Aykanat, T., Jacobsen, J. A., & Hindar, K. (2024). Ontogenetic variation in the marine foraging of Atlantic salmon functionally links genomic diversity with a major life history polymorphism. Molecular Ecology, e17465. https://doi.org/10.1111/mec.17465
One of the great challenges of biology is to understand how genes determine traits. Even seemingly simple traits like eye color have a complicated genetic basis. Scientists often learn about genetics by studying “polymorphisms,”, or traits that occur in two or more forms within a population. In humans, some obvious examples are eye, hair, and skin color. Comparing the genomes of individuals with different versions of a trait can help figure out which genes cause that trait. However, this is hard to do for wild animals, especially when the traits involved are not externally obvious.

Polymorphisms can be more subtle than color, and even include behaviors. One important behavioral polymorphism occurs in Atlantic salmon: their breeding age. Salmon are “anadromous,” meaning they spawn in freshwater rivers, migrate to the ocean, and then return to their spawning ground to reproduce. Salmon can spend anywhere from 1 to 3 years at sea before returning to freshwater, and this duration at sea is polymorphic. Some animals spawn after only one year at sea, while others wait 2 or 3 years. This trait is called the “age at maturity.”
Don’t be so (im)mature
Age at maturity is mostly determined by two genes. Salmon with “early” genotypes at these genes usually spawn after just one year at sea, while salmon with “late” genotypes spend 2-3 years at sea. However, scientists don’t know what these genes actually do. What is it about “early” or “late” fish that affects the time they spend at sea?
One possible mechanism could be diet. If genetic variation results in fish that have more efficient foraging strategies at sea, those fish might grow faster and mature earlier.
In this study, biologists tested the hypothesis that the “age at maturity” genes affect feeding behavior, causing variation in energy gain and therefore time to maturity. To do this, they took advantage of a large database of dietary data collected from Atlantic salmon.
The genetics of eating
Using some fancy math, the researchers estimated two aspects of foraging: foraging frequency (how often fish actively try to hunt a particular prey), and foraging outcome (how much of a prey item fish actually eat). Oceanic salmon mostly eat fish and crustaceans, so the researchers analyzed these two categories of prey.
Regardless of genotype, older salmon were more likely to seek crustaceans. However, younger salmon had more crustaceans in their stomachs, indicating higher foraging efficiency. The researchers suggest that older salmon have a “continuous” feeding strategy, where they try to forage more often but are less efficient than younger fish. This might be because older fish are larger, and thus need to spend more time foraging to support their larger body size.

This is where the genetics get a little complicated. Young salmon with the “early” genotype forage crustaceans more efficiently than young salmon with the “late” genotype, which might explain how they are able to grow up faster. However, this pattern was reversed in older salmon: old salmon with the “early” genotype were less efficient. These genes seem to represent a trade-off, where salmon feed more efficiently at a young age and have a chance to mature quickly. but have reduced efficiency at older ages.
They grow up so fast
Age at maturity is economically important to salmon fisheries because salmon must be eaten before they mature. Therefore, late-maturing fish have more meat when they reach market. For the salmon themselves, age at maturity comes with tradeoffs: younger and smaller animals might benefit from breeding sooner, but older animals can invest more in their offspring due to their large size.
This study explains how changes in feeding conditions could affect these population dynamics.
As the climate changes in the North Atlantic, we expect changes to marine food webs, which might result in new evolutionary pressure on salmon. The warming Arctic may reduce the amount of cold water entering Atlantic waters from the north, causing worse feeding conditions for fish. If the amount of crustacean prey goes down, the salmon that are more efficient crustacean hunters might do better, and selection might favor that genotype over others. And because the same genes contribute to feeding behavior and age at maturity, changes in the food supply might cause breeding salmon populations to become younger (or older) in the future.

I am a PhD student at MIT and the Woods Hole Oceanographic Institution, where I study the evolution and physiology of marine invertebrates. I usually work with zooplankton and sea anemones, and I am especially interested in circadian rhythms of these animals. Outside work, I love to play trumpet, listen to music, and watch hockey.