Trophoscapes of predatory fish reveal biogeographic structuring of spatial dietary overlap and inform fisheries bycatch patterns

Abstract

Trophic interactions are proximate drivers of ecosystem function, including predator-prey dynamics, and their spatio-temporal variability may reflect ecosystem shifts and changes in trophic transfer. We investigated biogeographic structuring of trophic interactions by analyzing multi-decadal time series of diet for Pacific hake Merluccius productus and Chinook salmon Oncorhynchus tshawytscha from a large marine ecosystem. We compared our predictions for spatio-temporal variability of hake and salmon trophoscapes (i.e. spatially explicit predictions of trophic relationships) to inform ecosystem dynamics and fishery bycatch patterns. We have 3 inter-related findings pertaining to the spatial coherence of the trophoscapes and the potential consequences to juvenile and sub-adult (i.e. after the first year at sea but prior to maturation) salmon when sharing foraging areas with Pacific hake. First, the spatial scale of Pacific hake diet represents coastwide variability, and the spatial variability of Chinook salmon diets differs across regions and demonstrates a broad diet. Second, the expectation for increased diet and spatial overlap of Pacific hake and Chinook salmon during low productivity periods (e.g. periods with low krill biomass, suboptimal upwelling) can inform fishery management challenges. In this regard, we explore the role of shared foraging habitats on increased predation, and consequentially reduced recruitment, by Pacific hake on juvenile salmon during sub-optimal upwelling conditions. Third, we show that above-average bycatch of sub-adult Chinook salmon was associated with later spring transition, potentially as a result of both Pacific hake and salmon sharing foraging areas and prey species on the shelf and shelf break.

Size‐selective predation effects on juvenile Chinook salmon cohort survival off Central California evaluated with an individual‐based model

“…While juveniles experience their greatest rate of growth during this period, growth and condition are dependent on environmental variables such as the onset of upwelling (Fiechter et al, 2015), food availability (MacFarlane, 2010; Sabal et al, 2020;Wells et al, , 2016, and temperature-dependent effects on bioenergetics (Daly & Brodeur, 2015). In this region, krill are indicative of the juvenile salmon forage base (MacFarlane & Norton, 2002, 2023, and the condition of juvenile Chinook in the GoF is associated with the availability and spatial distribution of krill (Fiechter et al, 2015;Henderson et al, 2019;. For instance, above-average years for juvenile growth and survival at sea have been linked to early season upwelling driving high krill abundances and good prey retention in nearshore waters where juvenile Chinook salmon enter the ocean (Fiechter et al, 2015;Wells et al, , 2016.…”