Title:
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The potential application of acoustic methods in North Sea demersal fish stock surveys
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In the North Sea, groundfish stock assessment relies on independent bottom trawl surveys
to tune the annual indices that are estimated from commercial landings. These surveys
have been carried out by several European fishery institutions since the mid 19605 under
standardized protocols which have allowed a long time series to be built for the main
commercial stocks. An echo-integration survey is an independent metliod with a high
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sampling rate that can be carried out underway. Although it is successfully used on some
stocks in the Barents Sea and off the North American Pacific coast, this method is not
commonly used for demersal fish quantification. This study investigates the use of echointegration
data, gathered during regular bottom trawl surveys in the North Sea~ in
providing tuning indices for commercial stocks. To do this, density values extracted from
bottom following layers in the acoustic dat{and partitioned by the corresponding catch
proportions were compared to the density values obtained from the bottom trawl catches.
Univariate and multivariate linear models including depth, position and time of day were
fitted and showed very low correlation for demersal species, while resulting in reasonable
to good correlation for aggregating pelagic species. As the acoustic method has a dead
zone next to the seabed, an analysis of the dead zone dimensions in the study area as well
as its mitigation was performed and a software routine was developed to optimize the data
USge close to the seabed. A different approach, which allocated acoustic backscatter to
targets according to visual interpretation of the echograms and scrutiny protocols, was
performed on the optimized data and the density values compared to those of the catches.
Correlation results did. not show significant improvement; however the scrutiny process
revealed that demersal species were not evident on the echograms, irrespective ofthe catch
in the bottom trawl. It is concluded that this discrepancy is due to the aggregation patterns
in groundfish, and not necessarily the dead zone alone. Quantification of pelagic species
can benefit from the use of echo-integration during bottom trawl surveys, however
modifications to the survey design to allow targeted trawls and a systematic survey track
are suggested to improve future studies.
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