Julie van der Hoop, Anna Nousek-McGregor, Douglas Nowacek, Susan Parks, Peter Tyack, Peter Madsen
Endangered North Atlantic right whales feed by swimming forwards with their mouths wide open – a strategy that allows them to capture large amounts of their small prey. However, this so-called “ram-feeding” strategy incurs considerable drag because of their mouth size and baleen. Right whales show morphological adaptations to their high-drag foraging mode, but do they also show biomechanical and behavioural adaptations, such as the low-speed, continuous swimming adopted by other ram-filter feeders? We still know very little about how a gigantic filter feeder with one of the highest predator-prey size ratios survives on a very specific resource that is constrained in both space and time.
We used multi-sensor bio-logging tags to ask the questions: How much prey-laden water is filtered by right whales over the course of a dive, and what information do they use, as breath-hold divers, to decide to forage or not?
We found that right whales do show similar swimming patterns as other ram-filter feeders, repeatedly pausing for ~3 seconds between ~50 second bouts of fluking while at the bottom of their 10-15 minute foraging dives in the Bay of Fundy, Canada. Though repeated, the timing of these bouts and pauses were not consistent, suggesting they may be extrinsically driven by prey density. During foraging dives, right whales descended at 1.4 m/s and slowed to 1.1 m/s while filtering. We suggest that, similar to observations in bowhead whales, right whales swim at consistently low speeds when foraging to reduce the drag of their open mouths. However, right whales only slow down by 25% (compared to 40% for bowhead whales), likely due to the smaller cross-sectional mouth area per body size; the faster swimming speed of right whales provides a higher filtration rate, but only partially compensates for the smaller mouth area. To acquire similar prey resources as bowheads, right whales must feed on higher density prey aggregations, on prey with higher energy density, and (or) for longer periods of time.