Frederiksen, M., Elston, D.A., Edwards, M., Mann, A.D. and Wanless, S.
||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 longterm
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.