Abstract
The average sizes of Pacific salmon have declined in some areas in the Northeast Pacific over the past few decades, but the extent and geographic distribution of these declines in Alaska is uncertain. Here, we used regression analyses to quantify decadal trends in length and age at maturity in ten datasets from commercial harvests, weirs, and spawner abundance surveys of Chinook salmon Oncorhynchus tshawytscha throughout Alaska. We found that on average these fish have become smaller over the past 30 years (~6 generations), because of a decline in the predominant age at maturity and because of a decrease in age-specific length. The proportion of older and larger 4-ocean age fish in the population declined significantly (P < 0.05) in all stocks examined by return year or brood year. Our analyses also indicated that the age-specific lengths of 4-ocean fish (9 of 10 stocks) and of 3-ocean fish (5 of 10 stocks) have declined significantly (P < 0.05). Size-selective harvest may be driving earlier maturation and declines in size, but the evidence is not conclusive, and additional factors, such as ocean conditions or competitive interactions with other species of salmon, may also be responsible. Regardless of the cause, these wide-spread phenotypic shifts influence fecundity and population abundance, and ultimately may put populations and associated fisheries at risk of decline.
Highlights
Size and age at maturity are important life-history traits for Pacific salmon (Oncorhynchus spp.), reflecting an assortment of evolutionary and ecological influences [1]
The average sizes of Pacific salmon have declined in some areas in the Northeast Pacific but the geographic distribution and species-specific extent of these declines in Alaska is unknown
Average length declined gradually since the early 1980s. We examined these overall trends in more detail by partitioning the length-at-age data by age class (2, 3- and 4-ocean fish)
Summary
Size and age at maturity are important life-history traits for Pacific salmon (Oncorhynchus spp.), reflecting an assortment of evolutionary and ecological influences [1]. The average sizes of Pacific salmon have declined in some areas in the Northeast Pacific but the geographic distribution and species-specific extent of these declines in Alaska is unknown. Several studies have shown that individuals in many areas are returning to spawn at an earlier age and at smaller sizes over the last few decades [2,3,4,5]. A reduction in size at maturity is important because it may influence individual reproductive potential and population fitness. For example, have higher fecundities allowing them to spawn with more than one male and to PLOS ONE | DOI:10.1371/journal.pone.0130184. For example, have higher fecundities allowing them to spawn with more than one male and to PLOS ONE | DOI:10.1371/journal.pone.0130184 June 19, 2015
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