Publication Abstract

Using acoustics to investigate changes in efficiency of a sandeel dredge

S. Mackinson, K. Turner, D. Righton and J. Metcalfe

Acoustics and a modified scallop dredge were used to sample sandeels (Ammodytes marinus), during 4 surveys conducted within a 27nm by 30nm area of the Dogger Bank in 2000 and 2001. The independent estimates of relative abundance were comparable, providing insight to changes in local stock size. We used simulations to include variability relating to sources of error in the absolute estimate of acoustic abundance, and estimated the efficiency of the modified scallop dredge that would be required to achieve the same abundance estimate as that derived from acoustics. The estimated efficiencies varied between surveys but were similar to those reported for conventional scallop dredges. A higher apparent efficiency in April suggests that sandeels were more readily available to the dredge, a result believed to be linked to changes in behaviour associated with emergence time and the size structure of the stock. Estimates were most sensitive to uncertainty in the value of reference target strength, which accounted for 66% of the total variation in acoustic abundance and corresponding apparent dredge efficiency. We show that the median value is the most reliable summary statistic of simulated distributions when there is high uncertainty in target strength.

Reference:

S. Mackinson, K. Turner, D. Righton and J. Metcalfe, 2005. Using acoustics to investigate changes in efficiency of a sandeel dredge. Fisheries Research, 71, 357-363.