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Marine Ecology Progress Series

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MEPS 432:137-147 (2011)  -  DOI:

Mechanisms of long-term decline in size of lesser sandeels in the North Sea explored using a growth and phenology model

Morten Frederiksen1,2,3,*, David A. Elston4, Martin Edwards3, Alec D. Mann5, Sarah Wanless1

1Centre for Ecology & Hydrology, Bush Estate, Penicuik EH26 0QB, UK
2Department of Arctic Environment, National Environmental Research Institute, Aarhus University, Frederiksborgvej 399,
4000 Roskilde, Denmark
3Sir Alister Hardy Foundation for Ocean Science, The Laboratory, Citadel Hill, Plymouth PL1 2PB, UK
4Biomathematics and Statistics Scotland, Craigiebuckler, Aberdeen AB15 8QH, UK
5Biomathematics and Statistics Scotland, The King’s Buildings, Edinburgh EH9 3JZ, UK

ABSTRACT: The lesser sandeel Ammodytes marinus is a key species in the North Sea ecosystem, transferring energy from planktonic producers to top predators. Previous studies have shown a long-term decline in the size of 0-group sandeels in the western North Sea, but they were unable to pinpoint the mechanism (later hatching, slower growth or changes in size-dependent mortality) or cause. To investigate the first 2 possibilities we combined 2 independent time series of sandeel size, namely data from chick-feeding Atlantic puffins Fratercula arctica and from the Continuous Plankton Recorder (CPR), in a novel statistical model implemented using Markov Chain Monte Carlo (MCMC). The model estimated annual mean length on 1 July, as well as hatching date and growth rate for sandeels from 1973 to 2006. Mean length-at-date declined by 22% over this period, corresponding to a 60% decrease in energy content, with a sharper decline since 2002. Up to the mid-1990s, the decline was associated with a trend towards later hatching. Subsequently, hatching became earlier again, and the continued trend towards smaller size appears to have been driven by lower growth rates, particularly in the most recent years, although we could not rule out changes in size-dependent mortality. Our findings point to major changes in key aspects of sandeel life history, which we ­consider are most likely due to direct and indirect temperature-related changes over a range of biotic factors, including the seasonal distribution of copepods and intra- and inter-specific competition with planktivorous fish. The results have implications both for the many predators of sandeels and for age and size of maturation in this aggregation of North Sea sandeels.

KEY WORDS: Ammodytes marinus · Growth model · MCMC · North Sea · Phenology · Sandeel

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Cite this article as: Frederiksen M, Elston DA, Edwards M, Mann AD, Wanless S (2011) Mechanisms of long-term decline in size of lesser sandeels in the North Sea explored using a growth and phenology model. Mar Ecol Prog Ser 432:137-147.

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