Southern ocean study offers detailed data on foraging patterns
By Summit Voice
SUMMIT COUNTY —The deep-diving elephant seals of Marion Island, in the southwestern Indian Ocean, are going to even greater depths to find prey like squid, as global warming heats up the water.
Scientists with the Alfred Wegener Institute for Polar and Marine Research who have been tracking the pinnipeds for the past few years say that warming in the upper levels of the ocean has pushed prey to greater depths than ever before, forcing the elephant seals to follow.
“The food in the sea is unevenly distributed. It is not worth the seals fishing anywhere and at any time. With the new data we hope to see the routes taken by the elephant seals of Marion Island and the water levels in which they find food“, said researcher Horst Bornemann.
The elephant seals are very close to reaching their physiological limits even in their dive behavior today. This leads the biologists to assume that this may reduce the survival rate of the seals in the long term.
The study was a collaboration between the German institute and Universities of Pretoria and Cape Town.
The southern elephant seals from Marion Island are extreme divers in the truest sense of the word. The animals spend more than 65 per cent of their lives in depths below 100 meters, diving far deeper than their fellow species in southern areas.
The maximum dive depth of these seals is more 2,000 meters. However, the water masses through which the elephant seals from Marion Island swim in search of food are becoming increasingly warmer due to climate change and are forcing the animals to dive deeper.
The Southern Ocean is warmed primarily in the water levels up to a depth of 1,000 meters, in those areas in which squid and fish ought to be found.
“This prey is moving down to greater depths presumably due to the increasing water temperatures and this is forcing the seals to follow them“, Bornemann said.
Over the course of several years, Bornemann and. Joachim Plötz together with Dr. Trevor McIntyre and other seal researchers from the Mammal Research Institute in South Africa have fitted over 30 elephant seals with satellite transmitters. The fist-size transmitters are attached to the head of the seals using artificial resin immediately after moulting and measure the dive depth, water temperature and salinity every time the animals dive.
When the animal resurfaces to breathe the transmitters send their data to the respective research institutes via satellite. The results show that the elephant seals need to dive deeper in warmer water so that they ultimately have less time to actually search for food.
”We therefore assume that the animals will find less prey in warmer water masses“, said Plötz.
The scientists will be going back to Marion Island in April to collect evidence for their theory. This time they wish to equip the animals with a “jaw movement” sensor which has been developed by Japanese biologists at the National Institute of Polar Research in Tokyo. It is not much larger than a small finger and notices when the seal opens its mouth.
“So far we can only derive from the dive profile whether an elephant seal was probably following a fish swarm. With this new measuring device we learn whether he has actually eaten,” Plötz said.
Using this forage data, the AWI biologists wish to draw conclusions as to the spatial and temporal distribution of particularly productive zones in the South Polar Sea.
The scientists also have spent days walking over the ”island of horizontal rain“ to achieve their research goal.
“The elephant seals of Marion Island are very loyal to their location. They return to this island time and again to moult and mate. This behavior gives us the opportunity to consistently fit measuring devices to the same animals thereby gaining an insight into the movement patterns of individual animals. Their movement and dive routes help us to find out where the oceanic food grounds of the Marion Island elephant seals are located,” Plötz explained.
The extent to which the animals of this rather northerly elephant seal colony are able to adapt to the warming of the ocean remains to be seen. The scientists from Germany and South Africa see only two alternatives for the animals: either the seals extend their hunting grounds to the colder water masses of the Antarctic or they must dive even deeper in future.
Filed under: biodiversity, climate and weather, Environment, global warming, Marine biology, Summit County news Tagged: | climate, elephant seals, global warming, Indian Ocean, Marion Island, Southern elephant seal, Southern Ocean