The right prey for right whales
Sometimes, in order to better understand and protect something very big, you need to focus on something very small. So was the case for a group of scientists from Wood’s Hole Oceanographic Institute and the National Marine Mammal Laboratory who spent two seasons in the Bering Sea investigating the prey, and the factors that amass that prey, of one of the world’s most endangered cetaceans: the North Pacific right whale. Their new paper was published last month in the journal Marine Ecology Progress Series (find it here).
North Pacific right whales made the news earlier this year when the first one was spotted in British Columbia in over 60 years by researchers with Fisheries and Oceans Canada. (You can read more about this amazing encounter here.) What made this sighting so spectacular is that right whales are critically endangered. This once abundant whale was hit hard by whaling in the 17th and 20th centuries. As a result, some scientists estimate there may be only 30 or so individuals left in the world.
Despite their dire situation, little is known about the factors that drive the distribution of these last remaining right whales. How and why do they choose to use certain habitats? To better understand these questions would allow a more comprehensive view on right whale ecology and how this imperiled species may react to changes in climate and human-induced stressors.
To begin search for answers, the research team spent the summer of 2008 and 2009 in the southeastern Bering Sea- an area of known use by North Pacific right whales. With extraordinary luck, they were able to locate right whales during both seasons and used a variety of techniques to assess the prey, as well as the environmental factors that affect prey.
Right whales are baleen whales, but employ a different feeding tactic than most. They are “ram filter feeders” that skim along the top of the water with their mouths agape, collecting food in their enormous baleen (upwards of 2 m long!) as they swim through prey patches. As a result of this unusual feeding technique, right whales do not target schooling fish and large euphasiids (krill) like other baleen whales. Instead, they appear to focus on areas of high copepod density. Copepods are small animals in the crustacean family (the same family as crabs and shrimp). This research team found that in the southeast Bering Sea, the main prey for these animals are a calanoid copepod called Calanus marshallae. This zooplankton species is known for its high lipid (fat) content, making it an appropriately rich food source for enormous right whales.
As copepods must be highly concentrated for right whales to feed efficiently, the research team also wanted to investigate the environmental processes that help amass and retain these animals. What causes these copepods to clump and stay together? The paper’s authors believe that copepods most likely aggregate around phytodetritus re-suspended from the bottom. Phytodetritus is basically dead plant material from tiny phytoplankton that settles on the ocean bottom. The copepods graze on this for food in areas where it has been redistributed back to the surface. Ultimately, the appearance of endangered North Pacific right whales in the southeastern Bering Sea may be intimately tied to the amount of dead plant material available in the water column for copepods to feed upon.
Like we said, sometimes, in order to better understand and protect something very big, you need to focus on something very, very small.