Abstract
With the current trends in climate and fisheries, well-designed mitigative strategies for conserving fish stocks may become increasingly necessary. The poor post-release survival of hatchery-reared Pacific salmon indicates that salmon enhancement programs require assessment. The objective of this study was to determine the relative roles that genotype and rearing environment play in the phenotypic expression of young salmon, including their survival, growth, physiology, swimming endurance, predator avoidance and migratory behaviour. Wild- and hatchery-born coho salmon adults (Oncorhynchus kisutch) returning to the Chehalis River in British Columbia, Canada, were crossed to create pure hatchery, pure wild, and hybrid offspring. A proportion of the progeny from each cross was reared in a traditional hatchery environment, whereas the remaining fry were reared naturally in a contained side channel. The resulting phenotypic differences between replicates, between rearing environments, and between cross types were compared. While there were few phenotypic differences noted between genetic groups reared in the same habitat, rearing environment played a significant role in smolt size, survival, swimming endurance, predator avoidance and migratory behaviour. The lack of any observed genetic differences between wild- and hatchery-born salmon may be due to the long-term mixing of these genotypes from hatchery introgression into wild populations, or conversely, due to strong selection in nature—capable of maintaining highly fit genotypes whether or not fish have experienced part of their life history under cultured conditions.
Highlights
Climate change, over-fishing and habitat alteration are suspected to be contributing to declines in Pacific salmon stocks to the point that some populations are seriously threatened [1]
Concern that hatchery-reared coho salmon were supplanting wild coho [7], [8] hit a peak during 2001, when 70% of the coho caught in the Strait of Georgia (SOG, Fig. 1) were reported to be of hatchery origin [9]
In the continuing debate of nature versus nurture, this study has found that the effects of rearing environment on phenotype and behaviour far outweighed the effects of any genetic differences existing between second-generation hatchery- and wild-born coho salmon in this system
Summary
Over-fishing and habitat alteration are suspected to be contributing to declines in Pacific salmon stocks to the point that some populations are seriously threatened [1]. Over the past 50 years, governments around the North Pacific, including Japan, Russia, Canada and the United States, have implemented salmon enhancement programs to increase the numbers of juvenile salmon released to oceanic conditions. These programs typically utilise the artificial propagation of returning mature adult salmon, rearing juveniles in freshwater culture conditions and releasing them as smolts. Despite the annual release of billions of hatchery-reared fish into the Pacific Ocean, the marine survival rates of many salmon populations continue to decline [2,3,4], and the effects of these introductions on wild populations are only beginning to be understood [5], [6]. The causes and effects of these differences are unclear; i.e. what are the relative roles of genotype and rearing environment on the observed phenotypic differences between wild and hatchery-reared salmonids?
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