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Earlier pupping in harbour seals, Phoca vitulina Reijnders, P.J.H.; Brasseur, S.M.J.M.; Meesters, E.H.W.G. (2017). Earlier pupping in harbour seals, Phoca vitulina, in: Brasseur, S.M.J.M. Seals in motion. How movements drive population development of harbour seals and grey seals in the North Sea. pp. 44-53 In: Brasseur, S.M.J.M. (2017). Seals in motion. How movements drive population development of harbour seals and grey seals in the North Sea. PhD Thesis. Wageningen University: Wageningen. ISBN 978-94-6343-612-0. 176 pp. https://dx.doi.org/10.18174/418009

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Beschikbaar in	Auteurs
VLIZ: Open access 308465 [ download pdf ]

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Trefwoord

Marien/Kust

Author keywords

pupping phenology; seals; forage base; fisheries impact

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Auteurs		Top
Reijnders, P.J.H. Brasseur, S.M.J.M. Meesters, E.H.W.G.

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Abstract

The annual reproductive cycle of most seal species is characterized by a tight synchrony of births. Typically, timing of birth shows little inter-annual variation. Here, however we show that harbour seals Phoca vitulina from the Wadden Sea (southeast North Sea) have shortened their yearly cycle, moving parturition to earlier dates since the early 1970s. Between 1974 and 2009, the birth date of harbour seals shifted on average by -0.71 d yr-1 , three and a half weeks (25 days) earlier, in the Dutch part of the Wadden Sea. Pup counts available for other parts of the Wadden Sea were analysed, showing a similar shift. To elucidate potential mechanism(s) for this shift in pupping phenology, possible changes in population demography, changes in maternal life-history traits and variations in environmental conditions were examined. It was deduced that the most likely mechanism was a shortening of embryonic diapause. We hypothesize that this could have been facilitated by an improved forage base, e.g. increase of small fishes, attributable to overfishing of large predator fishes and size-selective fisheries.

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