Effects of tidal wetland invertebrate export on juvenile salmon diets and growth potential in connected habitats

Background/Question/Methods

Tidal pulsing of nutrients into estuarine wetlands leads to high levels of primary productivity and subsequent secondary production of aquatic and terrestrial invertebrates in these habitats. Thus, densities of both aquatic and terrestrial prey available to aquatic consumers may be greater in tidal wetlands than in the adjacent river mainstem. Spatial subsidies from adjacent wetlands can enhance prey densities in the mainstem, creating conditions where foraging and subsequent growth rates of consumers in the mainstem are similar to those in the more productive wetlands. Our objective was to quantify the degree to which wetland prey export enhances salmonid foraging and growth rates in adjacent mainstem habitats. We examined patterns in the diets of juvenile Chinook, coho, and chum salmon in the Grays River estuary, Washington for signatures of wetland prey export. We captured juvenile salmon using a beach seine at five locations in the river mainstem: three adjacent to tidal wetlands, one upstream from the wetlands, and one near the river mouth. We then compared fish diets to prey assemblages in the wetlands.

Results/Conclusions

Although ration sizes were not significantly different along the length of mainstem, diets of all three salmonid species contained a higher proportion of energy-rich drift insects at sites closer to the wetlands. High levels of drift insect biomass exported from wetland tidal channels suggest that these diet patterns are a function of prey export. Supplemental high-energy prey in the mainstem can increase salmon growth potential, and may be critical for fish that do not utilize wetland habitat or that are forced out of shallow wetland channels at low tide. Diets differed among species, however, indicating that salmonids exploit exported prey to different extents. Our results show that tidal wetlands augment salmonid fitness both directly by providing habitat and indirectly by enhancing adjacent habitats. The degree to which estuarine wetlands subsidize connected habitats should therefore be considered in restoration efforts.