Into the deep: the functionality of mesopelagic excursions by an oceanic apex predator

Abstract

Comprehension of ecological processes in marine animals requires information regarding dynamic vertical habitat use. While many pelagic predators primarily associate with epipelagic waters, some species routinely dive beyond the deep scattering layer. Actuation for exploiting these aphotic habitats remains largely unknown. Recent telemetry data from oceanic whitetip sharks (Carcharhinus longimanus) in the Atlantic show a strong association with warm waters (>20°C) less than 200 m. Yet, individuals regularly exhibit excursions into the meso‐ and bathypelagic zone. In order to examine deep‐diving behavior in oceanic whitetip sharks, we physically recovered 16 pop‐up satellite archival tags and analyzed the high‐resolution depth and temperature data. Diving behavior was evaluated in the context of plausible functional behavior hypotheses including interactive behaviors, energy conservation, thermoregulation, navigation, and foraging. Mesopelagic excursions (n = 610) occurred throughout the entire migratory circuit in all individuals, with no indication of site specificity. Six depth‐versus‐time descent and ascent profiles were identified. Descent profile shapes showed little association with examined environmental variables. Contrastingly, ascent profile shapes were related to environmental factors and appear to represent unique behavioral responses to abiotic conditions present at the dive apex. However, environmental conditions may not be the sole factors influencing ascents, as ascent mode may be linked to intentional behaviors. While dive functionality remains unconfirmed, our study suggests that mesopelagic excursions relate to active foraging behavior or navigation. Dive timing, prey constituents, and dive shape support foraging as the most viable hypothesis for mesopelagic excursions, indicating that the oceanic whitetip shark may regularly survey extreme environments (deep depths, low temperatures) as a foraging strategy. At the apex of these deep‐water excursions, sharks exhibit a variable behavioral response, perhaps, indicating the presence or absence of prey.

Revealing the full ocean migration of individual Atlantic salmon

“…This development has provided novel Strøm et al Anim Biotelemetry (2018) 6:2 information for a variety of species, in addition to disentangling important concepts in behavioural ecology [e.g. 5,6]. For Atlantic salmon, a challenge has been to track the continuous horizontal migration of individuals in the open ocean.…”

Energy-Rich Mesopelagic Fishes Revealed as a Critical Prey Resource for a Deep-Diving Predator Using Quantitative Fatty Acid Signature Analysis

“…The main contributor to this biomass is found in the deep scattering layers of the mesopelagic (200-1,000 m) and the bathypelagic (1,000-4,000 m) depth zones (Sutton, 2013;Davison et al, 2015). The deep scattering layers are composed of dense aggregations of fishes, squids, and other micronekton (organisms 2-20 cm) that provide an abundant prey resource to those deep-diving predators able to access them (MacLeod et al, 2003;Harvey et al, 2013;Howey et al, 2016). Despite the ecological significance of the deep ocean, it has been understudied, with scant information for many species, communities, and life histories, as well as limited understanding of the complex trophic interactions occurring therein (Webb et al, 2010;St John et al, 2016).…”

Horizontal and vertical movements of Caribbean reef sharks ( Carcharhinus perezi ): conservation implications of limited migration in a marine sanctuary

“…The increased frequency of vertical excursions during the winter, compared to summer, could relate to food pulses associated with deepening of the mixed layer. Alternative hypotheses explaining the functionality of large vertical movements have been postulated for highly mobile sharks [47], such as navigation [63], energy conservation [64] and interactive behaviour [65]. Although such explanations cannot be discounted in this study, evidence for foraging-based movements appears most plausible, certainly for coastal shark species, and therefore alternative solutions are discussed no further.…”