The use of acoustic surveys in the assessment of the North Sea herring stock and a comparison with other methods
The application of the results of acoustic surveys to stock assessment is examined for herring (Clupea harengus L.) in the northern North Sea. Surveys designed to estimate spawning biomass have been carried out under ICES coordination in July each year since 1979. The results are reported annually in the form of estimated numbers-at-age and biomass in statistical rectangles. Total mortality estimates derived from the survey results are higher than those estimated by virtual population analysis (VPA). The reasons for this are explored by simulation techniques in which the relationship between target strength and length and the age-sampling procedure are varied. These indicate that the precision of the acoustic survey itself (as distinct from target-strength values and age sampling) dominates the random errors. It is not, however, clear whether the discrepancy between mortality estimates is due to a bias in the sampling for length compositions by trawl, or to inaccuracies in the VPA estimates. A comparison with other techniques (VPA and estimates of stock size derived from herring larvae surveys) indicates that the acoustic surveys are providing similar indications of changes in abundance. Acoustic surveys, moreover, are providing absolute estimates of stock size within about 30% of the estimates obtained from VPA.