Longitudinal gradients in threshold sizes for alternative male life history tactics in a population of Atlantic salmon (Salmo salar)

Abstract

Atlantic salmon (Salmo salar) males may mature early in life in freshwater, rather than maturing after a migration to sea, if their size is above a threshold value. We analyzed the spatiotemporal variation in size and incidence of the early maturity tactic among males over an 8-year period in six subpopulations on two branches of a river and collected environmental data on each site and across the river scape. A positive longitudinal trend in the frequency of early maturing males that was stable over the 8-year period occurred from the mouth to the head of the river. Threshold sizes for early maturation varied among subpopulations; size thresholds for male parr to mature were higher in downstream habitats and lowest upstream. This pattern was consistent in both river branches over the 8-year period and was not related to either the density of parr or site-specific abiotic habitat characteristics. However, the cumulative incidence of habitat features that could impede migration of large individuals increased with increasing upstream distance. Migration costs may contribute to the observed variation in threshold sizes.

Genomewide Identification of Genes Under Directional Selection: Gene TranscriptionQST Scan in Diverging Atlantic Salmon Subpopulations

“…In 1997, highly significant genetic differentiation at microsatellite loci (F ST ¼ 0.028 and 0.036 (two upstream sites), P , 0.0006) was revealed between salmon subpopulations from downstream and upstream of the fish ladder (Garant et al 2000), indicating partial reproductive isolation between them. Moreover, Aubin-Horth et al (2006) also reported a significantly higher proportion of precocious male parr (early maturing males) and a smaller size threshold for male parr to mature upstream rather than downstream from the fish ladder. Environmental conditions also differ between river sections.…”

Why age and size at maturity have changed in Pacific salmon

“…Open circles are observed data as in Fig. 1a Baum et al 2004, Aubin-Horth et al 2006. Baum et al (2004) and Aubin-Horth et al (2006) showed that the threshold size at maturity decreased with altitude and distance from the mouth of the river, respectively.…”

“…1a Baum et al 2004, Aubin-Horth et al 2006. Baum et al (2004) and Aubin-Horth et al (2006) showed that the threshold size at maturity decreased with altitude and distance from the mouth of the river, respectively. In addition to genotype, temperature, food availability, condition factors and growth history affect the maturation probability independent of body size at a given age (Bromage et al 1992, Silverstein et al 1998 (2000) compared the maturation schedule of white-spotted charr Salvelinus leucomaenis in below-dam river sections accessible to the migrant form with above-dam river sections inaccessible to the migrant form.…”

Old wine in new bottles: reaction norms in salmonid fishes

“…Although family-level differences in the incidence of parr maturity had been documented previously (for example, Saunders et al, 1982), it was approximately a decade after the threshold reaction-norm papers that field evidence of population differences in parr-maturation thresholds were documented in salmonids: Atlantic salmon in Scotland (Baum et al, 2004) and Québec (Aubin-Horth and Dodson, 2004;Aubin-Horth et al, 2006); and masu salmon, O. masou, in Japan (Morita and Nagasawa, 2010). The first experimental evidence of population genetic differentiation in threshold reaction norms in salmonids was provided by Piché et al (2008), based on common-garden experiments undertaken on crosses in Nova Scotia (Figure 8b), although Nicieza et al (1994) provided circumstantial evidence to this effect in their comparative study of differences in juvenile growth and patterns of length bimodality in Scottish and Spanish Atlantic salmon.…”