Boats coming and going out of Elliot Bay get in the way of small chum salmon trying to find a bite to eat along Seattle's shorelines. That;s the conclusion of researchers from the University of Washington (UW) after they examined exactly what these young fish needed to eat in order to survive to adulthood. Thescientists found that what's referred to as "built-up" or "armored" shorelines – those restored with the addition of large piers and concrete seawalls – might be changing the fish's preferred diet.
"Our study shows that armoring affects what species of prey are available," Stuart Munsch, lead author and a UW doctoral student in aquatic and fishery sciences, said in a news release.
After collecting young salmon from both armored shorlines as beaches restoried to reflect a more nature environment, researchers flushed out the fishes' stomachs to see what they had most recently eaten. They determined that young Pink and Chinook salmon, who normally feed on floating organisms, didn't have a problem feeding. However, other young salmon that prefer bottom-dwelling organisms had to adjust their diet near urbanized shorelines, such as Elliot Bay.
"Fish that normally eat those missing prey will feed on alternative species at armored sites, but we don't know what the costs of that change are to the fish," Munsch added in the release.
The researchers concluded that restored beaches house more tasty treats for the chum salmon, since they attract more diverse organisms such as small crustaceans. Chum salmon are dependent on these small organisms until they grow big and strong enough to swim out to sea. These small crustaceans are not as abundant along the urbanized shorelines, where less appetizing barnacles live instead.
"Engineered shorelines like these man-made beaches are going to have more components of a natural ecosystem than a featureless wall," Jeff Cordell, co-author and lead investigator on the project and a UW research scientist with aquatic and fishery sciences, said in a statement. "Man-made beaches will produce more diversity and numbers of the kinds of food juvenile salmon utilize."
The chum salmon were the only fish that reportedly changed their diets to adapt to the less abundant sources along "armored" shorelines. Even though their normal diet consists of easy prey, the large piers often block light and confuse the young salmon. Therefore, the fish that have to change their diets might have to use more energy to do so, the researchers noted.
"The [type of] copepods that chum salmon usually feed on are brightly colored and they're found near the bottom," Munsch explained. "We think that chum salmon along armored shorelines might have to spend more energy searching for prey that are harder to see, or chasing prey that are more evasive, when that energy should be allocated to growth or migration."
Their findings were recently published in the journal Marine Ecology Progress Series.
The researchers are also working with Seattle's Seawall Project, which is replacing the current waterfront wall with a fish-friendlier structure.
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