Whale carcasses influence community and ecosystem processes at large spatial and temporal scales
Baleen whales include the largest animals ever to have existed on the planet, and they undertake some of the longest mammalian migrations, swimming up to 8,300 kilometers between high-latitude feeding areas and low-latitude breeding grounds. Humpback whales in the Northern Hemisphere, for example, migrate from feeding grounds along the coastal regions of the Gulf of Alaska to breeding grounds in Hawaii. In the Southern Hemisphere, they migrate from feeding grounds off Antarctica to breeding grounds off the coasts of Australia, South America, and the west coast of Africa. What are the ecological consequences of such an enormous undertaking? In this study, we examine the equator-ward transport of nutrients in the form of whale carcasses, placentas, and sloughing skin. In addition to this biomass, whales typically fast on their winter breeding grounds but continue to metabolize reserves, releasing nitrogen in the form of urea into typically oligotrophic tropical or subtropical waters. We used allometric scaling relationships, derived from fasting and lactating elephant seals, to explore the potential transport of nutrients between whale feeding and breeding grounds.
In areas such as Hawaii and Brazil, parturient female humpback whales transport and release thousands of metric tons of biomass in the form of placentas, sloughing skin, and the carcasses of dead neonates. Whale falls from adults represent enormous nutrient pulses to these oligotrophic areas. In addition to these nutrients, the carcasses also support hundreds of macrofaunal species, including 60 species that appear to be whale-fall obligates. Our estimates indicate that the hunting of whales, and resulting depletion of whale fall habitat, may have caused some of the first anthropogenic extinctions in the deep sea, before humans even discovered their existence. Whale-fall habitats declined in range (through serial depletion), size (through selecting for the largest species and individuals), and number (through overharvest). One of the biggest ecological consequences of this relentless hunt could have been on the breeding areas of these migratory animals, where whale carcasses provided important nutrient subsidies. As a result of protection efforts, many great whale populations are now recovering—restoring habitat and nutrient subsidies to areas where, until recently, they were absent. In addition to carcasses and placentas, the equator-ward transfer of nitrogen in the form of excreted urea forms a “great whale conveyer belt.” Before whaling, blue whales may have increased primary productivity by 15% in their lower-latitude breeding areas.