Comparison between "underway" and "on station" acoustic measurements made during bottom trawl surveys
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It becomes in many cases, routine, to collect acoustic data during bottom trawl surveys, both during and between trawls providing “on station” and “underway” acoustic data. Still, the combination of trawl and acoustic data is not operational and could represent a cost-effective way of improvement: bottom trawl survey use samples taken from very small areas (say 2 n.mi.) as representative of much larger areas (the statistical rectangles) while an acoustic transect from one trawling station to the next one, covers a 20-30 n.mi. strip. This study uses 3 different surveys (Norwegian acoustic and bottom trawl survey for cod and haddock in 2000; IBTS (France) in 2002; Northern Irish bottom trawl survey in 2000) where “on station” data are integrated over the trawling sections while “underway” data are regularly integrated over 1, 0.5 or 0.1 n.mi. The objective of this exploratory data analysis is to evaluate the consistency between “underway” and “on station” acoustic data. Experimental means and variances of NASC values are first presented. Acoustic observations being autocorrelated, we analysed the consistency between acoustic data using geostatistical tools. The spatial structures (i.e. variograms) of the ‘underway’ acoustic data indicate that fish assemblages are autocorrelated in space. If the variograms of “underway” and of “on station” data indicate the same large distance structures, the “underway” acoustic data provide the opportunity to observe short scale structures that are smaller than the interstations distances. These structures appears as nugget effect on “on station”variograms and may explain why very few structure are observed sometimes “on station”.