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Why some Atlantic salmon spawn several times

Published: 04 April 2023
 Salmon in profile.

Atlantic salmon have the capacity to reproduce more than one time but very few individuals manage to become repeat spawners. Nevertheless, repeat spawners are important for population dynamics. In a new study, researchers from the Swedish University of Agricultural Sciences (SLU) and the Norwegian Institute for Nature Research (NINA) investigated the life history of more than 200 000 adult Atlantic salmon from Norwegian populations. The proportion of repeat spawners was on average 3.8 % and ranged between 0 and 26 percent across 179 populations.

The adult Atlantic salmon has grown large at sea, and it returns to the place in the river where it once hatched when it is time to spawn. Atlantic salmon are iteroparous, which means they can spawn several times, but not many individuals manage to become repeat spawners. In Norwegian rivers, only 3.8% of the fish were repeat spawners and the proportion varied between 0 to 26 percent across 179 populations.

"Repeat spawners are important for recruitment to the population, especially in years with low numbers of first-time spawners. They are on average larger than first-time spawners and females contribute with more eggs. They also contribute to genetic diversity since more cohorts spawn together when repeat spawners are present," says Lo Persson, researcher at the Swedish University of Agricultural Sciences (SLU) who was one of the principal investigators in this study.

Small salmon in large-salmon populations of salmon more often become repeat spawners

Fish that return to spawn after only one year at sea, called one-sea-winter fish (1SW), have a higher probability of being a repeat spawner compared to a fish that has spent multiple years at sea, called multi-sea-winter fish (MSW), before first spawning.

"It has been known that small fish, i.e. the ones that return after only one year at sea, invest relatively less energy into spawning compared to large fish. However, we found that small fish in large-salmon populations more often became repeat spawners compared to small fish in small-salmon populations, which could indicate local adaptation."

Fish in large-salmon populations were alternate spawners, they reconditioned for two years between each spawning event, whereas  fish in small-salmon populations were consecutive spawners and spawned again the year after the first spawning event.

"People believe that this can be explained by the relatively larger energy investment made by larger fish that then requires additional time to recondition at sea. However, we found a strong correlation between the time to recondition and the population mean sea age, were also small fish in large-salmon populations (high mean sea age) were alternate repeat spawners. The differences between populations indicate local adaption, which means that salmon over time has evolved to be fit in their home river and the marine environment where they feed. Based on our results, we think the repeat spawning trait has been subjected to selective pressures that differ among rivers -  giving rise to differences among populations," says Lo Persson.

Females are more often repeat spawners than males

Females were more often repeat spawners than males, the study confirms. That males have lower post-spawning survival than females has been observed previously. Injuries from aggressive interactions at the spawning ground are thought to contribute to lower survival of males.

In this study, the researchers were able to show that males lost more than females in terms of body mass when compared to first-time spawners. This could indicate that the ability to recondition is constrained in males, which could contribute to their lower survival.

"The analysis of lost body mass made me re-think my idea of what a repeat spawner looks like. Before this study, my idea was that they are really large fish but now I have a better understanding of the cost that spawning imposes on the fish."

When the fish matures and return to spawn, not only do they not eat for up to a year, they often invest more than 50% of their energy during the spawning migration and at the spawning grounds. If they would have stayed an additional year at sea they would have roughly doubled their weight.

"Of course some of the largest fish are repeat spawners, for example, we had a 12 year old female in our data that was caught on her fourth return to the river and she weighed 15 kg. However, the majority of repeat spawners are much smaller than that."


 Close-up of a salmon scale.Life history of salmon can be obtained from their scales

This study is based on data from analysis of salmon scales called “scale reading”. The scales have been collected from adult salmon caught in Norwegian rivers between 1989 and 2017. The analysis reveals how many years the fish spent in freshwater before it migrated to the sea, how many years it spent at sea before maturation and it's first spawning, its likely age at additional spawning events, and the time between spawning events. From more than 200 000 salmon scales, 141 different life histories were identified. This is the highest number of unique life histories recorded and was only possible because of the huge number of scales that were read.

"One aspect of repeat spawning that is not possible to detect with the current scale-reading techniques, is precocious maturation of males who then can spawn as parr before they migrate to sea. These males are technically also repeat spawners but that part of their life history could not be included in this study, says Lo Persson. Overall, though, repeat spawners are important for life-history variation in Atlantic salmon and they contributed to 75 percent of the 141 unique life histories we found," says Lo Persson.


Photo: Jörgen Wiklund, SLU
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