Using acoustics to investigate changes in efficiency of a sandeel dredge

Acoustics biomass measurements and a sandeel dredge tows were used to sample sandeels (Ammodytes marinus), during six surveys conducted on the Dogger Bank, North Sea between 2000 and 2002. Presented here, is an approach that uses the acoustically determined biomass to estimate the apparent efficiency of the dredge. Simulations were used to quantify the main sources of error in the estimate of acoustic biomass. The estimated dredge efficiencies varied between surveys but were comparable in 2000 and 2001. The dredge appeared to have performed poorly in June 2002. In principle, the use of 'acoustically derived' estimates of dredge efficiency will make it possible to calculate the biomass of sandeels at times when they are not readily available to acoustics (e.g. in April and November). However, before this can be achieved with confidence it is necessary to determine what proportion of sandeels may remain buried in the sediment during daylight hours and better define the target strength of sandeels at 120 kHz.