Acoustics vs. bottom trawling fish estimates: A comparative analysis of the surveys on the reproductive stock of common hake (Merluccius hubbsi) in the nor-patagonic area

ALVAREZ COLOMBO, G.; MADIROLAS , A.; HERNANDEZ, D.; CASTRO MACHADO, F.; MACCHI, G.J.

Bergen

Simposio; Symposium on the Ecosystem Approach with Fisheries Acoustics and Complementary Technologies; 2008

The main spawning ground of argentine hake (Merluccius hubbsi)have been subject to annual acoustic assessment surveys since 1988. After more than a decade of intensive fishing on the stock, the surveys clearly shown changes in fish aggregation patterns and relative abundance. A sharp break in the traditionally observed pattern occurred after 1996. In 2001, the surveys were re-designed and survey timing and area were reviewed. The new survey strategy allowed the direct comparison of both, acoustic and bottom trawl fish assessment methods. Results from five combined surveys are compared. The fish distribution pattern as observed by both methods was coincident. A relationship equal to +2.75 was obtained for the averaged fish densities of the bottom-trawl/echointegration fish density ratio. Biomass densities calculated from acoustic data resulted systematically lower than those from catch results. Several factors known to affect each method´s performance are discussed. The acoustic dead zone and the underestimation of the area effectively swept by the trawl, are considered as the most plausible explanation for the obtained relationship. These findings are thought to encourage the complementary use of the two assessment techniques, offering a valuable tool for the reciprocal control of the results obtained from combined acoustic-bottom trawl surveys.