Reproductive strategies of Atlantic salmon: Ecology and evolution

Atlantic salmon (Salmo salar, Salmonidae) show a diversity of life history, behavioural and morphological adaptations for reproduction which have evolved as an outcome of competition to maximize reproductive success. Reproductive traits of females have been shaped principally by natural selection for offspring production and survival, those of males by sexual selection for access to matings. Female Atlantic salmon invest approximately six times more energy in offspring production (i.e, gonads) than males and face an important trade-off between number and size of eggs to produce that will maximize the number of surviving offspring. Timing of breeding and the construction of nests appear adapted to increase offspring survival. The most important determinant of female breeding success is body size because it affords high fecundity, access to breeding territories and decreased probability of nest destruction. Asynchronous female spawning and the male ability to spawn rapidly and repeatedly results in male-biased operational sex ratios that generate intense male competition for mates. This has likely been responsible for the evolution of elaborate male secondary sexual characters associated with fighting and status signalling. Furthermore, it has given rise, through frequency-dependent selection, to two alternative male breeding phenotypes: (1) large, anadromous males; and (2) small, mature male parr. Anadromous males invest heavily in behavioural activity on the spawning grounds, searching and fighting for mates and courting them, with body size being an important determinant of their breeding success. This behavioural activity carries a heavy cost, as anadromous males have significantly reduced survival relative to females. In contrast, mature male pall invest proportionally more in testes for sperm competition and attempt to 'sneak' access to matings. While this behaviour also carries costs in terms of subsequent growth and survival, male parr are more likely to breed again, either prior to or following a migration to sea, than anadromous males. While knowledge about the breeding of Atlantic salmon is detailed, we are only beginning to understand the ultimate causes and/or functional significances of their reproductive strategies. Predictive models of the life history variation are developing, focusing on the need for empirical study and testing of life history and reproductive patterns.