon, where he worked with Bill Bradshaw and Mike Lynch on life-history evolution in container-breeding mosquitoes and fruit flies. He grew up in Alaska and worked for a decade at a remote research station on Baranof Island in the Alexander Archipelago, conducting field studies on pink, chinook, chum, coho, and sockeye salmon. His current research focuses on the population biology and ecological genetics of fishes, with an emphasis on characterizing the quantitative genetic architecture of phenotypic variation and its responses to natural and human-induced selection.