Restored beaches are a boon for young salmon

Somewhere in Puget Sound’s Elliott Bay near Seattle, a young chum salmon gobbles up a crustacean larva near the seafloor. That makes sense. It’s what juvenile chum salmon are supposed to eat. But elsewhere in the bay, another chum salmon decides to feast on plankton near the water’s surface instead. The two fish both manage to eat their fill, but they’re eating different prey, despite being located in some cases within one kilometer of each other. It all comes down to real estate: the first fish swims around near a restored beach. The second one lives near an armored seawall. It turns out that when we cover the coastline with artificial barriers, we don’t just protect the boardwalk; we change what’s on the menu for nearby fish.

In some ways, it’s actually surprising that the fish manage to survive at all in Elliott Bay, which is already crowded with container ships and tourists. But all that boat traffic isn’t the main factor that disrupts their habitat. Salmon hatch from their eggs in freshwater rivers and streams, and eventually swim down into the salty Puget Sound, where they dine on tiny ocean critters until they’re ready to swim back upstream to spawn. But 99 percent of the bay is artificially armored (a whopping one third of the entire Puget Sound is armored), to protect important seaside commercial interests. The remaining one percent is made of restored, naturalistic pocket beaches.

That the bay’s salmon were finding a way to scrape by was already clear, but what University of Washington researchers Stuart H. Munsch, Jeffery R. Cordell, and Jason D. Toft wanted to know was just how the un-natural seawalls affected them. The first thing they did was sample the prey available to salmon at three sites along seawalls and at three sites near beaches. Then, they trawled for juvenile salmon with large fishing nets at the same sites. Once the fish had been landed, the researchers flushed out their stomach contents using a technique known as “gastric lavage.” That allowed them to analyze the entirety of their stomach contents without any negative impacts on the fish, which, after measurements were taken, could be released back into the bay.

In the prey sampling part of the study, the researchers collected some 65,000 individual invertebrates. Most of them were copepods and barnacle larvae, though they also found amphipods, and polychaete worms in lesser abundance, and least common were crab larvae and insects. While most of those critters were found at both seawalls and beaches, there was variation in richness between site types.

From the stomachs of 459 juvenile chum, pink, and Chinook salmon, the researchers flushed 33,000 prey items. Salmon near both types of sites all ate their fill, and all preferred insects and crab larvae and avoided barnacle larvae.

The pink and Chinook salmon both preferred feeding on prey floating in the water column or on the water’s surface, and that allowed them to eat a similar diversity of critters whether near seawalls or beaches. But chum salmon tend to look for their food closer to the seafloor, and they did show a diet change according to shoreline type. When caught near armored seawalls, the chum salmon were forced to look for floating invertebrates, like the pink and Chinook salmon did. It was only near beaches where the chum salmon were able to find their preferred prey: bottom-dwelling crustaceans.

“Our study contributes to a growing recognition that shoreline infrastructure changes the feeding ecology of fish in shallow habitats,” say the researchers. While the habitat no doubt becomes changed when the shoreline changes, what is not yet clear is whether habitat quality also becomes modified. For example, do chum salmon suffer energetic costs when forced to hunt alternative prey?

This isn’t just a potential problem for fish like the area’s Chinook salmon, which are classified as threatened under the Endangered Species Act. Coastal development has occurred across most of the globe, and its implications for coastal and estuarine species are in many ways unknown. What is heartening, however, is that despite its artificial nature, restored beaches provided fish with a habitat close enough to natural that they were able to find the food they expected. And what more does a hungry fish need? – Jason G. Goldman | 02 October 2015

Source: Munsch SH, Cordell JR, Toft JD (2015) Effects of seawall armoring on juvenile Pacific salmon diets in an urban estuarine embayment. Marine Ecology Progress Series 535, 213-229. DOI: 10.3354/meps11403.

Header image: Young salmon photographed at the Seattle Aquarium, via shutterstock.com

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