Ocean warming has a negative impact on the condition of elephant seals, reveals a study published in the Open Access journal BMC Biology. High ocean temperatures observed from 1975 to the late 1990s are correlated with a 28% decrease in the weight of elephant seal pups. Elephant seals are shown to be sensitive to ocean temperature changes associated with both long-term 25-year cycles and short-term 3-4 year cycles such as those caused by El Niño.
Sea lions and fur seals that feed near their rookeries at the surface of the ocean are known to be very sensitive to water temperature changes, as an increase in water temperature usually causes their prey to migrate to cooler areas, depleting local food resources and resulting in pup starvation. Elephant seals, which feed in deep waters, were previously thought to be better buffered against ocean temperature changes than other sea mammals.
A study by Dr Burney Le Boeuf and Dr David Crocker from the University of California in Santa Cruz, shows that as waters get warmer the average weight of an elephant seal pup decreases. Weaning weights are a direct indicator of food availability to mother seals during their pregnancy. Because their prey has migrated to colder waters, female seals have to swim further and spend more time searching for food. As a result, they gain less body mass while pregnant and have fewer reserves to draw on during nursing. Nursing pups get all nourishment from their mothers; changes in ocean temperature therefore directly affect the weight, and the survival rate, of the pups.
To assess the impact of temperature changes on food availability to elephant seals, Le Boeuf and Crocker monitored the weight of 2750 elephant seal pups over a period of 29 years, from 1975 to 2004, during which the ocean temperature generally increased before starting to cool again in 2000. The pups, from a rookery located in central California, were weighed within 10 days of weaning. Le Boeuf and Crocker’s results show that the pups’ mean weaning weight declined from 146 kg in 1976 to 115 kg in 1999. This coincided with an increase in mothers’ foraging time of about 36% (as well as a decrease in mass gained). After 1999, however, the declining trend abruptly stopped and the weaning weight of the pups increased to eventually reach 130 kg. This change coincided with the cooling of the ocean at the beginning of the new century.
“Despite ranging widely and foraging deeply in cold waters beyond coastal thermoclines in the northeastern Pacific, elephant seals are impacted significantly by ocean thermal dynamics”, conclude the authors.
Le Boeuf and Crocker add that monitoring the mean weaning weight of elephant seal pups can therefore help track changes in ocean temperature cycles.
From Biomed Central