Friedlaender, A. S., Goldbogen, J. A., Nowacek, D. P., Read, A. J., Johnston, D., & Gales, N. (2014). Feeding rates and under-ice foraging strategies of the smallest lunge filter feeder, the Antarctic minke whale (Balaenoptera bonaerensis). The Journal of experimental biology, 217(16), 2851-2854.
Body size and feeding strategy are two important traits that determine a whale’s foraging efficiency. A larger body size typically confers advantages in terms of foraging efficiency and diving capacity (longer dives) but also decreased maneuverability and acceleration.
One fundamental rule is that the energetic costs of finding and catching prey must be balanced or exceeded by the energy benefit of prey consumed. The same idea behind exercise and weight loss except, whales do not want to lose weight.
Whale feeding is most effective when prey, Antarctic krill (a type of small, swimming crustacean; Fig. 2), is closely aggregated. Because tight krill clusters typically occur at deeper depths, the rate at which large baleen whales (e.g., Humpback) are able to feed should be limited by their capacity to dive for long periods of time. Several species of baleen whale use a lunge feeding strategy that involves intermittently swallowing large volumes of prey. Each lunge uses a lot of energy. And larger whales have exponentially greater (many times more) energetic costs per lunge than smaller whales.
Antarctic minke whales are the smallest lunge-feeding baleen whales and are found abundantly in the Southern ocean. They provide a unique test for the exploring how foraging strategies manifest in smaller whales.
To monitor the depth, duration and number of lunges (feeding attempts) per dive, scientists used suction cup tags (not cheap, for those interested) attached to the backs (Fig. 1) of two separate whales in February 2013. The tags stayed on the whales for 18h and 8h respectively. After recording data and releasing from the whales, the tags were collected from the water and the data uploaded from each tag.
Results and Significance
The two tags recorded a total of 2831 feeding lunges and 649 foraging dives. Average dive depth was 18m (+/-0.5m) and average duration was 1.4minutes (+/-0.4min). Maximum depths reach 105m and the longest dives were about 7.2minutes.
The whales exhibited three distinct types of foraging dive: short surface dives (Fig.3C), long shallow dives (Fig. 3D), and long deep dives (Fig. 3E). Shallow dives accounted for 73% of all dives but just 34% of lunges. This is because the deeper dives had a much greater number of lunges. This makes sense because diving deeper requires much more energy, so whales need to make deep dives worth the energy costs by consuming large amounts of krill.
Compared to other, larger baleen whales, the Antarctic minke whale appears to exhibit much shallower dives with extremely high feeding rates (Fig. 4). It is possible that the minke whales, being smaller and more maneuverable, tend to feed under sea ice where krill aggregate. This may also contribute to resource partitioning (species using different portions of the same resource) between minke and larger baleen whales that dive deeper for krill (Friedlaender et al. 2009).
Although minke whales are found abundantly in all major oceans, there has been little study of their behavior. Recent advances in tagging technology made this study possible. The authors of the study suggest future research efforts should address how whale foraging strategies may adapt to changes in prey type and density to develop a better understanding of the connections between feeding mechanics and ecological roles (e.g., depth partitioning of krill between minke and humpback whales).