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

Abstract

In the context of delta restoration and its impact on salmonid rearing, success is best evaluated based on whether out‐migrating juvenile salmon can access and benefit from suitable estuarine habitat. Here, we integrated 3 years of post‐restoration monitoring data including habitat availability, invertebrate prey biomass, and juvenile Chinook salmon (Oncorhynchus tshawytscha) physiological condition to determine whether individuals profited from the addition of 364 ha of delta habitat in South Puget Sound, Washington, United States. Productivity in the restored mudflat was comparable to reference sites 3 years after dike removal, surpassing a mean total of 6 million kJ energy from invertebrate prey. This resulted from the development of a complex network of tidal channels and a resurgence in dipteran biomass that was unique to the restoration area. Consequently, a notable shift in invertebrate consumption occurred between 2010 and 2011, whereby individuals switched from eating primarily amphipods to dipteran flies; however, dietary similarity to the surrounding habitat did not change from year to year, suggesting that this shift was a result of a change in the surrounding prey communities. Growth rates did not differ between restored and reference sites, but catch weight was positively correlated with prey biomass, where greater prey productivity appeared to offset potential density‐dependent effects. These results demonstrate how the realized function of restoring estuarine habitat is functionally dependent. High prey productivity in areas with greater connectivity may support healthy juvenile salmon that are more likely to reach the critical size class for offshore survival.

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

“…; Chittenden et al. ; Davis et al., in press). Stable isotopes determined that long‐term prey contributions and patterns of residency were at least partially responsible for these physiological differences.…”

“…; David et al. ; Davis et al., in press). In contrast, fish captured in EFT and freshwater tidal marshes (the majority of which were unmarked) demonstrated a greater reliance on terrestrial insects and their larvae.…”

“…Meanwhile, terrestrial insects in freshwater and mesohaline marsh may remain underutilized except by wild salmon. As fish grow larger in June and July, trophic overlap between the two cohorts decreases because wild fish likely reside longer in the estuary before moving to the nearshore (Davis et al., in press). The broader trophic niche and longer delta residence times of wild juvenile Chinook Salmon may allow them to exploit resources better and thus to have higher bioenergetic growth potentials.…”

“…; Chittenden et al. ), yet wild fish appear to excel in terms of growth and energy consumption in comparison with hatchery fish of the same cohort (Davis et al., in press). Evidence for differential habitat use suggests that there are undetected ontogenetic aspects that reduce growth rates for hatchery stocks after they are released (Goertler et al.…”

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

“…Collective monitoring efforts in the Nisqually River Delta have shown that salmon habitat use and growth potential are affected by changes in biophysical and morphological characteristics in restored tidal channels (David et al ; Ellings et al ; Davis et al ). Our findings expand upon these results by demonstrating the added value of restoration in enhancing foraging capacity for multiple salmon species.…”