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

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