Scientists suggested the removal of large fish species by fishing boats was allowing populations of smaller species - favoured by the seals - to thrive.
The findings appear in the Royal Society journal Biology Letters.
"We report on a continual shift in the birth date of harbour 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 harbour seals (Phoca vitulina) in other parts of the Wadden Sea, we conclude that the factor causing this shift has acted on the entire... population."
It is understood that the mating season runs from spring until autumn. Males may mate with more than one female, and females give birth to one pup in the spring, about 9-11 months after mating. Read the full report here.
Source:Â Mark Kinver, Science and environment reporter, BBC News, UK
