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

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.

Multiscale relationships between humpback whales and forage species hotspots within a large marine ecosystem

“…As spatial scales increased, some models became nonlinear or decreased as hotspots increased. The largest concentrations of predators are not always associated with the largest concentrations of prey (e.g., Benoit‐Bird et al, 2013; Boyd et al, 2020; Fall et al, 2021; Hammond et al, 2013), and at certain levels of prey density, functional response curves level off (Mehlum et al, 1999). This suggests a potential density‐dependent relationship with the number of prey hotspots in an area limiting the number of predators that can exploit prey at any given time.…”

Foraging movements of humpback whales relate to the lateral and vertical distribution of capelin in the Barents Sea

“…Overall, the negative relationship between light intensity and whale move persistence suggests that whales display area-restricted foraging behaviour at higher sun intensities. This could reflect the whales diving deeper during the day to reach the capelin that migrate to the deep when light intensities are stronger (Dalpadado and Mowbray, 2013;Skaret et al, 2020;Fall et al,…”

“…During summer months, shoals of capelin migrate to central and northern areas of the Barents Sea to feed, primarily on copepods and krill (Dalpadado et al, 2012;Dalpadado and Mowbray, 2013). While in these summer feeding grounds, capelin undertake diel vertical migrations, with a tendency to aggregate at deeper depths during the day and disperse towards the surface at night (Dalpadado and Mowbray, 2013;Skaret et al, 2020Fall et al, 2021. This pattern is believed to be linked to variations in light intensity.…”

Visual predation risk and spatial distributions of large Arctic copepods along gradients of sea ice and bottom depth

“…Foraging aggregations over shallow topographies may attract larger predators that profit from the same mechanisms of topographic blockage. In the Barents Sea, cod ( Gadus morhua ) benefit from capelin coming near the seafloor (Fall et al 2021), and studies from Newfoundland show that cod have a higher amount of capelin in stomach samples from shallower areas (Fahrig et al 1993). Shallow banks in the northern Barents Sea are also central foraging grounds for cetaceans feeding on capelin and other prey (Skern‐Mauritzen et al 2011).…”