For the Atlantic Salmon's Sex Life, Size Does Matter

Each summer sees Atlantic salmon by the tens of thousands migrating from the Barents Sea to the Teno River in Finland to breed in the streams from which they hatched, and it seems that size matters, and the larger they are, the better their chance for sex life or to find a mate to breed with.

They will need extreme endurance to spawn. They stop feeding and swim upstream against the rapid waters, overcome obstacles, and survive the myriad predators and fishing hooks and nets before finally reaching spawning grounds. If they survive their journey, they need to fight others for the chance to have sex and hopefully spawn offspring.

A new study has been published in the Molecular Ecology journal, which discovered that the fishes with the largest size have the most number of offspring. It also found that far fewer larger fishes are in spawning grounds compared to smaller and younger competitors. The research team from the University of Helsinki, as well as the Natural Resources Institute in Finland, conducted the study as part of an extensive monitoring effort.

To identify the fishes, they collected fin tissues and scales to use in genetic fingerprinting of over 5,000 juveniles and adults. The adults also get an identification tag. The researchers used the scales to record growth.

Lead author Kenyon Mobley said they took great care to avoid harming the fishes. They recaptured the adults and previous juveniles that have become adults that return to spawn many years after sampling.

Many Teno River salmon are in the sea one to four years before migration, and the more time they spent in the sea, the bigger they become. In general, females take two to three years to become mature, while males take just one year. For each year they spend in the sea, the females gain more than four kilos in weight and have 60% more spawn. For males, the rate is almost 5 kilos and 200% more spawn per year spent in the sea. The downside of being in the sea, however, is more exposure to predators, diseases, fishing, and, consequently, death. Indeed, very few older and larger fishes return.

According to Natural Resources Institute Finland professor Jaakko Erkinaro, knowing these facts help in making accurate reproduction models and developing management guidelines. The study found that larger fishes, particularly males, get more mates and reproduce more offspring.

Females are also rarer than males, with a maximum ratio of seven males per female, at least at Utsjoki River's spawning ground. It is not yet clear why there are fewer females here since, in other parts of the Teno River, the ratio is more balanced.

Juveniles often spend three to five years of life in freshwater, and the fewer years spent in the sea, the smaller the adult sizes become. Smaller females lay fewer eggs and consequently have fewer offspring. Meanwhile, males are seemingly unaffected by time spent in freshwater. Moblew said knowing these details in Atlantic salmon life history is needed for their conservation.