The scale of population structuring in cod Gadus morhua from the west of Scotland (ICES stock area VIa) and northern North Sea was investigated using a combination of non-genetic methods. Site fidelity of spawning aggregations was examined using historic tag-recapture experiments and individual geolocation estimates from data storage tagged fish. The extent of movements from spawning areas indicated by these two tagging approaches was broadly similar. Between 67 and 97% of adult cod remained within 100 km of spawning areas throughout the year, suggesting resident spawning groups. A small proportion of cod, however, did appear to stray between spawning areas. A comparison of the elemental signature of the otoliths of 0 year-group and the 0 year-group component from the same year-class at age 2 years indicated that most adults originate from local nursery areas. Moreover, there did not appear to be detectable exchange between spawning areas >200 km apart, such as the Clyde and Minch or Shetland and the Inner Hebrides. As such, population processes may operate at a smaller spatial scale than the stock level, with spawning aggregations functioning as local populations within a metapopulation. The relevance of this scale of dynamics is discussed in relation to recently imposed closed areas in the region. # 2006 Crown copyright

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Sandeel abundance and sediment particle size data from 2886 grab samples collected during 14 surveys undertaken in the Wee Bankie/Marr Bank region of the north-western North Sea between 1998 and 2003 are analysed. Odds Ratio, an analysis that compares the proportion of all samples assigned to a particular sediment category, with the proportion of samples containing sandeels assigned to the same category, reveals clear patterns of either selection for, or avoidance of, seabed habitats containing varying proportions of 8 sediment particle size categories. As the proportion of Coarse Gravel, Fine Gravel, Fine Sand, Coarse Silt, Medium Silt and Fine Silt in the seabed habitat increases, sandeels show reduced selection for and increased avoidance of the habitat. Conversely, as the proportion of Coarse Sand and Medium Sand in the sediment increases, sandeels show reduced avoidance of and increased selection for the habitat. Classification Tree and Principal Components Analysis define 8 types of seabed habitat on the basis of their sediment particle size composition that vary in sandeel occupancy rate, sandeel density and the size of sandeels that occupy the habitat. Variation in sandeel density across these 8 habitat types is primarily influenced by the silt content in the sediment. Variation in the size of sandeels occupying each habitat type is mainly dictated by the coarseness of the sediment. The value of such results for the design of grab surveys intended to monitor variation in local sandeel abundance is discussed.KEY WORDS: Ammodytes marinus · Sandeel · Habitat preference · Sediment particle size Resale or republication not permitted without written consent of the publisher

Boulcott, P., Wright, P. J., Gibb, F. M., Jensen, H., and Gibb, I. M. 2007. Regional variation in maturation of sandeels in the North Sea. – ICES Journal of Marine Science, 64: 369–376. The current assessment of lesser sandeel (Ammodytes marinus) in the North Sea assumes a single stock and a knife-edge maturity ogive. However, there is evidence that the North Sea stock consists of several reproductively isolated components, raising the possibility of demographic differences among regional aggregations. We examine regional variation in size- and age-at-maturity in four components of the North Sea stock. Surveys in 1999 indicated pronounced regional differences in length- and weight-at-age, implying a disparity in growth rate across the North Sea. Logistic regression revealed that the onset of maturity was significantly related to regional distribution, in addition to length and age, with a tendency for fish off the UK's northeast coast to mature later and smaller than elsewhere. No significant effect of year on either growth or length-at-maturity was revealed from a comparison with other data collected in 2004. The results show that important regional phenotypic variation not currently represented in stock assessments could have implications for the local sustainability of sandeel aggregations.