Small-scale spatial structuring of burrowed sandeels and the catching properties of the dredge

The lesser sandeel (Ammodytes marinus) remains burrowed for long periods in the seabed in sandy areas, and its habitat preference is reflected in a large-scale (10 km) patchy distribution in the North Sea. The small-scale spatial structure of sandeels in the seabed is less known. In this study, it was found that sandeel catches in grabs and dredges fit gamma distributions with a common shape parameter on both a 10 m and a 100 m scale. These spatial models were used to examine the catching properties of the dredge, which, in the new ICES sandeel assessment, is used to estimate recruitment and maturity curves. Analyses of the empirical data and simulations of the dredge and grab catch rates show very low dredge catchability (� 5%), but no length-dependent catchability in the dredge was found. However, analyses of the catch rates of parallel dredge hauls indicated increasing dredge catchability with sandeel density. The observed small-scale patchiness and the low and seemingly density-dependent dredge catchability result in biased abundance estimates with low precision. Therefore, we recommend a joint international effort to develop a more catch-efficient sampling dredge, where the effect of density on catchability is further examined.