Life history variation and growth rate thresholds for maturity in Atlantic salmon, Salmo salar

Based upon published and unpublished data compiled for 275 populations, we describe large-scale spatial and temporal patterns in Atlantic salmon, Salmo salar; life history and model these data to evaluate how changes to life history influence optimal growth rate thresholds for sea age at maturity. Population means (ranges in parentheses) describe the following for salmon throughout its range: smelt length = 14.8 cm (10.5-21.5 cm); smelt age = 2.91 years (1.04-5.85 years); egg-to-smelt survival = 1.5% (0.2-3.2%); grilse length = 56.8 cm (48.5-70.0 cm); sea age at maturity = 1.60 years (1.00-2.64 years); smolt-to-grilse survival = 7.4% (1.3-17.5%). Growth rate thresholds specify the length increase between the smelt and grilse stages above which reproduction after one winter at sea is favoured over later maturity. Our simulations indicated that increased growth generally favours earlier, but never delayed, maturity. Optimal growth rate thresholds for sea age at maturity are highly sensitive to survival but only moderately sensitive to fecundity, smelt size, and smelt age. Depending on an individual's growth rate at sea, early maturity is favoured by decreased smelt age or by increased smelt length, fecundity, or survival (freshwater or marine). We suggest that future Atlantic salmon life history research focus upon reaction norms and growth rate thresholds for age at maturity, demographic and genetic consequences of male parr maturation, and the origin and maintenance of coexisting anadromous and nonanadromous life history polymorphisms.