Acoustic data collected during and between bottom trawl stations: consistency and common trends

Abstract

Acoustic data are often collected during bottom trawl surveys. Their use can potentially improve the precision and accuracy of fish abundance estimates if acoustic data collected between trawl stations are consistent with those collected during trawling operations. This question is addressed here through the analysis of 20 bottom trawl surveys (three survey areas and five different survey series) with coincident acoustic measurements during and between trawl stations. Firstly, on-station and underway acoustic data were compared using statistics computed globally over each survey (average vertical profiles, global indices of collocations, and spatial structures) for various combinations of depth layers. Secondly, we focussed on underway acoustic data recorded in the vicinity of stations, distinguishing between data recorded before and after the tows. On-station and underway acoustic data were highly consistent, and no systematic perturbation of the acoustic sign due to the presence of the gear a few hundred metres behind the vessel was observed.Résumé : On récolte souvent des données acoustiques durant les inventaires faits au chalut de fond. Leur utilisation peut potentiellement améliorer la précision et la justesse des estimations d'abondance des poissons, si les données acoustiques récoltées entre les stations de chalutage sont compatibles avec celles récoltées durant les opérations de pêche. Nous examinons la question en analysant 20 inventaires faits au chalut de fond (trois zones d'inventaire et cinq séries différentes d'inventaires) pour lesquels il existe des mesures acoustiques coïncidentes obtenues dans et entre les stations de chalutage. Nous avons d'abord comparé les données acoustiques obtenues en route et dans les stations à l'aide de statistiques calculées globalement pour chaque inventaire (profils verticaux moyens, indices globaux de collocation et structures spatiales) selon diverses combinaisons de couches de profondeur. Ensuite, nous nous sommes intéressés aux données acoustiques obtenues en route près des stations, en distinguant entre les données enregistrées avant et après le chalutage. Il existe un excellent accord entre les données acoustiques obtenues dans les stations et celles enregistrées en route; on n'observe pas de perturbation systématique du signal acoustique due à la présence des engins de pêche à quelques centaines de mètres derrière le navire.[Traduit par la Rédaction] Bez et al. 180

Can the precision of bottom trawl indices be increased by using simultaneously collected acoustic data? The Barents Sea experience

“…The higher on-station means support the findings of Bez et al (2007). They demonstrated a generally high consistency between on-station and between-station acoustics for several surveys, including the Barents Sea winter survey, using tools like the variogram and a global index of collocation.…”

A comparison of acoustic backscatter between a trawling and a free-running vessel for eastern Bering Sea walleye pollock (Theragra chalcogramma)

“…Of the five known studies (including this one) that directly compared the acoustic backscatter between a trawling and a freerunning vessel, one found no difference between the two survey modes (Bez et al, 2007), two reported higher acoustic backscatter while free-running (Ona and Godø, 1990;De Robertis and Wilson, 2006), and two reported higher acoustic backscatter while trawling (this study and . It is not clear why the results from the different studies are so divergent, but it is important to question the implied assumption that any observed differences in the acoustic backscatter between trawling and free-running reflect differences in fish behavior.…”

“…Of the studies that have directly compared the acoustic backscatter between a trawling and a free-running vessel, only one reported no difference between the two survey modes (Bez et al, 2007). All others found significant differences but with contradictory results.…”

Estimating gear efficiency in a combined acoustic and trawl survey, with reference to the spatial distribution of demersal fish

“…Besides differences in avoidance reactions due to different light levels or fish diel activity (accounted for by the trawl efficiency coefficient), estimates of b<1 in fact represent a net reduction of the amount of hake biomass available to the trawl, which could be explained by a higher trawl selectivity for the smaller hake present at night-time. Spatial information in acoustic data has been used in the past to increase the precision and accuracy of trawl-based abundance estimates (Bez et al, 2007). The originality of our approach resides in the quantitative study of fish spatial distribution prior to trawl efficiency computations, to control for fish availably effects.…”