Phenotypic divergence between hatchery pink and coho salmon and their wild counterparts

Abstract

Interbreeding between hatchery and wild Pacific salmon (Oncorhynchus spp.) is an ongoing management concern, particularly when hatchery Pacific salmon possess divergent phenotypes. This project investigated the potential domestication of hatchery Pacific salmon from the relaxation of natural and sexual selection; such divergence might reduce wild Pacific salmon fitness if strays interbreed with natural-born fish. In Southeast Alaska, U.S.A., we compared two years (even- and odd-year lines) of hatchery pink salmon (Oncorhynchus gorbuscha) and one year of hatchery coho salmon (O. kisutch) to their wild source populations to characterize divergence in phenotypes associated with mating and reproductive success. Specifically, we examined secondary sexual traits in males and fecundity and egg quality traits in females. Due to the relaxation of natural and sexual selection in the hatchery during the spawning period, we expected to observe hatchery male Pacific salmon with shorter snouts and smaller humps and hatchery female Pacific salmon with a larger gonadosomatic index (GSI), smaller eggs, and lower egg energy and lipid density when compared to their wild counterparts. Hatchery-wild differences in GSI and egg size in coho salmon females and snout length and body depth (a proxy measurement for hump size) in odd-year pink salmon males supported our hypotheses. However, hatchery coho salmon males had larger snouts and body depths than the wild coho salmon males and odd-year pink hatchery eggs were larger than the eggs of their wild counterparts, contrary to our hypotheses. No differences were observed in coho salmon egg composition, even- and odd-year female pink salmon GSI, even-year pink salmon egg size, and even-year pink salmon male snout and body depth. While the results were mixed, we found evidence that hatchery rearing alters reproductive traits. Our results underscore the need to better understand the possible biological consequences of altered reproductive traits on wild populations, including comprehensive marking of hatchery releases to identify strays, monitoring of phenotypes in hatchery populations, and consideration of hatchery practices that will reduce domestication.