A young coho salmon. Photo credit: Morgan Bond.

A young coho salmon. Photo credit: Morgan Bond.

Every year, millions of salmon are born in freshwater streams and rivers. The fish spend their early years there, learning the scents of their particular setting. Young salmon then voyage to the sea, returning to the streams they were born in a few years later. The salmon must now choose between dozens of similar-looking waterways. Because the fish are better equipped for handling the conditions in their childhood stream, choosing the wrong one could have disastrous consequences. So, the fish rely on the memory of the smells they learned as juveniles to make their way back to the streams they were born in. However, how these scent memories are formed is still unknown. This led Andrew Dittman, David Baldwin and Nathaniel Scholz of the National Marine Fisheries Service, USA, Darran May of the University of Washington, USA, and Marc Johnson of the Oregon Department of Fish and Wildlife, USA, to try to understand how coho salmon (Oncorhynchus kisutch) form memories of their childhood streams and use them when they return from life at sea.

The team reared fish in the lab until the winter of their second year. Around this time, the fish begin to undergo a change that preps them for their life in a marine environment. These changes also allow them to imprint the smells from their home in their memory. But how? Dittman and colleagues think that part of forming memories could be due to having more scent receptors, which allow the salmon to sense some of the odours that could be present in the water. They were particularly interested in one receptor, the basic amino acid receptor – which responds to the amino acid l-arginine – because salmon can remember this scent. First though, the researchers needed to make sure that the salmon even had these receptors and where they were. It turns out that coho salmon do have these smell receptors in their nose and tongue, as the researchers expected. Could these receptors be the answer the scientists were looking for?

To find out, the researchers began adding the amino acid l-arginine to some of the fish's water while the salmon were undergoing their transition into their ocean-ready stage and ready to start remembering scents. After the fish had experienced the amino acid for several weeks, the researchers measured how sensitive the salmon's noses were to l-arginine by recording the electrical impulses of the nerve cells in the nose when the salmon encountered the amino acid. Every fish sensed the amino acid, but the electrical activity was even higher in salmon that already had l-arginine in their water. This suggests that the salmon are remembering this smell when they encounter it. But why is the response so much bigger for fish that have sensed l-arginine before?

The scientists thought that the larger response could be because there were more receptors in the fish that had previously smelled the l-arginine. To find out, they measured the levels of amino acid receptor mRNA the salmon had in their noses. Although the amount of mRNA for the receptors increased in all fish as they aged, the amount of receptor mRNA in fish that had l-arginine added to their water was seven times higher at the end of the experiment than it was at the beginning. Three weeks after the researchers stopped adding l-arginine to the fish's water, the mRNA levels remained high, meaning that the fish could be more sensitive to amino acids they recognize long after they stop experiencing them. If this is true, Dittman and colleagues are hopeful that this knowledge could be used to help conservation efforts and hatcheries. Although there is still lots of work to be done, it seems likely that salmon know the way home by following their nose.

Dittman
,
A. H.
,
May
,
D.
,
Johnson
,
M. A.
,
Baldwin
,
D. H.
and
Scholz
,
N. L.
(
2024
).
Odor exposure during imprinting periods increases odorant-specific sensitivity and receptor gene expression in coho salmon (Oncorhynchus kisutch)
.
J. Exp. Biol.
227
,
jeb247786
.