The effect of vessel avoidance of wintering Norwegian spring spawning herring

The significance of vessel avoidance behaviour for the acoustic abundance estimation of Norwegian spring spawning herring has been investigated during acoustic surveys in the herring wintering area in Northern Norway. The relative variation in echo energy values measured from a submerged 38 kHz transducer during passage of a survey vessel was analysed. All systematic changes in acoustic densities around the time of passing were interpreted as avoidance reactions and the most important behavioural changes are believed to be body tilting and vertical and horizontal swimming. A quantification of the loss of echo energy due to vessel avoidance is presented. Experiments were grouped into nighttime and daytime experiments. While a considerable drop in echo energy around the time of passage was observed at night, insignificant loss in echo energy was found during the day. There was a high degree of variation between experiments, but a general reaction pattern was identified in all the nighttime experiments. The magnitude of the vessel avoidance reaction was highly dependent on depth. Coefficients that quantified avoidance reactions were calculated for separate depth layers between 40 and 400 m. Between 40 and 100 m depth, 24 nighttime experiments showed a mean loss of echo energy of 84%. The effect of avoidance reactions faded out below 100 m, but was observed down to 150 m at night. Below 150 m no significant avoidance reaction was seen, and this may explain the absence of avoidance reaction during the day when the main part of the biomass was distributed below 150 m. Consequences for uncertainty in abundance estimation are discussed, and a rough estimate of the acoustic density lost during acoustic surveys in the wintering area is made. (C) 2002 Elsevier Science B.V. All fights reserved.