Observing behaviour of over-wintering Herring (Clupea harrengus L.) in Ofoten with Acoustic Current profiler

2003

This report is not to be cited without prior reference to the authors

In this paper we present results and interpretation of velocity and signal strength data

collected with a ‘Continental’ Current Profiler on over-wintering herring in the Ofoten in the

North of Norway. The instrument was lowered to a depth of 150 m and kept stationary with

the transducer upward looking during a period of two months. In the same area and period we

had two bottom-mounted echo sounder of the type EK60 at 500m and 400m depths. Data

from the echo sounders were used to demonstrate that the Continental signal strength could

clearly measure the backscattering of herring. First, we used threshold and contour extraction

on the EK60 data. The extracted contours were then overlaid the backscattered data from the

Continental. The distribution and density dynamics of the two data sets matched very well. By

using the extracted contours for analyzing the speed and direction, we concluded that herring

behaviour could efficiently be observed from the Continental. We could not observe the

diurnal vertical migration due to the lack of depth coverage of the migration range of herring.

Exchange between fish layers was well reflected in the data from the Continental. The

horizontal speed and direction showed that the herring aggregations mainly floated on the

tidal current during the over-wintering. These observation contrasts the expected rheotaxis,

but fits well with the fact that herring is in a state of energy saving and thus would avoid

unnecessary swimming.

ICES

ICES CM Documents;2003/Q:6