Migratory Fish and their Role in the Greater Ecosystem

We talk quite a bit about “restoring fish passage” and “fish habitat” when we talk about restoration at American Rivers. However, beyond making the connection to the meditative joys of recreational angling or an elusive reference to a salmon steak, we rarely discuss the larger role fish play in the world. As we celebrate the inaugural World Fish Migration Day, we thought we would dig a little deeper, let our river nerd flag fly, and talk about another way in which migratory fish are critical to a healthy ecosystem.

Osprey | Ken Lee

Osprey | Ken Lee

Specifically, let’s look at the predator-prey relationship between terrestrial and aquatic species. Throughout the late 1990s and early 2000s, Mary F. Willson and her compatriots published several journal articles [PDF] examining the role that anadromous fish (i.e., migratory fish that are born in rivers, but spend most of their life in the ocean before returning to rivers to spawn) play in the food web. Rather than focus on the more commonly considered question of the effects of predators (wildlife) on fish populations, their study examined how populations of anadromous fish influence wildlife populations.

You see, the silvery salmon may seem a bit innocuous— just another happy fish getting fat off bugs— but salmon (all inclusive) and their other migratory brethren could be an important link in the geographic distribution and abundance of wildlife. In Anadromous Fish as Keystone Species in Vertebrate Communities [PDF], Wilson and Halupka note that:

“Food resources for almost all kinds of animals are variable in space and time, but the anadromous fish system is an extreme case in which prey is temporarily very abundant, spatially constrained, relatively easy to capture, and more or less predictable.”

They go on to draw parallels between the spawning runs of anadromous fish and the migration of antelope across Africa. The ability of wildlife to anticipate the arrival of a theoretically abundant food source has also led scientists to posit that some species, like the mink, may even time reproduction to coincide with the availability of this high-energy food source. This means that declines in anadromous runs from issues like overfishing and fragmented habitat have ramifications that extend beyond these discrete aquatic communities.

This, of course, is just the tip of the iceberg! We haven’t even begun to talk about the link decaying carcasses play in populations of invertebrates (fish food) and in providing nutrients for streambank vegetation.

The bottom line is that the migratory fish runs we work so hard to restore are important to the health of the bears, minks, eagles, and other wildlife we all love.

If you want to continue to see us dig deeper and get geeky here on the blog, leave a comment below and let us know.