A slight decrease in the amount of prey available can seriously affect the reproductive success of elephant seals. This was reported by the University of California at Santa Cruz.
The female elephant seal gives birth to a calf each year in the winter. They spend four weeks on the beach, relying entirely on their stored fat, until their young are weaned and returned to the ocean to feed. After the winter breeding season, they go to sea for two months before returning to the colony to moult. They then embark on a long migration, traveling thousands of miles across the North Pacific Ocean over a period of seven to eight months.
During these foraging trips, the seals use different strategies, cover different distances from the shore, dive to different depths and pursue different prey (various fish and squid species). The researchers found that diving depth was the strongest predictor of mass gain. Deep-diving elephant seals had a more energy-rich diet than shallow-diving elephant seals.
The resulting weight gain directly affects the elephant’s ability to produce offspring. A female who has not gained enough weight will not give birth to a calf after returning to shore. The threshold is 205 kg: animals gaining less than 260 kg rarely breed, while those gaining more than 260 kg almost always give birth.
Survival also depends on weight gain, and the longer a female lives, the more offspring she can produce. Previous research has shown that relatively few long-lived female elephants in the long run produce most of the offspring in a colony. While these “supermothers” can live up to 23 years and give birth to more than 15 offspring in their lifetime, most females do not live that long and produce far fewer offspring. But the strategies that allowed these mothers to be so successful have long been unknown.
“We found that an additional 5 percent foraging success resulted in a 300 percent increase in lifetime offspring production due to the effect of increased mass on both survival and the birth and rearing of offspring each year,” the scientists explain.
In this regard, scientists are concerned about the fishing industry’s plans to develop the deep mesopelagic zone, where seals find the most high-calorie food. As the study shows, even a small change in the amount of food can seriously affect the population.