Evolution of age and length at maturation of Alaskan salmon under size‐selective harvest

Abstract

Spatial and temporal trends and variation in life-history traits, including age and length at maturation, can be influenced by environmental and anthropogenic processes, including size-selective exploitation. Spawning adults in many wild Alaskan sockeye salmon populations have become shorter at a given age over the past half-century, but their age composition has not changed. These fish have been exploited by a gillnet fishery since the late 1800s that has tended to remove the larger fish. Using a rare, long-term dataset, we estimated probabilistic maturation reaction norms (PMRNs) for males and females in nine populations in two basins and correlated these changes with fishery size selection and intensity to determine whether such selection contributed to microevolutionary changes in maturation length. PMRN midpoints decreased in six of nine populations for both sexes, consistent with the harvest. These results support the hypothesis that environmental changes in the ocean (likely from competition) combined with adaptive microevolution (decreased PMRNs) have produced the observed life-history patterns. PMRNs did not decrease in all populations, and we documented differences in magnitude and consistency of size selection and exploitation rates among populations. Incorporating evolutionary considerations and tracking further changes in life-history traits can support continued sustainable exploitation and productivity in these and other exploited natural resources.

Non-random sampling for reproductive status induces bias in probabilistic maturation reaction norm midpoints

“…Mature male parr of both species were captured in all 10 tributaries, but no mature female parr were collected in any of the tributaries, suggesting that only male parr of both species mature (Table S3 in Sahashi and Morita, 2013). In addition, mature and immature male parr of both species were captured in all 10 tributaries (Table S3 in Sahashi and Morita, 2013), suggesting that maturing and immature fish coexist in a given environment (if all samples from a population are mature and the immature fish size distribution is reconstructed using a back calculation method (e.g., spawning ground samples of sockeye and chum salmon, Morita et al, 2005;Kendall et al, 2014), bias in the PMRN midpoint will be induced by the number of immature fish determined by setting mortality rate during the reconstruction). In this study, the PMRN midpoint was defined as the fork length at which the probability of maturing at the first age of male parr maturity is 50% (L 50, male, first ) (first age of male parr maturity, char: 1+, salmon: 0+).…”

“…In particular, the midpoint of the PMRN, which is the age-specific size at which the probability of maturing is 50%, has been used as an index of the maturation reaction norm. Changes in the PMRN midpoint have been used to test for fisheries-induced evolution in many fish populations over the past 10 years (e.g., Atlantic cod: Olsen et al, 2004; American plaice: Barot et al, 2005; sockeye salmon: Kendall et al, 2014).…”

Trends and management implications of human‐influenced life‐history changes in marine ectotherms

“…Kuparinen et al 2009;Garcia et al 2012). For example, the fisheries-imposed selection differential on growth rate of two whitefish species in Swiss lakes was À7% to À9% per generation (Nussl e et al 2011), selection for sizeat-age of Alaska sockeye salmon was around À5 to À10 mm per year (Kendall et al 2014), and selection for earlier migration dates of Columbia River sockeye salmon was around À1 day/year between 1950 and 2010 (Crozier et al 2011). For many stocks, fishing mortality of individuals recruited to fisheries can be considerably higher than natural mortality, which means that fishing will act as a stronger selective force than natural processes for that life stage (Darimont et al 2009).…”

Illegal tusk harvest and the decline of tusk size in the African elephant

“…; Diaz Pauli and Heino ; Kendall et al. ), but few studies have tested this proposition in mammals (Mysterud et al. ; Prowse et al.…”