During a literature search for some dissertation-related stuff I stumbled upon quite possibly the most awesome paper ever for a shark fan. It’s well-known that sharks, especially large, migratory species, supplement their diet of fish and sea mammals by scavenging on whale carcasses. It’s been theorized that whale carcasses are a very important food source for migrating sharks, but it seems like a relatively unreliable food scource: most large sharks don’t hunt cooperatively and even a 20-foot great white is highly unlikely to take down a 40-foot whale, so the sharks have to wait for something else to kill the whale. Or not. Apparently some sharks have taken matters into their own jaws and rather than waiting for whale carcasses, they create them.
Taylor et al. (2012) documented four confirmed attacks, three of them fatal, on Northern right whale calves by white sharks along the U.S. east coast. These potential whale killers are members of my favorite white shark population. It should be noted that none of these attacks were directly observed (the ocean is a big place, even for animals as large as great whites and right whales) and had to be inferred using rather clever means. For other large, mobile, mammalian prey, predation rate by sharks is estimated by the appearance of bite scars on the individuals that got away (this method had been used to estimate the risk of shark attacks on dolphins). Sharks, especially great whites, have very distinctive bites. The stereotypical half-moon shape seen on plenty of surfboards during Shark Week specials are also found on young whales, and are easily distinguished from orca bites, which look more like rows of straight lines (from the orca’ teeth raking across the whale’s flesh… gruesome). At least four right whale calves have been found with those “surfboard bites” on their flanks and tails.
Bites on live whale are obvious proof that sharks are willing to attack young right whales while they’re still alive. Granted, an attack on a vulnerable, inexperienced calf is not quite as badass as an attack on a fully-grown adult, but considering that right whales are born around the average size at maturity for great whites (10-15 feet), these sharks are making solo attacks on prey that is the same size as them or possibly larger. Never let it be said that white sharks are cowards.
Only one case study analyzed by Taylor et al. (2012) involved a still-living whale. On a whale found dead, it can be tricky distinguishing the fatal bites made while the whale was alive from the bites of the army of sharks and other creatures scavenging the carcass. In one case, a necropsy revealed no other trauma or possible causes of death other than a deep shark bite on the tail that severed a major artery. The other two cases involved a combination of entanglement in fishing gear and shark attack, but at least a few of the bites on these unfortunate whales occurred before they were dead. But how did the researchers tell which bites occurred before death?
The answer lies in a method that would make Al proud. Like many large marine creatures, right whales are absolutely crawling with parasites. One in particular, the orange cyamid (a species of whale louse), colonizes wounds very quickly. The amount of louse coverage can give an estimation of when the wound occurred. In the three fatal cases at least a few of the shark bites had louse coverage that suggests the bites occurred while the whale was still kicking. This is an awesomely gross and effective method, analogous to the use of maggots to estimate time of death on land.
So aside from the overwhelming badassery of sharks attacking large whales, there are some rather serious implications of the findings of this paper. The most obvious is that North Atlantic right whales are among the most critically endangered marine species on earth. Whaling hit this species hard, and in the early-mid 20th century the population may have been below 100 individuals. The right whale population has shown evidence of recovery in the last few years, with a record 39 calves born in 2008-2009 (yes, this species is so endangered that almost 40 births is a huge deal). Two of the four cases analyzed by Taylor et al. (2012) occurred during this period, meaning that at least 5% of the right whale calf population was subject to shark attacks. This may seem like a bad thing, but sharks and other predators tend to preferentially attack prey that is abundant. An increase in shark attacks on whale calves is actually a good sign, since it means there are now enough calves being born to represent a worthwhile food source for their predators.
Why right whales? Right whales were named by whalers for being the “right” whale to hunt due to their behavior: they tend to stay on the surface, move slowly, and float when dead. Behavior also makes right whale calves the “right” whale for sharks to hunt: unlike other whales such as humpbacks that travel in large pods, mother and calf right whales travel in pairs, meaning there is only one angry adult for a shark to get past to get to the calf. The authors theorize that most shark attacks on whales occur at or shortly after birth, when the calf is most vulnerable and inexperienced and the mother is somewhat incapacitated by, well, giving birth. Never let it be said that white sharks are dumb.
This paper is hot off the presses (as far as I know, this blog might be the first coverage it’s gotten) but it presents evidence of a predatory relationship with huge implications for both whale and shark conservation. That said, this paper, unlike other papers on shark predation, doesn’t pretend to cover more ground than it does. It doesn’t need to. Here we have evidence that Carcharodon carcharias may still have a little bit of Carcharodon megalodon in it.
Taylor, J., Mandelman, J., McLellan, W., Moore, M., Skomal, G., Rotstein, D., & Kraus, S. (2012). Shark predation on North Atlantic right whales (Eubalaena glacialis) in the southeastern United States calving ground Marine Mammal Science DOI: 10.1111/j.1748-7692.2011.00542.x