Long-term changes in size of lesser sandeels Ammodytes marinus in the Wee Bankie aggregation in the northwest part of the North Sea were investigated using fish collected from Atlantic puffins Fratercula arctica feeding chicks. Between 1973 and 2002, 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 per fish. 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 recruits. We tentatively suggest that changing environmental conditions, acting through effects on early growth and/or hatch date, may have contributed to the long-term decline in size of sandeels in this area. KEY WORDS: Climate change . Environmental monitoring . North Sea . Industrial fishery . larval growth rate . Atlantic puffin Running head: Decline in size of sandeels