Date Published: May 15, 2019
Publisher: Public Library of Science
Author(s): Donald A. Larsen, Deborah L. Harstad, Abby E. Fuhrman, Curtis M. Knudsen, Steven L. Schroder, William J. Bosch, Peter F. Galbreath, David E. Fast, Brian R. Beckman, Josep V. Planas.
In many salmonid species, age and size at maturation is plastic and influenced by the interaction between genetic and environmental factors. Hatchery reared salmon often mature at an earlier age and smaller size than wild fish. Modern salmon conservation efforts have focused on managing the level of gene flow between hatchery and natural origin fish to minimize potential genotypic and phenotypic change. In salmonids, maturation probability is dependent on exceeding a genetically set threshold in growth rate and energetic status (and by association, body size) referred to as the probabalisitic maturation reaction norm (PMRN). Over fourteen years, we monitored the frequency of age-2 precocious male maturation (common term: age-2 minijack rate) and the PMRN of natural founder (FNDR), integrated natural-hatchery (INT), and segregated hatchery (SEG) broodlines of spring Chinook salmon, Oncorhynchus tshawytscha. The average age-2 minijack rate (± SEM) of the FNDR, INT and SEG broodlines was 48.2 ± 5.2%, 41.9 ± 3.6% and 30.9 ± 4.7%, respectively. Additionally, the PMRN WP50 (predicted weight at 50% maturation) of the SEG broodline was significantly greater (20.5 g) than that of the FNDR/INT broodlines (18.2 g). We also conducted a common garden experiment exploring the effects of less than one [INT (0–1)], one [SEG (1)] or two [SEG (2)] generations of hatchery culture on the age-2 minijack rate and PMRN WP50. Growth was not significantly different among broodlines, but age-2 minijack rates were significantly lower following two consecutive generations of hatchery culture: [INT (0–1): 68.3 ± 1.7%], [SEG (1): 70.3 ± 1.8%] and [SEG (2): 58.6 ± 0.4%] and the PMRN WP50 was significantly higher by 6.1 g after two generations of SEG culture. These results indicate that managed gene flow reduces phenotypic divergence, but may serve to maintain potentially undesirably high age-2 minijack rates in salmon conservation hatchery programs.
Declines in salmon abundance throughout the United States, coupled with the regulatory requirements of the Endangered Species Act (Federal Register 70:37160), have resulted in efforts to enhance and restore threatened and endangered salmon populations. Salmon hatcheries have been in common use for the past century as a familiar response to the threat of declining stocks . While these programs may produce significant quantities of returning adults, various negative effects associated with hatchery culture of salmonids have been documented . One major concern is evidence demonstrating reduced reproductive fitness of hatchery fish when spawning in the natural environment [3, 4, 5, 6] with the fitness loss occurring in as little as one generation in culture [7, 8]. Domestication selection, genetic drift, and inbreeding depression are possible mechanisms responsible for this fitness loss [9, 10, 11, 12].
A series of studies conducted over the past two decades have examined the causes [27, 43, 50, 51], magnitude [22, 28, 29] and implications [6, 22, 28, 43] of high rates of precocious male maturation in hatchery spring Chinook salmon. Over a seven year period, an average of 41% of all FNDR/INT broodline male fish released from the CESRF were estimated to be initiating maturation as age-2 minijacks and this rate was approximately 10-fold higher than that estimated for naturally rearing fish in the Yakima River . A fundamental conclusion drawn from these earlier studies was that hatchery culture conditions can have significant effects on size and age at maturation. In the current investigation we endeavored to determine how integrated and segregated hatchery management strategies affect precocious male maturation in spring Chinook salmon. We provided two lines of evidence to support our initial hypothesis that the age-2 minijack rates and the PMRN of spring Chinook salmon may change as a result of segregated hatchery culture. Furthermore, we have attempted to determine what proportion of the variance in age-2 minijack rate and the PMRN is attributable to genetic factors (broodline, brood year and age-3 jack contribution), rearing environment (reflected in size and growth rate), as well as other potential unknown factors. Navigating this analysis with so many potential sources of variation presented some clear challenges. But, the fact that significant differences were detected, despite such variation, provides strong evidence to support our conclusions.