Following a Foraging Fish-Finder: Diel Habitat Use of Blainville's Beaked Whales Revealed by Echolocation

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.

Discrimination of fast click series produced by tagged Risso's dolphins (Grampus griseus) for echolocation or communication

“…A stable ICI during the search and approach phases has been interpreted by Madsen et al (2005) as an echolocation strategy to organize the acoustic input of echoes from multiple simultaneous targets, while maintaining a long range for acoustic detections. Grampus griseus and Blainville's beaked whales (Mesoplodon densirostris) have been reported to forage on deep-water bottomdwelling organisms, amongst other prey (Kruse et al, 1999;Blanco et al, 2006;Santos et al, 2007;Arranz et al, 2011). It is possible that both species may be feeding in comparable habitats, where stable ICIs would facilitate gathering concomitant biotic and abiotic cues to locate prey and to orient near the seabed.…”

“…Foraging buzzes have been associated with the deepest part of foraging dives in other odontocetes (Miller et al, 2004;Rasmussen et al, 2013). The shallowest maximum dive depth of dives (37 m) and the range of buzz depths (28 to 493 m) points at a potentially large variation in prey type or prey location in the water column, and relatively shallow foraging depth for Grampus with respect to other deep-diving odontocetes foraging at comparable latitudes Watwood et al, 2006;Aguilar Soto et al, 2008;Arranz et al, 2011). As 5% of buzzes were recorded at or near the surface (<20 m depth; Fig.…”

Gait switches in deep-diving beaked whales: biomechanical strategies for long-duration dives

“…At least two entire deep dives were recorded from each whale ( Table 1). As described elsewhere (Hooker and Baird, 1999;Baird et al, 2006;Tyack et al, 2006;Arranz et al, 2011), these beaked whales typically perform long and deep foraging dives (Md: means of 49 min and 844 m; Zc: 59 min and 1044 m; Ha: 49 min and 1572 m in the data analysed here). For Md and Zc, ascents from deep dives were about twice as long in duration as descents and were performed at proportionally lower vertical speeds and absolute pitch angles (Table 1).…”

“…In either case, recruitment of anaerobic muscles must necessitate a post-dive recovery interval to process lactate, and this may help explain the prolonged periods between deep dives reported for the beaked whale species studied here (Hooker and Baird, 1999;Tyack et al, 2006). Thus, this strategy comes at the cost of a lower time available for foraging: although many other factors influence diving rates, the proportion of time spent actively searching for prey on a daily basis by beaked whales [17-25% Arranz et al, 2011)] is well below that of sperm whales [53% (Watwood et al, 2006)]. …”

Amazon river dolphins (Inia geoffrensis) modify biosonar output level and directivity during prey interception in the wild

“…Tags allow researchers to obtain information on individual echolocation clicks over many hours under circumstances where array recordings are often unattainable (Madsen et al, 2002;Johnson et al, 2004) and, depending on tag placement and the species tagged, returning echoes from actively pursued prey may be recorded (Johnson et al, 2004;Arranz et al, 2011;Wisniewska et al, 2016). However, as echolocation clicks are highly directional (Au, 1993;Koblitz et al, 2012), tag recordings provide a highly distorted perspective on biosonar clicks from the tagged animal (Au et al, 2012) and therefore only allow for relative adjustments of source parameters .…”