Fish behaviour during sampling as observed from free floating buoys - application for bottom trawl survey assessment
Abstract
Varying fish distribution and behaviour during bottom trawl surveys has long been
considered important for the reliability of survey abundance estimates. Size and species
dependent behaviour may bias the estimates and the problem may be augmented by
within and between survey variation in the natural conditions. Qualitative descriptions of
behavioural effects are numerous, but a quantitative methodology driven by observations
is still lacking. In this paper we look into the application of data from a newly developed
acoustic buoy based on experiments from saithe (Pollachius virens) off northern Norway.
Time series plots of collective as well as individual fish behaviour during vessel passage
show avoidance reaction. Time series plots of vertical fish velocity vectors show periods
in which both speed and direction appear to be random alternating with periods with clear
synchronous co-ordinated movement even when the fish are not simulated by vessel
noise. The data uncover substantial variability in natural fish behaviour which is an
obstacle for drawing firm conclusion as well as for modelling vessel/trawl affected
behaviour for use in survey stock assessment. Bergen Acoustic Buoy (BAB) is an
improved sampling tool, which has the potential to supply the data needed for resolving
these problems.
Publisher
ICESSeries
ICES CM documents1999/J:10