Size matters in the sex life of salmon

June 23, 2020

Every summer, tens of thousands of Atlantic salmon migrate from the Barents Sea to the Teno River, Finland, to spawn in the streams where they were born. This journey is a feat of endurance: salmon stop feeding and must navigate fast flowing water, leap over obstacles, and avoid predators, hooks, and fishing nets to arrive at their spawning grounds.

The marathon doesn't stop there though: once they arrive at their spawning grounds, they must fight for the possibility to mate with members of the opposite sex. Who are the winners of this evolutionary competition? It turns out that the largest fish produce the most offspring, but there are far fewer of these fish on the spawning ground battling for reproductive success than their younger - and smaller - competitors, according to researchers at the University of Helsinki and the Natural Resources Institute Finland.

The study, recently published in the scientific journal Molecular Ecology, is part of a long-term monitoring program. A small piece of fin tissue was removed for genetic fingerprinting of more than 5000 adults and juveniles before they were released back into the wild. Adults were also fitted with a unique identification tag after a few scales were carefully sampled. The scales are particularly valuable, as they record annual growth cycles, much like tree rings.

"Great care was taken to not harm the fish," explains Dr. Kenyon Mobley, lead author of the article. "In fact, we have recaptured adults returning to spawn several years later and juveniles returning to spawn as adults."

Larger salmon have more offspring

Most salmon in Teno River spend between one and four years at sea before migrating back to breed. The more time salmon spend at sea, the larger they grow. Females generally take between 2-3 years to mature, but most males return after just one year at sea.

Mobley's study showed that for every year spent at sea, females gain over 4 kilograms of body weight and produce 60% more offspring. Males, on the other hand, gain nearly 5 kilograms of body weight and produce 200% more offspring for every year they spend at sea.

However, spending more time at sea comes with a significant cost. Very few of these older larger fish return to spawn. "This is presumably because spending more time at sea exposes fish longer to predators, fishing, and diseases, and thus a higher risk of death before having a chance to spawn," explains Mobley.

"Knowing the reproductive contributions of different sized fish in this river section can help us to develop more accurate models of offspring production. These are needed for developing Teno salmon management guidelines," says Professor Jaakko Erkinaro from Natural Resources Institute Finland. "It also helps our ongoing research aimed at predicting how many large adults may survive at sea to return to spawn," Mobley adds.

Larger salmon have more mating partners

Like most animals in nature, salmon are not monogamous and can have up to eight mating partners, the study shows. Having more mating partners ensures successful fertilization of eggs and passing on their genes to the next generation.

Nearly all females captured in the study produced offspring, mating on average with more than two males, and gained 35% more mates for each year they spent at sea. Males have, on average, less than one mate, indicating that many males are excluded from mating presumably through strong competition by bigger males. For each year spent at sea, males gain 60% more mates. This means that larger salmon, in particular males, have a distinct advantage when it comes to finding mates.

Where are the females?

In the study population, females are a rare commodity. There are up to seven males for every female at the spawning ground near the entrance of the Utsjoki River. This pattern is consistent across all years of the study. Having a high number of males likely increases fights among males for opportunities to mate with the few available females. Why so few females return to this particular site remains a mystery, as other locations in the Teno River have a more balanced mix of males and females.

Early life-history affects female reproduction

Prior to entering the sea, juvenile salmon usually spend between 3-5 years in freshwater. The researchers were surprised to find that the longer the females stay in freshwater, the fewer years they spend at sea, and return to spawn at a much smaller size. Because these females are smaller, they have fewer eggs and produce less offspring. Males, on the other hand, do not seem to be affected by spending more time in freshwater.

"These results show how overlooked aspects of salmon life-history are important to the long-term conservation of these fish," said Mobley.

University of Helsinki

Related Salmon Articles from Brightsurf:

Alaska's salmon are getting smaller, affecting people and ecosystems
The size of salmon returning to rivers in Alaska has declined dramatically over the past 60 years because they are spending fewer years at sea, scientists report.

Chinook salmon declines related to changes in freshwater conditions
A new University of Alaska-led study provides the first evidence that declines in many of Alaska's chinook salmon populations can be attributed in part to climate-driven changes in their freshwater habitats.

Size matters in the sex life of salmon
For Atlantic salmon, size matters when it comes to love.

What does drought mean for endangered California salmon?
Droughts threatens California's endangered salmon population -- but pools that serve as drought refuges could make the difference between life and death for these vulnerable fish.

Salmon provide nutrients to Alaskan streambanks
Nutrient cycling of stream ecosystems dependent on portion of salmons' lifecycle.

Melting glaciers will challenge some salmon populations and benefit others
A new Simon Fraser University-led study looking at the effects that glacier retreat will have on western North American Pacific salmon predicts that while some salmon populations may struggle, others may benefit.

Bigger doesn't mean better for hatchery-released salmon
A recent study in the Ecological Society of America's journal Ecosphere examines hatchery practices in regards to how Chinook salmon hatcheries in the PNW are affecting wild populations over the past decades.

Salmon get a major athletic boost via a single enzyme
A single enzyme anchored to the walls of salmons' blood vessels helps reduce how hard their hearts have to work during exercise by up to 27%.

Salmon are shrinking and it shows in their genes
Male salmon are maturing earlier and becoming smaller, and it shows in their genes.

Young salmon may leap to 'oust the louse'
A study by Simon Fraser University aquatic ecologists Emma Atkinson and John Reynolds reveals that young salmon may jump out of water to remove sea lice.

Read More: Salmon News and Salmon Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to