Impact of herring spawning behaviour on acoustic abundance estimates

Abstract

Spawning dynamics of Norwegian spring spawning herring was studied in south western Norway 29 March to 3 April 2000 using hydroacoustics. The horizontal distribution of the spawning layers shifted in a south-easterly direction during the study period indicating directional spawning. A diurnal spawning pattern was found, with layers of spawning herring recorded at night from 18 to 24 UTC (= local time - 2 hours), few herring recordings during the night from 24 to 06, and most herring recorded pelagically during the day. Recorded fish density was highest in the period 15 to 18 and lowest from 21 to 03, and schools staying pelagically had higher density than bottom layers. The observed behaviours are likely to influence acoustic abundance estimates, particularly when surveying during the night towards the end of the spawning season.