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Title: The potential application of acoustic methods in North Sea demersal fish stock surveys
Author: Greig, Antonio Bernardo
ISNI:       0000 0001 3518 6800
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2007
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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 ' 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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available