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Biological oceanography

An arms race: Foraging Fish vs. Rorqual Whales

Reference: Cade, D.E., Carey, N., Domenici, P., Potvin, J. and Goldbogen, J.A., 2020. Predator-informed looming stimulus experiments reveal how large filter feeding whales capture highly maneuverable forage fish. Proceedings of the National Academy of Sciences, 117(1), pp.472-478.

DOI: 10.1073/pnas.1911099116

The evolution of filter feeding predators and their prey

Filter feeding baleen rorqual whales, like blue and humpback whales, evolved relatively recently, along with their feeding strategies. These large baleen whales evolved to engulf huge amounts of water to take advantage of changing ocean conditions ~ 500,000 years ago. This filter feeding strategy evolved around the same time ocean processes shifted to increased upwelling, or cold nutrient rich waters from the deep ocean, that move upward and fertilize the ocean surface. These areas of upwelling result in the growth of primary producers (phytoplankton) and krill, small animals that consume the microscopic photosynthesizers. As upwelling events started to occur more frequently, large whales were able to take advantage of an increase in krill by engulfing large numbers of these small animals and filtering water out using their unique baleen feeding structures.

Image of Baleen from a filter feeding baleen whale. Image credit: NOAA Fisheries – Southwest Fisheries Science Center (SWFSC).

Baleen feeding structures act like a comb; whales engulf water and then push the water back out through their baleen. Anything in the water that is too large to slip through the baleen remains in the whale’s mouth to eat. Many of these large whales feed on krill that are extremely small and survive by consuming large groups of these organisms.

Krill are small and move relatively slowly allowing larger and faster baleen whales to easily consume large quantities. However, these filter feeding whales can also consume small foraging fish, like anchovies. These foraging fish evolved more than 100 million years ago with fast swimming speeds and high maneuverability. Large filter feeding whales must swim fast enough and evade detection long enough to consume large quantities of these fish. One way these whales may overcome this difficulty is by slowing their approach speed and waiting to open their mouths to keep the prey fish unaware of their presence for as long as possible. When these whales open their mouth, the drag from water slows them down so the timing of their approach is critical. The authors of this study were interested in linking these two seemingly contradictory ideas: how can a predator which moves slowly when catching its prey manage to feed on small, fast and easily maneuverable fish?

Video of whale preying on schooling anchovies from video taken on the back of a humpback whale in a study by Cade et al. 2020).

How to study prey engulfment

The authors used three main phases during this study to understand the filter feeding behavior of rorqual whales: 1) Tag whales in the wild by attaching a transmitter with a suction cup to the top of the whale along with a video recorder to determine body size, swimming speed, and engulfment strategies. Many of these videos can be viewed online and show different feeding strategies for eating krill or anchovies. 2) After data on wild populations were collected, these data were used in a laboratory setting to create the shape of an approaching predator and measure the response of anchovies, a small foraging fish. These tanks were set up by covering all but one side to obscure the view outside the tank. One side of the tank was set up to show a video of the shape of an approaching whale. The whale body shape was approximated to an oval, which expanded in size at different rates until the final approach where the size drastically grew to approximate the shape of an engulfment or opening mouth. Videos were taken from above and the front so full a view of the predator approach (expanding oval) and prey avoidance by the anchovy were captured (video showing experimental tanks). Finally, after using data from whales in the wild and from lab experiments, models were created to determine how the swimming speed of the whale, opening of the whale mouth, and prey response occurs.

Image showing the experimental design by Cade et al. (2020). The numerals on the left side of the image show the approximated shape of the approaching predator linked with a dashed line to the feeding behavior. On the top right side of the image, the fish response is shown in two still shots from the side and top of the tank. The bottom right shows the tank used for experiments with a camera on one side and the top of the tank.

How does it work?

These authors discovered that rorqual whales have unique feeding behaviors when consuming anchovies vs. krill. While eating krill their speed and time to mouth opening are less critical for effective consumption of prey and therefore speeds were slower. On the other hand, consumption of anchovies must be properly timed to result in efficient feeding. These whales were described as using a ‘hide in plain sight’ strategy where they were inconspicuous against the background until they opened their mouth. Meanwhile the speed these whales obtained before opening their mouth must be fast enough to catch foraging prey. Whales that timed their mouth opening with the point of their peak speed were more likely to efficiently catch prey (~60% of a school).

The authors also found that using this method of lunge feeding (quickly opening the mouth to engulf prey) was seven times more energy efficient, even though it is more costly to move quickly enough to engulf fish than it is to feed on krill. Interestingly, the foraging fish have adapted to avoiding smaller individual predators by schooling. Schooling behavior is taken advantage of by more recently evolved lunge feeding rorqual whales that can engulf 30-60% of prey. The strategies of the filter feeding whale allow more flexibility in diet and potentially more energy efficient feeding strategies.

What does this mean for filter feeding whales?

Large filter feeding baleen whales have evolved to take advantage of large groups of prey, such as swarms of krill or schools of fish. Ironically, even though schooling strategies evolved to prevent predation by appearing bigger to a potential predator, these schools also make the perfect meal for large filter feeders. By successfully consuming many different types of prey (krill and foraging fish), rorqual whales are better able to deal with changing ocean conditions. These whales don’t rely on just one organism to survive but can feed on several types of prey with different movement strategies. Therefore, even amidst continued global change, the future looks hopeful for these resourceful hunters.

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