Monday May 22, 2023

Cormorant tossing a fish into its mouth. Flickr Creative Commons, Veit.

Salmon and trout face numerous threats on their long and perilous journeys from their birth rivers and streams to the ocean. Predation, often by non-native fish, is a major source of mortality in out-migrating salmonids. Another obstacle these young fish face that is not discussed as frequently is predation from the sky. Birds, especially waterbirds that nest in a colony, are skilled hunters when it comes to pecking away at vulnerable juvenile fish populations. What makes salmonids so susceptible to being eaten by these bird species, and what can fisheries scientists learn from these interactions? The authors of a literature review published in the North American Journal of Fisheries Management addressed these questions by digging through more than 20 years of published studies to see what factors influence avian predation on juvenile salmonids. They found that the susceptibility of salmon to becoming bird food is influenced by many factors, including the bird species, the salmon species, and the environment. By taking a broad look across multiple bird and salmon species in different settings (marine versus freshwater), this synthesis found commonalities among predator-prey interactions, as well as important differences that determine whether juvenile salmon will get consumed by birds. 

The review gathered studies that used mark-recovery methods of sockeye salmon (Oncorhynchus nerka), Chinook salmon (O. tshawytscha), coho salmon (O. kisutch), and steelhead (O. mykiss) consumed by Caspian terns (Hydroprogne caspia; terns), double-crested cormorants (Nannopterum auritum; cormorants), and multiple species of gulls (Larus spp.; gulls), because these were the most frequently studied predators. Most of the studies reviewed occurred in the Columbia River basin. The authors discussed the findings of all these studies according to four categories of factors considered to influence fish susceptibility to bird predation: (1) salmonid prey species (2) environmental conditions (3) densities of prey and predators, and prey migration timing, and (4) prey characteristics. 

A seagull on a rock.

The study results revealed that not all salmonids were equally susceptible to being eaten by birds. For example, steelhead were consistently at greater risk of being eaten by terns and gulls than Chinook salmon. But this was not the case for all birds, since cormorants were not very specific about which salmon species they eat. 

Some of the environmental factors that influenced avian predation included distance from bird colonies, habitat location (up-river versus estuary, reservoirs), and turbidity. In most cases, being closer to a bird colony increased the chances of being eaten. Individual terns and cormorants tended to consume more salmon in freshwater up-river habitats compared to estuaries. This is likely due to higher abundances of salmon and fewer non-salmonid prey species upstream from estuaries, making them the dominant food source in these habitats. In reservoirs, predation by terns was higher out in open water, while predation by gulls was concentrated near dams. Turbidity (a measure of water clarity) appears to be a double-edged sword. Highly turbid, or cloudy, waters increased the risk of tern predation by decreasing young salmon reaction time. Conversely, low turbidity increased susceptibility to cormorant predation because greater visibility led to higher prey encounter rates. 

A pair of terns.

For predator and prey species, the number of individuals makes a difference when assessing prey susceptibility. In some cases, when juvenile salmonids were in high density, the risk of predation decreased. This is known as ‘predator swamping’, meaning that there is strength in numbers when it comes to avoiding becoming lunch. Not surprisingly, the authors reported that predation risk increases when there are many avian predators present. In addition, salmon migrating later (May and June) were more susceptible to predation by terns and cormorants. 

 Salmon size was the most frequently studied prey characteristic. Terns and gulls prefer salmon between 175 and 275 millimeters in length, presumably because smaller fish are not worth the effort and larger fish are more difficult to capture. Cormorants are not as picky. Salmon in poor health are also more likely to get consumed.

Overall, this review shows the complexities in assessing how susceptible young salmon and trout are to bird predation. There are several factors that influence these dynamic relationships, some of which may still be unexplored by researchers. The juvenile life stage is vital to maintaining viable salmonid populations, but it is also subject to high mortality. Therefore, identifying and understanding where and how juveniles die can provide information for management agencies to take steps in minimizing different sources of mortality. 

This post was featured in our weekly e-newsletter, the Fish Report. You can subscribe to the Fish Report here.

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