The oceans cover more than 70% of the Earth’s surface. Nevertheless, we probably know more about the surface of the moon than we do about the ocean floor despite the oceans’ enormous importance for climate change. Marine biologists are now trying to remedy this by equipping marine animals with high-resolution biologgers that measure light intensity, prey availability, and ocean temperature. A new study of elephant seals’ dives through 7 months of migrating 10,000 kilometres provides unprecedented insight into the life deep in the oceans that risks being affected by global climate change.
You only see them when they are on the beach for a few months, tending to their pups or moulting. The rest of the time, elephant seals (Mirounga angustirostris) spend 7 months in the ocean travelling to and from their foraging grounds off the coast of Alaska. However, observing their behaviour first-hand is very difficult because they spend most of their time underwater. High-resolution biologgers have recently enabled researchers to obtain new insight into both what the elephant seals experience and how the oceans affect them.
“Our new study provides fascinating and important knowledge about the diving behaviour of elephant seals as they explore the open ocean. But perhaps even more importantly, the recent development and use of instruments such as biologgers provides invaluable insight into species interactions in the marine ecosystem. We hope that this knowledge will help the public and politicians make environmentally conscious decisions,” explains Roxanne Beltran, Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, United States.
Diving requires a lot of energy
The marine biologists aimed to study the fascinating 7-month migration of elephant seals from their breeding grounds in California 10,000 kilometres north across the North Pacific, mostly underwater – only interrupted by short minute-long trips to the surface for air before the seals again forage in the twilight zone. The researchers monitored the seals in great detail using the biologgers.
“The high-resolution biologgers enabled us to continuously measure the fat percentage and 3D movements of 71 female elephant seals during their foraging migrations. After shedding all their fur during the annual moult, seals set off on this migration – thin and debilitated – and dive continuously to nearly 1 kilometre underwater,” explains Roxanne Beltran.
One reason elephant seals dive so deep and dive almost continuously throughout their 7-month trip is to avoid predators, including white sharks and killer whales, which tend to remain at the surface rather than in the dark and cold depths. As a result, the seals use a lot of energy diving into the deeper and colder water.
“Our measurements showed that seals always balance acquiring energy and ensuring safety, so they rest when it is dark and forage at depth when it is light. Throughout the migration, their fat percentage improved from 21% when they embarked on the migration to 32% at the end. As they gained more fat, they rested more than 100 metres deeper, where it was 300 times darker, so seals with a high fat percentage gave priority to safety over conserving energy,” says Roxanne Beltran.
To Hell and back again
The new study is not the first study using biologgers to measure both the swimming depths and fat percentages of marine animals. The innovation is that the researchers simultaneously monitored the activity timing of the mammals with great accuracy and especially the light intensity during the drift phase, in which they rest.
“The measurement took place over a long period of time and across thousands of kilometres, and was therefore far from easy. The statistical models needed to account for the fact that time is circular, meaning that 23:00 is equally close to 01:00 as it is to 22:00,” explains Roxanne Beltran.
The researchers used circular mixed-effects regression models to estimate how fat percentage, photoperiod and location contributed to the timing of rest. The marine biologists from the United States allied with a biostatistician, Jolien Cremers, from the Department of Public Health at the University of Copenhagen. The result was unprecedented insight into how marine mammals actively use the lightscape to determine whether to spend more or less time in the surface water, which is brighter and warmer but full of predators.
“We are not sure, but the seals are probably especially vulnerable during drift dives when they either sleep or forage for food. The female seals have just been to Hell and back again within months of suckling their pups and mating and nursing. They are at their absolute thinnest and need to regain their fat during the 7- month migration. Our results show that they constantly fine-tune their diving patterns to use the safety of darkness to rest deep below the surface,” says Roxanne Beltran.
90% of world trade is seaborne
The researchers hope that many other research groups can use the new method, since it enables more comprehensive understanding of the relationship between behaviour and risk, which is extremely important for all animals, especially when their habitats are changing.
“The deep water in which the elephant seals seek refuge from predators will probably soon change because there is great interest in extracting resources from these habitats, which have limited protection from fishing regulations. Now is the time to understand how physiology and ecology interact to shape the fundamental behaviours of marine animals,” explains Roxanne Beltran.
In the coming years, Roxanne Beltran and her colleagues hope to use the high-resolution biologgers for other purposes than measuring animal behaviour. By having many different marine animals carry the instruments, the researchers hope to map the oceans in terms of their noise, oxygen concentration, salinity and temperature.
“Many of my colleagues are actually quantifying global warming in 3D using temperature sensors attached to seals. We may also be able to use animal-borne video cameras to watch how species interact in the open sea. We can detect bioluminescence using light-sensitive tags. The possibilities are virtually endless, and leveraging new technologies can help us answer some really important questions about the oceans and how climate change will affect them,” says Roxanne Beltran.
As much as 90% of world trade is transported on ships that emit noise in the open sea. We do not yet know whether this noise affects animals, frightening them to behave differently. It is too early to say whether this knowledge will change human behaviour, but Roxanne Beltran is optimistic.
“Convincing people to protect something they cannot see and do not understand is difficult. Bringing this to people’s attention may help them to understand the possible consequences of their actions and hopefully motivate them to make different choices, whether buying things that are more sustainable or choosing to buy things that do not need to be transported quite as far,” explains Roxanne Beltran.
