Enhanced invertebrate prey production following estuarine restoration supports foraging for multiple species of juvenile salmonids (Oncorhynchus spp.)

Abstract

Estuaries provide crucial foraging resources and nursery habitat for threatened populations of anadromous salmon. As such, there has been a global undertaking to restore habitat and tidal processes in modified estuaries. The foraging capacity of these ecosystems to support various species of out‐migrating juvenile salmon can be quantified by monitoring benthic, terrestrial, and pelagic invertebrate prey communities. Here, we present notable trends in the availability of invertebrate prey at several sites within a restoring large river delta in Puget Sound, Washington, U.S.A. Three years after the system was returned to tidal influence, we observed substantial additions to amphipod, copepod, and cumacean abundances in newly accessible marsh channels (from 0 to roughly 5,000–75,000 individuals/m2). In the restoration area, terrestrial invertebrate colonization was dependent upon vegetative cover, with dipteran and hymenopteran biomass increasing 3‐fold between 1 and 3 years post‐restoration. While the overall biodiversity within the restoration area was lower than in the reference marsh, estimated biomass was comparable to or greater than that found within the other study sites. This additional prey biomass likely provided foraging benefits for juvenile Chinook, chum, and coho salmon. Primary physical drivers differed for benthic, terrestrial, and pelagic invertebrates, and these invertebrate communities are expected to respond differentially depending on organic matter exchange and vegetative colonization. Restoring estuaries may take decades to meet certain success criteria, but our study demonstrates rapid enhancements in foraging resources understood to be used for estuary‐dependent wildlife.

Integrated Diet Analyses Reveal Contrasting Trophic Niches for Wild and Hatchery Juvenile Chinook Salmon in a Large River Delta

“…Woo et al. (in press) did not observe Surf Smelt Hypomesus pretiosus , Pacific Herring Clupea pallasii , or other forage fish when they sampled the Nisqually River prey field during the 2010, 2011, and 2012 out‐migration seasons. Conversely, Duffy et al.…”

Gauging resource exploitation by juvenile Chinook salmon (Oncorhynchus tshawytscha) in restoring estuarine habitat

“…). Greater prey biomass at Madrone resulted in an overall energy availability that was comparable to both reference sites, despite 50% less channel area (Woo et al ; Fig. ).…”

“…Fulton's condition factor was influenced by aquatic prey biomass ( F [1,284] = 4.423, p = 0.036), but this variable only explained 16% of variation in body condition. Because invertebrate prey biomass also varied seasonally and by site (Woo et al ), site‐specific differences had an indirect effect on Chinook size ( F [1,284] = 152.362, p < 0.001; F [1,284] = 11.387, p < 0.001).…”

“…A comparison of invertebrate prey communities in Chinook gut contents to those found in surrounding habitat (PSI) supported this finding, and showed that the shift in dietary similarity between 2010 and 2012 was a direct result of juvenile salmon switching their targeted prey from benthic to terrestrial invertebrates in the restoring marsh. This may be because terrestrial invertebrate biomass more than tripled at the recently restored Madrone site between 2010 and 2011/2012, while benthic and pelagic invertebrate biomass remained steady (Woo et al ). Despite notable shifts in available and consumed prey communities, PSI values were consistent from year to year at all four sites.…”

“…Nisqually's post‐restoration monitoring framework has guided the ongoing collection of physical and biological data (Ellings ) including intensively monitored fish populations, and assessments of habitat availability (Ellings et al ), invertebrate prey availability (Woo et al ), and growth potential (David et al ). Woo et al () observed substantial increases in benthic, terrestrial, and pelagic invertebrate biomass within 3 years of dike removal; however, different invertebrate species can have different energy densities, and may vary in size or ease of capture (Brey et al ; Gray ). Furthermore, this energy is of little utility if juvenile salmon cannot access and forage in prey‐rich sites, or if thermal conditions present metabolic limitations.…”