Observing behaviour of over-wintering Herring (Clupea harrengus L.) in Ofoten with Acoustic Current profiler
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http://hdl.handle.net/11250/106499Utgivelsesdato
2003Metadata
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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.