Why age and size at maturity have changed in Pacific salmon

Abstract

Over the last few decades, the size at which Pacific salmon Oncorhynchus spp. attains maturity has decreased in many populations, whereas the age at maturity has increased. Both fisheries-induced evolution and environmentally-induced phenotypic plasticity could contribute to the changing age and size at maturity of Pacific salmon. We evaluated the potential for genetic changes in the maturation schedule of Japanese chum salmon using the probabilistic maturation reaction norm (PMRN) method. We found that the recent decrease in size at maturity, and increase in age at maturity, of Japanese chum salmon can be largely attributed to a phenotypic response to a reduced growth rate, but that fisheries-induced evolution should not be ruled out. Recent claims concerning fisheries-induced evolution of the maturation schedule are based on the decline in the age-specific body size at which the probability of maturing is 50%, a feature of PMRNs. However, the PMRN could change with changing environmental conditions. Therefore, a genetic change cannot be diagnosed only by the PMRN method.

Marine growth patterns of southern British Columbia chum salmon explained by interactions between density-dependent competition and changing climate

“…As reported for many other salmon populations (Bigler et al 1996;Morita et al 2005;Morita and Fukuwaka 2007), the average size-at-age of Big Qualicum chum salmon decreased the past four decades, and the proportion of older maturing individuals increased. The application of mixed-effects modelling allowed us to examine individual variation in growth rates that led us to conclude that reduced chum salmon size is likely a response to increased salmon abundance, which in recent years has been largely due to higher numbers of hatchery salmon being released (Eggers 2009;Ruggerone et al 2010a;Irvine and Ruggerone 2016).…”

“…To estimate biomass, Irvine and Ruggerone (2016) multiplied speciesspecific mass-per-individual estimates by numerical run sizes (catch plus escapement) for chum, sockeye, and pink salmon. Using biomass rather than numerical abundance estimates to evaluate competition effects gives more influence to heavier than light salmon species (e.g., chum versus pink salmon) and also incorporates changes in size-at-age found in many salmon populations (Bigler et al 1996;Morita and Fukuwaka 2007), which could decrease the demand for resources per individual. We used aggregate biomass estimates of mature North American chum, sockeye, and pink salmon in most of our analyses and included Asian chum salmon in one analysis.…”

“…However, by remaining at sea, salmon also increase their risk of mortality before reproducing. Thus, similar to other semelparous species (Sebens 1987), chum salmon populations exhibit a phenotypic plastic response by maturing at an age that potentially maximizes fitness (Roff 1992;Kozłowski 1992;Morita and Fukuwaka 2007). This led to our fourth hypothesis (H4); chum salmon that have slower early marine growth will mature at an older age.…”

“…To test the hypothesis (H4) that slow early growth delayed ageat-maturity (Morita and Fukuwaka 2007), we performed a linear regression between the annual scale growth between the first and third winters at sea (i.e., summed growth SW2 and SW3) and the proportion of chum salmon that matured at SW5 in southern BC. We used the annual scale growth between the first and third winters at sea to represent cumulative early growth because Morita and Fukuwaka (2006) found that growth during the second and third year at sea best predicted the probability of maturation for chum salmon.…”

“…populations (Pyper and Peterman 1999;Agler et al 2013;Jeffrey et al 2017), which are often accompanied with increased age-at-maturity (Bigler et al 1996;Morita et al 2005;Morita and Fukuwaka 2007). Relatively high abundances of salmon, especially during periods of low marine productivity, have been hypothesized to cause reductions in size-at-age and salmon survival (e.g., Irvine and Akenhead 2013;Yasumiishi et al 2016).…”