The study by Le Boeuf and Crocker 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 this time the ocean temperature increased to the highest on record in 1997-98, after the most intense El Nios of the century in 1982-83 and 1997-98, before cooling 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