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Acoustic Detection and Attraction of Tuna in New Zealand Waters





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    Project
    Acoustic biomass estimates of pelagic forage species in the offshore waters of the Lesser Antilles
    Scientific Basis for Ecosystem-Based Management in the Lesser Antilles Including Interactions with Marine Mammals and Other Top Predators (LAPE)
    2008
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    A recently completed survey of the Lesser Antilles pelagic ecosystem used a combination of multi-frequency acoustics and pelagic trawling to locate and estimate the biomass of forage species. Stratified zig-zag transects were used to investigate an area of 610 000 km2 in the waters east and west of the Lesser Antilles from Antigua to Trinidad. Distinct pelagic layers and aggregations observed acoustically were sampled using a multiple (3) codend pelagic trawl. Pelagic organisms were identified to the lowest taxon possible and categorized by the presence or absence of a swim bladder. The acoustic data were grouped into nine broad categories based on the multi-frequency returns considering backscattering strength, aggregation appearance, depth and time of day. The resulting acoustic density was converted to indicative biomass density (kg/m2) by application of target strength estimates from published sources. Relative abundance and composition of the nine acousti c categories for the 18 and 38 kHz frequencies are presented. The spatial distribution of these broad categories displayed several distinct patterns. There was an inshore-offshore segregation of some groups throughout the survey area, but there was little sign of latitudinal gradients in distribution of these broad groupings. In the open pelagic waters, diel vertical migrations were observed by several identifiable acoustic categories, particularly mesopelagic fish (largely Myctoph ormes and Stomiiformes), and squids. Dusk and dawn vertical excursions were observed daily between depths from 500 m to less than 100 m.

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