Freshwater Tidal Forests and Estuarine Wetlands May Confer Early Life Growth Advantages for Delta-Reared Chinook Salmon

Large river deltas are complex ecosystems that are believed to play a pivotal role in promoting the early marine growth and survival of threatened Chinook Salmon Oncorhynchus tshawytscha. We used a fish bioenergetics model to assess the functional role of multiple delta habitats across a gradient of salinities and vegetation types, where consumption and growth rate potential (GRP) were considered as proxies for habitat quality. We subsequently compared our model output to empirical, or realized, growth estimates from scale circuli. In terms of consumption, prey energy density (EDprey) was 46-86% higher in tidal freshwater forest than in any other habitat type, while estimated consumption rates (expressed as proportion of maximum daily consumption; P-cmax) were positively correlated with FL. These size-related differences in P-cmax led to a noticeable increase along a freshwater-to-saline gradient from roughly 0.25 in tidal freshwater forest to 0.55 in the offshore subtidal zone, yet despite higher observed P-cmax values in nearshore and offshore habitats, the tidal freshwater forest and emergent salt marsh demonstrated the highest modeled GRP values. Similarly, realized growth rates for fish caught in tidal freshwater forest were up to 0.5% higher per day than for fish caught in the offshore area, but habitat-level differences were overshadowed by allometry and rearing origin. Scales from unmarked fish (assumed to be of wild origin) indicated that they grew, on average, 11% faster than did hatchery fish; however, these differences were subtle and were more obvious at fork lengths <100 mm. Our results suggest that tidal forests and emergent marshes may offer early life growth advantages for wild Chinook Salmon, but that wild and hatchery fish can compensate as they move seaward by opportunistically consuming greater quantities of low-energy density prey, taking advantage of pulses of larval forage fish, or by spending time in multiple interconnected habitat types.