Abstract
Simultaneous high resolution sampling of predator behavior and habitat characteristics is often difficult to achieve despite its importance in understanding the foraging decisions and habitat use of predators. Here we tap into the biosonar system of Blainville's beaked whales, Mesoplodon densirostris, using sound and orientation recording tags to uncover prey-finding cues available to echolocating predators in the deep-sea. Echolocation sounds indicate where whales search and encounter prey, as well as the altitude of whales above the sea-floor and the density of organisms around them, providing a link between foraging activity and the bio-physical environment. Tagged whales (n = 9) hunted exclusively at depth, investing most of their search time either in the lower part of the deep scattering layer (DSL) or near the sea-floor with little diel change. At least 43% (420/974) of recorded prey-capture attempts were performed within the benthic boundary layer despite a wide range of dive depths, and many dives included both meso- and bentho-pelagic foraging. Blainville's beaked whales only initiate searching when already deep in the descent and encounter prey suitable for capture within 2 min of the start of echolocation, suggesting that these whales are accessing prey in reliable vertical strata. Moreover, these prey resources are sufficiently dense to feed the animals in what is effectively four hours of hunting per day enabling a strategy in which long dives to exploit numerous deep-prey with low nutritional value require protracted recovery periods (average 1.5 h) between dives. This apparent searching efficiency maybe aided by inhabiting steep undersea slopes with access to both the DSL and the sea-floor over small spatial scales. Aggregations of prey in these biotopes are located using biosonar-derived landmarks and represent stable and abundant resources for Blainville's beaked whales in the otherwise food-limited deep-ocean.
Highlights
Foraging animals must locate food resources that are often patchy, and that change in composition and density with time and space [1,2]
Most biomass below the photic layer concentrates in two vertical strata: the deep scattering layer (DSL) [7] and the benthic boundary layer (BBL) [8]
Species inhabiting the deeper part of the DSL tend to have reduced locomotion and perform limited or no vertical migrations [11,12] resulting in diel stability of a portion of the DSL
Summary
Foraging animals must locate food resources that are often patchy, and that change in composition and density with time and space [1,2]. Most biomass below the photic layer concentrates in two vertical strata: the deep scattering layer (DSL) [7] and the benthic boundary layer (BBL) [8]. The BBL is considered to extend from the sea-floor to some 200 m altitude above it [13]. It holds most of the biomass in abyssal waters (1000–3500 m depth) and comprises typically species with low locomotory capacity [14]
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