Evolution of iteroparity is shaped by the trade-off between current and future reproduction. Atlantic salmon is an iteroparous species, but individuals show great variation in life history, including the number of reproductive events. There is large variation in time spent in rivers and at sea before first reproduction both within and among populations, but most individuals stay 1–4 years in freshwater (smolt age) and one to three winters at sea (sea age).
Some individuals survive for a second spawning, and a few survive to spawn more than two times. Repeat spawners migrate to sea between each spawning event to regain energy to develop new gonads. The recondition period is usually one or two years (i.e., fish return as consecutive or alternate spawners).
Variation in iteroparity among 205 050 individual Atlantic salmon caught in 179 rivers spanning 14° of latitude was reported in a new publication. The study was based on fish scales collected by anglers. Previous spawners were recognised by patterns in their scales.
The proportion of repeat spawners (iteroparous individuals) averaged 3.8% and ranged from 0% to 26% across rivers. Females were more often repeat spawners than males and had lower cost of reproduction in terms of lost body mass between spawning events.
Proportion of repeat spawners for a given sea age at maturity, and the ratio of alternate to consecutive repeat spawners, increased with increasing population mean sea age at maturity. By combining smolt age, sea age at maturity, and age at additional spawning events, 141 unique life-history types were identified, and repeat spawners contributed 75% of that variation.
Anadromous females and males that matured at a young age and a small size (fish that had stayed one year at sea) were more likely to become repeat spawners than larger, late-maturing fish (fish that had stayed more than one year at sea). This is expected, as smaller fish invest relatively less energy at spawning than larger fish, and the chance to survive to spawn a second time increases with decreasing energy investment at first spawning.
The alternate strategy of reconditioning two or more years at sea before next spawning, instead of one year, was more common in late-maturing large fish than in early-maturing small fish.
This new study shows that repeat spawners are important for life-history variation and suggest that the association between mean sea age and the frequency of repeat spawning is adaptive rather than a pleiotropic side effect arising from selection on sea age.
Read more in the new publication:
Persson, L., Raunsgard, A., Thorstad, E.B., Østborg, G., Urdal, K., Sægrov, H., Ugedal, O., Hindar, K., Karlsson, S., Fiske, P. & Bolstad, G. 2022. Iteroparity and its contribution to life-history variation in Atlantic salmon. Canadian Journal of Fisheries and Aquatic Sciences doi/10.1139/cjfas-2022-0126. https://cdnsciencepub.com/doi/full/10.1139/cjfas-2022-0126