Evidence for decrease in size of lesser sandeels Ammodytes marinus in a North Sea aggregation over a 30-yr period

摘要

Long-term changes in size of the lesser sandeels Ammodytes marinus in the Wee Bankie aggregation in the northwest part of the North Sea were investigated using individuals collected from Atlantic puffins Fratercula arctica feeding chicks. Between 1973 and 2002, the average size, on a given date, of fish hatched that year (0-group) declined by 11.1 mm. Over the same period, older (predominantly 1-group) sandeels showed an overall reduction in size on a given date, of 19.4 mm fish~(-1). In both cases the change in length corresponded to a 40% decline in energy content. These long-term trends in size-at-age are likely to have had major demographic consequences for this aggregation in terms of delayed sexual maturity and lower age-specific fecundity. While there was no evidence that the decrease was associated with the start of an industrial sandeel fishery in the area in 1990, the observed decline in size-at-age could potentially make this aggregation more vulnerable to collapse because of its reduced capacity to produce eggs. We tentatively suggest that changing environmental conditions from the early 1980s onwards, acting through effects on early growth and/or hatch date, may have contributed to the long-term decline in size of 0-group sandeels in this area. More data are needed to elucidate the reasons for the decline in the size of older sandeels.