Pacific salmon don’t feed in fresh water – everybody knows that, right?
Wrong. Brian Morrison, fisheries biologist and author of several recent posts on fish lifecycles in Western Alaska, is back today to tell us about some recent research findings that prove conventional wisdom wrong.
Salmon Feeding in Fresh Water?
Many anglers have long known that mature Pacific salmon (Oncorhynchus spp.) will strike at objects that resemble eggs while in fresh water. Concurrently, there has been a deeply entrenched belief held by both scientists and anglers that mature Pacific salmon cease feeding in fresh water, with numerous explanations having been developed to explain this behaviour, such as ‘instinct’, ‘aggression’, or a fish being ‘territorial’. It has even been stretched as a way for adult salmon to eliminate potential competitors that their offspring might encounter as juveniles. As a much simpler explanation, Pacific salmon continue to feed because it provides them with nourishment.
A group of researchers set out to test whether Pacific salmon do in fact feed, and try to unravel some of the myth around why they may feed. They looked at the feeding on eggs within adult Chinook Salmon, Chum Salmon, and Coho Salmon. Overall, 13% of stomachs examined contained eggs (up to 30% observed for Coho Salmon), and feeding rates were estimated at up to 14 eggs per day. Feeding experiments in a lab revealed that mature salmon could digest eggs, as fed salmon maintained significantly higher body mass than unfed salmon. The researchers in this study determined that the energy from consumed eggs could potentially allow salmon to migrate up to an additional 3.8 km per day of feeding, or extend the duration of spawning activity by up to 12%.
The researchers found that the energetic gains associated with egg consumption in Pacific salmon may thus be particularly important for precocial males (jacks). Alternatively, one of the most surprising findings was that large individuals were no less likely to feed than their smaller counterparts. The highest feeding prevalence was observed in adult female Coho Salmon from the Quinsam River, BC, in which 40% of the sampled females had consumed eggs. Importantly, female salmon may continue to feed in fresh water not only to offset the relatively higher costs of breeding that they incur, but to obtain physiologically important resources, such as carotenoids and dietary protein.
Although there is still a lot of uncertainty on why adult Pacific salmon will feed while in fresh water, the researchers conclude; given that Pacific salmon stocks show high homing to their stream of birth, it is possible that many populations have evolved local adaptations to their specific migration and spawning requirements that include nutrients from salmon eggs from their own species or other salmon species. Such a dependence on supplemental energy from freshwater feeding could explain why some populations fail to recover even when released from fishing pressure, as lower abundances of salmon (all species) may remove feeding opportunities that some populations and species depend on to successfully complete migration and spawning.
Want even more details? Here’s a link to the original research paper.