Diel vertical migration and central place foraging in benthic predators

Abstract

Diel vertical migration (DVM) is a widespread behaviour among many pelagic species, from zooplankton to sharks and has been widely studied in both marine and freshwater environments. Usually, DVM comprises repeated daily vertical movements through the water column, from shallower at night to deeper during the day. Consequently, DVM is perhaps unexpected in benthic predators. Nonetheless, DVM has been observed in benthic sharks and freshwater teleosts, where it comprises inshore-offshore migrations over the substrate. However, there is no clear evidence of this behaviour in large temperate benthic predators, such as skates. Here we present new observations of DVM in 4 species of skate (Raja brachyura, R. clavata, R. microocellata and R. montagui) that identify it as a general behaviour in this clade. Analysis of 89 depth recording archival tags yielded 674 clear DVM events where skates left daytime deeper waters for shallower nighttime areas before returning to within 2.5 m of starting depths. Interestingly, these events closely resemble those of central place foragers, where shallow areas are foraging and deeper areas are refuging locations. Behaviour such as this has not been previously recorded in marine benthic predators, and the findings suggest that DVM might occur in many other benthic species. A broader understanding of DVM in benthic animals will be important in the design of effective boundaries for marine protected areas. These findings also have implications for trophic coupling between deep and shallow benthic zones. Further characteristics of this unexpected behaviour are presented and hypotheses for its occurrence are discussed.