Abstract
Spatial overlap between predator and prey is a prerequisite for predation, but the degree of overlap is not necessarily proportional to prey consumption. This is because many of the behavioural processes that precede ingestion are non‐linear and depend on local prey densities. In aquatic environments, predators and prey distribute not only across a surface, but also vertically in the water column, adding another dimension to the interaction. Integrating and simplifying behavioural processes across space and time can lead to systematic biases in our inference about interaction strength. To recognise situations when this may occur, we must first understand processes underlying variation in prey consumption by individuals. Here we analysed the diet of a major predator in the Barents Sea, the Atlantic cod Gadus morhua, aiming to understand drivers of variation in cod's feeding on its main prey capelin Mallotus villosus. Cod and capelin only partly share habitats, as cod mainly reside near the seafloor and capelin inhabit the free water masses. We used data on stomach contents from ~2000 cod individuals and their surrounding environment collected over 12 years, testing hypotheses on biological and physical drivers of variation in cod's consumption of capelin, using generalized additive models. Specifically, effects of capelin abundance, capelin depth distribution, bottom depth and cod abundance on capelin consumption were evaluated at a resolution scale of 2 km. We found no indication of food competition as cod abundance had no effect on capelin consumption. Capelin abundance had small effects on consumption, while capelin depth distribution was important. Cod fed more intensively on capelin when capelin came close to the seafloor, especially at shallow banks and bank edges. Spatial overlap as an indicator for interaction strength needs to be evaluated in three dimensions instead of the conventional two when species are partly separated in the water column.
Highlights
Predation structures natural ecosystems (Schmitz et al 2017)
Foraging in predatory fish depends on a wide range of intrinsic and extrinsic factors such as light, temperature, bathymetry, local prey density and predator size that act on different spatial and temporal scales (Stefansson and Palsson 1997, Johannesen et al 2012, Lopez-Lopez et al 2015, Deroba 2018)
Capelin made up 59% of the total prey mass consumed by cod
Summary
Predation structures natural ecosystems (Schmitz et al 2017). At the scale of populations, the intensity of predation is often characterised by the average predator’s consumption of prey in relation to prey density, or the functional response (Holling 1959). To understand effects of lower-scale variation on the population-level functional response and potential biases introduced by aggregating across time and space, we must first identify the scales, patterns and drivers of variation in the feeding of individual predators (Englund and Cooper 2003, Railsback et al 2020). Foraging in predatory fish depends on a wide range of intrinsic and extrinsic factors such as light, temperature, bathymetry, local prey density and predator size that act on different spatial and temporal scales (Stefansson and Palsson 1997, Johannesen et al 2012, Lopez-Lopez et al 2015, Deroba 2018). Within regions at the scale of local habitats, feeding opportunity is affected by local bathymetries, water temperatures or fronts that either act directly on predator physiology or indirectly through effects on prey distributions (Sims et al 2003, Genin 2004, Ciannelli and Bailey 2005, SkernMauritzen et al 2011). Disentangling the effects of these diverse sources of variability at different spatial and temporal scales is a major challenge in the study of predator–prey interactions (Hunsicker et al 2011, Fouzai et al 2019)
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