A giant humpback whale, attacking small fish at speeds about as fast as a person jogs, is able to eat enough fish to sustain itself for a while. Based on previous field studies, laboratory experiments and mathematical modeling, researchers at Stanford University have found a surprising answer to this seemingly paradoxical feat -- Whales capture fish using stealth and deception.
The work also derived the first quantitative estimates of how many fish humpbacks consume in a single feeding event and it will provide future conservation efforts. "Lunge-feeding whales need dense concentrations of prey to forage effectively, yet fish schools could easily disperse and render lunge-feeding ineffective if they sensed a threat," said David Cade, lead author of the study.
Like other animals, humpbacks engage in lunge-feeding. This means they lunge after prey, take in a volume of water that can be larger than their own body (thanks to expandable oral cavities) and then filter out the excess water before gulping down their catch.
Opening the mouth costly
Opening the mouth, then, is hydrodynamically costly - like opening a parachute at high speeds, and feeding on fish requires the whales to time their lunging in ways that can be energetically costly. However, these costs are outweighed by the high energetic gains from captured prey. Researchers estimate that, for humpbacks, stealthy fish feeding is seven times more energetically efficient per lunge than feeding on krill.
The study, published in Proceedings of the National Academy of Sciences, made use of off recordings from whale-mounted video tags that the researchers deployed in Monterey Bay and Southern California made and used results from these experiments to predict how many fish would escape from an oncoming whale based on their reaction times.
Predator data developed
"One of the innovations of this study was to use predator data to inform the models we played back to fish," said Cade, who was a graduate student in the lab of Jeremy Goldbogen, assistant professor of biology at Stanford, during this research.
They found the range at which a fish responds to an oncoming predator. All fish "passed nearly simultaneously at a point when the whale opens its mouth, suggesting that by precisely timing its engulfment, the whale can avoid triggering escape responses."
The fish have been evolving to avoid being eaten by smaller predators for at least 100 million years, but lunge-feeding is a relatively new feeding strategy, evolutionarily, said Cade.
