No room for dessert: A mechanistic model of prey selection in gut‐limited predatory fish

Abstract

Predatory fish structure communities through prey pursuit and consumption and, in many marine systems, the gadoids are particularly important. These predators have flexible feeding behaviours and often feed on large prey items. Digestion times of large prey are usually longer than handling times, and gut processing limits feeding rate at high prey density. Optimizing the gut content mix can therefore be an important behavioural strategy. Here, we develop a foraging model that incorporates gut processing and use the model to disentangle internal and external limitations on feeding in the omnivorous cod (Gadus morhua, Gadidae). We extend the traditional definition of prey profitability to consider prey digestive quality, which we quantify for prey of Northeast Atlantic cod populations. We find an important role for gut limitation; within a range of ecologically relevant temperatures and prey densities, predicted feeding rates were strongly reduced compared to feeding constrained by external factors only, and the optimal diet composition under gut limitation differed from predictions from traditional foraging theory. Capelin, a main prey of Northeast Arctic cod, had the highest digestive quality of all prey across ecosystems, but the cold temperatures in the Barents Sea strongly limited feeding rate by slowing down digestion. Baltic cod fed on a higher proportion of poor‐quality prey compared to the other populations, contributing to its slow growth in relation to water temperature. Gut limitation is particularly important to consider in foraging models for fish with many alternative prey species or fish occupying cold waters where digestion is slow.

Habitat complexity and individual variation in diet and morphology of a fish species associated with macrophytes

“…Accordingly, there may be phenotypic plasticity to exploit a greater amount of resources, but individuals from the stands with intermediate biomass seem to invest in the consumption of more energy-efficient resources than in a wide variety of prey, as predicted by the Optimal Foraging Theory. Accordingly, the consumption of Diptera pupae and aquatic plants, for example, may provide the greatest nutritional benefit because they are easy to access (Fall & Fiksen, 2019).…”

Buried in the sand: Uncovering the ecological roles and importance of rays

“…Specifically, we need to know: (a) predator–prey interaction strength (Heupel et al., 2014; Roff et al., 2016); (b) prey profitability (i.e. the energetic value of a prey item once prey digestive quality and time to chase, capture and handle prey have been considered; Fall & Fiksen, 2020; MacArthur & Pianka, 1966; Sih & Christensen, 2001), which is influenced by a variety of predator–prey tactics (Heithaus, 2004); and (3) the dispersal capabilities of both predator and prey.…”

Predator–prey overlap in three dimensions: cod benefit from capelin coming near the seafloor

“…2 and Skaret et al 2020). However, capelin vertical migrations are diurnal, while it takes days for cod to digest a stomach full of capelin in the cold waters of the Barents Sea (Fall and Fiksen 2020). Hence, we are not able to reliably detect feeding periodicity Figure 1.…”

“…a loglinear relationship is non-linear at raw scale), or by averaging consumption across individuals in a sample. Due to the slow digestion times in the cold waters of the Barents Sea (Fall and Fiksen 2020), we hypothesise that the functional response fitted to individual-level data reaches saturation at high capelin density.…”

“…In the Barents Sea, a North Atlantic shelf sea bordering the Arctic Ocean, cod is an important predator on the pelagic schooling fish capelin Mallotus villosus. While capelin is the most profitable prey for cod in this region (Fall and Fiksen 2020), the population-level diet also includes many other prey species (Holt et al 2019). Cod diet changes with size (Dolgov et al 2011, Holt et al 2019, but the influence of other biological and physical factors on diet variation has not been quantified.…”