Blue whales increase feeding rates at fine-scale ocean features

Abstract

Marine predators face the challenge of reliably finding prey that is patchily distributed in space and time. Predators make movement decisions at multiple spatial and temporal scales, yet we have a limited understanding of how habitat selection at multiple scales translates into foraging performance. In the ocean, there is mounting evidence that submesoscale (i.e. less than 100 km) processes drive the formation of dense prey patches that should hypothetically provide feeding hot spots and increase predator foraging success. Here, we integrated environmental remote-sensing with high-resolution animal-borne biologging data to evaluate submesoscale surface current features in relation to the habitat selection and foraging performance of blue whales in the California Current System. Our study revealed a consistent functional relationship in which blue whales disproportionately foraged within dynamic aggregative submesoscale features at both the regional and feeding site scales across seasons, regions and years. Moreover, we found that blue whale feeding rates increased in areas with stronger aggregative features, suggesting that these features indicate areas of higher prey density. The use of fine-scale, dynamic features by foraging blue whales underscores the need to take these features into account when designating critical habitat and may help inform strategies to mitigate the impacts of human activities for the species.

Submesoscale coupling of krill and whales revealed by aggregative Lagrangian coherent structures

“…ACCESS surveys overlapped with the sampling footprint of the US Integrated Ocean Observing System High Frequency (HF) Radar Network (IOOS HFRNet, [ 62 ]), which provides continuous, high-resolution measurements of ocean circulation and structure at fine and intermediate scales [ 63 , 64 ] In this study, HF Radar surface current vectors are used to calculate the backward-in-time, finite-time Lyapunov exponent (hereafter FTLE), which is a scalar measure of the rate of attraction of simulated particle tracers advected using empirically measured surface current flows. Ridges of elevated FTLE values identify Lagrangian coherent structures (LCS), which represent barriers to transport, such as fronts and eddies [ 65 ] and have been linked to krill predator movement and foraging behaviour in the same region [ 33 , 43 , 45 ].…”

“…R. Soc. B 291: 20232461 used in [45] for identifying Lagrangian habitat features. To evaluate the scale of ecological processes and dynamical features influencing the relationships in our study (e.g.…”

“…Recent research has delved into the relationships between pelagic predator habitat selection and surface current features (e.g. fronts and eddies), for pinnipeds [38], sharks [39,40], turtles [41], seabirds [37,42] and cetaceans [43][44][45]. These surface current features reflect processes that affect productivity through the transport of nutrient-rich water [46] and aggregation of organisms operating at low (10 −2 ) and intermediate (10 2 -10 3 ) Reynolds numbers through physical forcing [37].…”

“…We use hourly high-frequency (HF) radar surface current measurements to calculate a time-dependent Lagrangian modelled proxy that reflects coherent aggregative ocean transport features at submesoscales [ 51 , 52 ]. Blue whales in the CCS have been shown to increase feeding rates at aggregative surface current features [ 45 ], suggesting that these features may correspond to increased krill density, but the mechanism by which these features would influence krill distributions has not been explained. We hypothesize that the surface current features identified as important to krill predators will have (1) both a surface and subsurface oceanographic expression, and (2) either increased krill abundance or density.…”

Swimming with the current improves juvenile survival in southern elephant seals

“…For example, female southern elephant seals from Kerguelen typically swim downstream, to the east of the island, where they come across a greater number of eddies [65]. These oceanic features play a role in clustering prey into denser aggregations, vital for seals and other apex marine predators during their foraging excursions [65][66][67][68]. In contrast, juveniles that forage in areas with weak eddy activity face higher chances of mortality [6].…”