Harbor Seal Pupping Season Starting Ealier
Changes in the marine ecosystem are effecting reproduction in harbor seals (Phoca vitulina) causing them to give birth to their pups much earlier than usual, according to a recent study.
The study found that the mammals are birthing their pups 25 days earlier compared to 35 years ago.
Scientists believe that removal of large fish species by the fishing industry was allowing populations of smaller species to thrive. These smaller species are favored by the seals.
“We report on a continual shift in the birth date of harbor seals in the Dutch Wadden Sea,” the team of researchers from the Netherlands Institute for Marine Resources and Ecosystems Studies (IMARES) wrote.
“Given the similar results found for harbor seals in other parts of the Wadden Sea, we conclude that the factor causing this shift has acted on the entire… Population,” wrote the team.
The mating season for harbor seals runs from spring through the fall and males usually mate with more than one female. The female gives birth to one pup in the spring, nine to eleven months after mating. The gestation period includes a phase of delayed implantation, when the fertilized egg stops growing and floats in the female’s uterus for up to three months. The delayed implantation allows the mother to recover from her previous pregnancy and to birth the new pup when conditions are best suited for its survival.
“A possible mechanism for this shift in pupping could be a shortening of one or several stages in the reproductive cycle, including lactation, delayed implantation and placental gestation,” the team wrote.
Older females normally give birth earlier in the year, but the lack of a noticeable change in the age structure of current seal populations led the team to rule that out as a possible reason for the shift in pupping.
They also found there was no noticeable variation in the placental gestation period.
However, the team said previous research into harp seals had identified a link between the females’ condition and implantation dates.
“We therefore suggest earlier timings of implantation as the most likely mechanism explaining the observed shift,” the researchers noted, adding that the “nutritional condition” of the mother after lactation is of importance.
Females undergo rapid weight loss during the weaning and mating seasons, which led the team to suggest that the mammals need to acquire enough fat before implantation begins.
“In other words, the better the food acquisition during and after lactation, the earlier they regain the mass needed for implantation,” they explained.
The seals particularly prefer a variety of small, bottom-dwelling fish species, and surveys in the North Sea revealed that the abundance of small fish had “increased steadily and significantly from 1977,” the team said.
“Intense fishing of larger fish in the North Sea has caused both a shift to smaller species and a decrease in larger predator fish, hence predation on small fish has decreased,” they said, adding that it has led to an abundance of small fish available to the seals, allowing them to reach their ideal weight more speedily and shortening the delayed implantation.
“It is interesting to see whether this shift (in delayed implantation) might be reversed when this exponentially growing (seal) population approaches the carrying capacity of the area.”
The findings appear in the Royal Society journal Biology Letters.
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