Are female northern fur seals finding enough food to help their pups survive?
The first few months at sea are a precarious time in the life of a young northern fur seal. After spending its first four months onshore, sustained by its mother’s milk, a juvenile fur seal must carry enough fat reserves to survive while it learns to catch fish—or it may starve.
New Consortium research highlights the important role of the mother’s foraging habits in preparing the young for weaning. Led by Dr Tiphaine Jeanniard du Dot (The University of British Columbia), the researchers studied the foraging strategies of lactating female northern fur seals. The results were published in the journal Marine Ecology Progress Series.
ONSHORE VS. OFFSHORE
“The northern fur seal population has been declining by six percent per year for the last 20 years,” says Jeanniard du Dot. “During the breeding season, the mothers are challenged to capture enough food to sustain themselves and to feed their pups. Lactation is very expensive energetically. I wanted to understand where the mothers forage and how efficient they are at finding and capturing prey.”
The researchers equipped 20 lactating female fur seals from St Paul Island with biologgers that tracked their location and their diving behavior. Using a variety of methods, the researchers calculated the amount of energy each seal gained and expended during a foraging trip.
“We discovered that the females on these rookeries displayed two different foraging strategies,” says Jeanniard du Dot. “One group made frequent short trips to a shallow shelf near the shore, where they fed on low-quality prey like walleye pollock and squid. The second group foraged in the deep Bering Sea basin further from shore, where they found energy-rich prey like northern smoothtounge, but were spending longer time at sea.”
“The females that foraged offshore in the deep basin had a higher foraging efficiency,” says Jeanniard du Dot, meaning they consumed more energy per unit of time than they spent; however, they spent much more time away from their pups, which were forced to fast in their absence. The onshore group nursed their pups more frequently but consumed lower quality prey, and therefore gained less energy per feeding trip than they spent.
“It’s a bit like the dilemma faced by many working parents,” explains Jeanniard du Dot. “If you spend more time working you get less time with your kids, but you get more money to put food on the table. It’s the same trade-off with seals: females who spent more time at sea came back with milk that was likely higher in energy content than females that stayed onshore and fed on lower energy fish. We’re not sure whether the higher quality milk is enough to compensate for the fact that the pups are not fed as often.”
Each strategy required the female to compromise on either time or energy, but neither strategy maximized efficiency on both, says Jeanniard du Dot. The results did not indicate whether one strategy might give some juveniles an advantage under certain conditions during their first year at sea, or whether both strategies would lead to a similar survival rate amongst juveniles.
“Under the current environmental changes in the North Pacific Ocean,” she says, “it’s difficult to tell which foraging strategy will be more successful. Conditions may change that could favor one strategy over the other, which is probably why both strategies still exist in the population, as females tend not to change behavior from year to year.”
Jeanniard du Dot says that while the results of her study do not provide clear-cut links between foraging strategies and the northern fur seal population decline, it provides strong clues as to why first-year pups might not return in as many numbers.
“The decline in northern fur seals is not due to one factor,” she explains. “Environmental changes affect the population by impacting the weakest part: the lactating females and growing pups who require the most energy.” Consequently, she believes that conservation efforts should focus on two key periods: the reproductive season for females, and the first year at sea for pups.
“An increase in ocean temperature means fish will be found deeper, making it harder for both females and pups to access those fish,” she says. “The pups need to have really good energy reserves to survive while they are learning to dive and developing diving capacity to reach the deep fish.”