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FAIRBANKS, Alaska -- Female harbor seal pups whose blubber falls below average levels may be at higher risk of delayed sexual maturation or death, even if they get enough fat in their diets later on, according to a new study sponsored by The American Physiological Society and presented at Experimental Biology 2006.
The study found that harbor seals pups that were heavier when captured from the wild continued to gain weight and grow regardless of whether the researchers placed them on a high fat or low fat diet. But smaller seals, regardless of which diet group they were put in, continued to lag behind in weight gain and even lost blubber depth.
"Our longitudinal study is leading to a better understanding of the role that fat consumption plays in the life of the harbor seal and could tie the declining population of seals and other marine mammals with the change in availability of seal prey, including herring," lead author Lori Polasek said.
The research is part of a much larger effort to find out why harbor seals and other marine mammals are disappearing from the chilly waters off the Alaskan coast. The population decline, which has become a focus of research in the past 10-15 years, is potentially due to a variety of factors, Polasek said, including:
Herring, capelin and eulachon are good sources of much-needed fat in the diet of harbor seals, but they have decreased in numbers in some areas where seals live, Polasek explained. "We are examining the importance of fatty fish in the diet of young, growing seals to see if our experimental seals reflect changes observed in wild populations and could explain the rapid population declines seen since the 1970s."
Low fat or high fat diet
The study began in 2004 with four one-month-old newly weaned female harbor seals, the animals most susceptible to changes in prey availability. In 2005, the researchers added four more newly weaned one-month-old females to the study.
For marine mammals, most energy is provided by fat and protein, Polasek said. And fat is the easiest to convert to energy and blubber. Herring is one potential source of fat for harbor seals, she said. After weaning, pups must learn to effectively fish and accumulate sufficient fat stores for the winter, but changes observed in the distribution, composition or abundance of fish may make it harder to ingest and store enough fat. The study aimed to find out what effect the fat content of the harbor seals' diets has on their ability to survive and reproduce.
At the beginning of the study, the pups weighed between 21 and 33 kilograms (9.5 to 15 pounds). The researchers placed four of the seals in the high fat diet group and four in the low fat group. Both groups received an identical base diet, including pollock, squid, and capelin, all of which contain less than 4% fat. The remainder of the diet was herring.
The low fat group received herring that contained less than 6% lipids, while the high fat group received herring that was 16% lipids. (Herring have a higher fat content in winter before laying their eggs, but lose about one-third of their fat after releasing their eggs.)
Except for the fat content of the herring, the nutrient content was the same for both groups, Polasek explained. The researchers monitored the animals in the low fat group to ensure fat intake was not falling below minimal levels, she said. "We don't want to put them in nutritionally stressed situation that would harm them," Polasek said. "We don't want to drop the fat too low, but we want to know how seals respond to a restricted fat diet."
Starting weight most important factor
The results so far have been surprising: growth rates, seasonal fluctuations in body mass, and percent body fat have been generally independent of the seal's diet. Instead, the starting weight has been the critical predictor of whether the seal would add body mass and body fat, Polasek explained.
Seals weighing less than 26 kilos (about 12 pounds) showed little progress in gaining weight and even lost blubber, regardless of which diet they were on. On the other hand, the study's largest seal, which weighed 33kg when she began in 2004, has shown good growth despite being on the low fat diet, Polasek said. A seal that weighed 26kg at the start of the study, on the high-fat diet, has also continued to grow in a similar manner.
These results fit with a recent study by a team from Göteborg University that found wild seal pups smaller than 26 kg when they were weaned were less likely to survive the winter. It seems that mother seals, which produce high fat milk, must impart enough fat to get pups over the 26kg mark, Polasek noted. And once weaned, they must ingest sufficient food to get through the winter when food is less abundant.
Seals rely on blubber both for insulation and as a source of energy. Nutritionally stressed adolescent females may take longer to reach sexual maturation and mature females that are nutritionally stressed may not reproduce, offering clues to why some harbor seal populations are falling, Polasek said.