Oceans are vast and home to many magnificent creatures, including colossal whales, which few of us have ever seen in real life but encounter frequently on our screens. Rorquals are a family of large whales that have a unique way of dining: they lunge forward, filling their mouths with water and food before filtering out the water and swallowing what remains. One species, Rice's whale (Balaenoptera ricei), is found only in the Gulf of Mexico, with a population of less than 100. Little is known about their eating habits and how they use energy when moving and searching for food, but this knowledge is essential for us to identify possible threats, such as entanglement in fishing gear, which could be modified to preserve the endangered species. This motivated a collaborative team of scientists from the Scripps Institution of Oceanography, USA, the University of California San Diego, USA, and the National Marine Fisheries Service, USA, to collect information about the lifestyle of two Rice's whales in their natural habitat.
Annebelle Kok (Scripps Institution of Oceanography) with six colleagues named the two animals Milky Way and Edna and collected information about their maneuvers using suction-cup tags, which recorded the whales’ movements, water pressure and sound of the water rushing past as they moved over several years. The team then used the pressure information to determine how deep the whales were diving and the movement information indicated how the animals changed direction, either by moving their head upwards or rolling their bodies. Then, the team used the sound recordings to determine how often the whales breathed by identifying the sound of exhalation when the whales reached the surface. Finally, Kok and colleagues calculated how much energy the whales consumed when foraging by estimating each animal's mass, their acceleration, velocity and the time they spent diving, breaking down the time underwater into four major events: time descending, at the bottom of the dive, while lunge-feeding and returning to the surface.
From this, the team were able to identify how often Milky Way and Edna performed deep dives to feed, which told them what kinds of food the whales were dining on; fish tend to reside at depth and are more calorific than small animals, such as krill and zooplankton, which live nearer to the surface. The scientists determined that prior to feeding, the whales circled around their prey before speeding up to engulf a colossal mouthful of water containing a school of fish. Sometimes the whales almost reached the bottom of the sea when they embarked on their deepest dives, which they mainly undertook during the day, completing no more than two dives at night, when they mainly stayed close to the surface feeding off krill and zooplankton. According to Kok and the team, the two whales spent most of their energy when circling their prey and lunging to engulf as many fish as possible. Interestingly, although both mammals behaved similarly during the day and night, Edna dived more frequently and consumed more high-energy food than Milky Way did. Kok and colleagues think that these differences may arise because Milky Way was caring for a calf, thus requiring lower energy consumption, as previously reported, whereas Edna might be leaner with higher energy needs.
The ability to collect information about animals in the wild and learning in detail about their foraging patterns around the clock is important if we want to protect them from extinction. Kok and colleagues also argue that Rice's whales are in danger of injury by boats at night, in addition to being at risk of becoming entangled in bottom longline fishing gear, since fishers are also trying to collect the fish that these mammals are looking for.
