Risso's dolphins perform spin dives to target deep-dwelling prey

Abstract

Foraging decisions of deep-diving cetaceans can provide fundamental insight into food web dynamics of the deep pelagic ocean. Cetacean optimal foraging entails a tight balance between oxygen-conserving dive strategies and access to deep-dwelling prey of sufficient energetic reward. Risso's dolphins ( Grampus griseus ) displayed a thus far unknown dive strategy, which we termed the spin dive. Dives started with intense stroking and right-sided lateral rotation. This remarkable behaviour resulted in a rapid descent. By tracking the fine-scale foraging behaviour of seven tagged individuals, matched with prey layer recordings, we tested the hypothesis that spin dives are foraging dives targeting deep-dwelling prey. Hunting depth traced the diel movement of the deep scattering layer, a dense aggregation of prey, that resides deep during the day and near-surface at night. Individuals shifted their foraging strategy from deep spin dives to shallow non-spin dives around dusk. Spin dives were significantly faster, steeper and deeper than non-spin dives, effectively minimizing transit time to bountiful mesopelagic prey, and were focused on periods when the migratory prey might be easier to catch. Hence, whereas Risso's dolphins were mostly shallow, nocturnal foragers, their spin dives enabled extended and rewarding diurnal foraging on deep-dwelling prey.

Sowerby's beaked whale biosonar and movement strategy indicate deep-sea foraging niche differentiation in mesoplodont whales

“…The high speeds are a result of locomotor strokes which are ∼50% faster than those of the slightly smaller Blainville's beaked whales. Both tagged Sowerby's beaked whales targeted prey resources over a broad depth range, in -shaped dives that are similar to those of pilot whales and Risso's dolphins ( Grampus griseus ), both high-speed mesopelagic foragers ( Aguilar de Soto et al, 2008 ; Visser et al, 2021b ). This hints at selective foraging on a broadly distributed energetic fauna.…”

“…The development of echolocation and breath-hold diving capabilities has enabled the radiation of toothed whales into a broad range of marine and aquatic niches, including the deep sea, where they tap into a stable, diverse and abundant fauna ( Johnson et al, 2004 ; Kooyman, 2009 ; Ramirez-Llodra et al, 2010 ; Visser et al, 2021a , b ). But despite targeting broadly similar fish and squid resources, some cosmopolitan deep-foraging toothed whales have very different diving and foraging styles ( Visser et al, 2021a ).…”

Movements and diving behavior of Risso’s dolphins in the Southern California Bight

“…Described as dynamic foragers, Risso's target different prey layers, switching between shallow water generalist and deep water specialist strategies (Benoit-Bird et al, 2019) and appear to consider optimal energy investments and tradeoffs prior to the next dive (Arranz et al, 2018;Visser et al, 2021). Jumbo squid are at depths > 250 m during the day, travel to near-surface waters at dusk, and then demonstrate highly variable dive behavior until they presumably find a zone to exploit for food (Gilly et al, 2006).…”

“…In Hawai'i, the mesopelagic layer migrates from the 400-700 m depth during daylight hours to 0-400 m at night, though pilot whales appear to target the deeper 400 m layer at night, potentially in search of higher caloric prey (Owen et al, 2019). In the Azores, Risso's target two deep scattering layers, a broad one at 500-700 m and a narrow one at 400 m, with both layers performing diel vertical migration (Visser et al, 2021). Off Santa Catalina Island, Risso's were detected in three layers: a deep static layer at 450 m, a mid-water migrating layer at 100-300 m, and an intermittent shallow layer at 50 m along with scattered patches consisting of fish and squid (Benoit-Bird et al, 2019).…”

“…They regularly switch predation tactics between a nearsurface generalist and deep-water squid specialist even within a single dive (Benoit-Bird et al, 2019). They plan foraging dives based on knowledge attained on previous dives (Arranz et al, 2018, Visser et al, 2021 and use a spin dive strategy to decrease transit time to exploit the diurnal mesopelagic layer (Visser et al, 2021).…”