How relative size and abundance structures the relationship between size and individual growth in an ontogenetically piscivorous fish

Abstract

While individual growth ultimately reflects the quality and quantity of food resources, intra and interspecific interactions for these resources, as well as individual size, may have dramatic impacts on growth opportunity. Out‐migrating anadromous salmonids make rapid transitions between habitat types resulting in large pulses of individuals into a given location over a short period, which may have significant impact on demand for local resources. We evaluated the spatial and temporal variation in IGF‐1 concentrations (a proxy for growth rate) and the relationship between size and concentration for juvenile Chinook salmon in Puget Sound, WA, USA, as a function of the relative size and abundance of both Chinook salmon and Pacific herring, a species which commonly co‐occurs with salmonids in nearshore marine habitats. The abundance of Chinook salmon and Pacific herring varied substantially among the sub‐basins as function of outmigration timing and spawn timing, respectively, while size varied systematically and consistently for both species. Mean IGF‐1 concentrations were different among sub‐basins, although patterns were not consistent through time. In general, size was positively correlated with IGF‐1 concentration, although the slope of the relationship was considerably higher where Pacific herring were more abundant than Chinook salmon; specifically where smaller individual herring, relative to Chinook salmon, were more abundant. Where Pacific herring were less abundant than Chinook salmon, IGF‐1 concentrations among small and large Chinook salmon were more variable and showed no consistent increase for larger individuals. The noticeable positive effect of relative Pacific herring abundance on the relationship between size and individual growth rates likely represents a shift to predation based on increased IGF‐1 concentrations for individual Chinook salmon that are large enough to incorporate fish into their diet and co‐occur with the highest abundances of Pacific herring.

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

“…; Chamberlin et al. ), were scarcely observed in the Nisqually diet samples except for in the offshore area in May. Consequently, this was the only month during which modeled GRP in the offshore habitat reached values that were equivalent to that in the tidal marshes, regardless of P cmax .…”

“…Although plankton and aquatic macroinvertebrates can be highly abundant in nearshore and offshore areas, piscivory is believed to bolster salmonid growth by providing a large, high-energy alternative to smaller crustaceans such as amphipods and crab larvae (Brodeur 1991;Daly et al 2009;Litz et al 2017). Forage fish, which are a major source of prey for some Puget Sound Chinook Salmon as they move to the offshore (Duffy et al 2010;Chamberlin et al 2017), were scarcely observed in the Nisqually diet samples except for in the offshore area in May. Consequently, this was the only month during which modeled GRP in the offshore habitat reached values that were equivalent to that in the tidal marshes, regardless of P cmax .…”

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

“…A more thorough analysis relating Puget Sound juvenile salmon diets to the historic seasonal prevalence of forage fish and fish larvae would be needed to determine the relationship between piscivory and prey availability. Such an analysis would represent a pivotal scientific contribution to the salmonid literature, as there is currently little historic data to draw upon in the Pacific Northwest (but see Chamberlin et al ).…”

“…(Brodeur 1991), but digested fish were detected in only 2 of our 244 stomach samples. We suspect that the lack of piscivory is indicative of reduced availability of fish larvae and small forage fish in the Nisqually River delta, since juvenile Chinook Salmon as small as 75 mm have been known to consume other fish (Daly et al 2009; Chamberlin et al 2017).…”

Ecological implications of changing hatchery practices for Chinook salmon in the Salish Sea

“…These potential interactions will likely vary regionally and/or locally, as suggested by previous studies of juvenile Chinook in the Salish Sea (Chamberlin et al 2017). These potential interactions will likely vary regionally and/or locally, as suggested by previous studies of juvenile Chinook in the Salish Sea (Chamberlin et al 2017).…”