Echotrace classification and spatial distribution of pelagic fish aggregations around drifting fish aggregating devices (DFAD)

Abstract

International audienceThis work presents a method to observe pelagic fish around rifting fish aggregating devices (DFADs). A triple-frequency vertical echosounder was employed to observe fish distributions in the vicinity of DFADs. Surveys were conducted in a star pattern that was centred at the DFADs. The objective of the study was to define a methodology for future acoustic studies. This goal was pursued by (i) studying the spatial distribution of fish aggregations, (ii) developing concepts for the grouping of observed aggregations and (iii) developing specifications for future autonomous acoustic tools. For this purpose 5 cruises were carried out in the western Indian Ocean. The multi-frequency approach proved useful as a means of separating acoustic detections into sound-scattering layers (e. g. plankton and micronekton), fish aggregations and individual fish. Fish target strength (TS) was measured. Four types of aggregations were found near DFADs: ( i) dense structure (ii) medium structure (iii) loose structure and (iv) structure consisting of separated targets. More than 90% of these structures were found within a radius of 400 m and about 75% within 200 m of the DFADs. The spatial configuration of DFAD fish aggregations appeared to be more dynamic compared to aggregations near moored FADs. The spatial distribution and structure of DFAD aggregations have direct implications for their catchability by tuna purse-seiner. We have carried out the first quantitative acoustic recordings around DFADs, and obtained a better understanding of the spatiotemporal dynamics of fish aggregations around DFADs in the Indian Ocean. Based on this knowledge we are now working on specifications for instrumented buoys that are intended as autonomous data recording observatories for such pelagic environments

Effect of current and daylight variations on small-pelagic fish aggregations (Selar crumenophthalmus) around a coastal fish aggregating device studied by fine-scale acoustic tracking

“…This approach was supported by previous acoustic survey measurements (Josse et al 2000;Doray et al 2006;Moreno et al 2007), where the fish spatial distribution along z was not affected by the presence of the FAD but instead depended on the fish species.…”

Three‐dimensional internal spatial structure of young‐of‐the‐year pelagic freshwater fish provides evidence for the identification of fish school species

“…Other parameters may also be involved in such changes (Brehmer et al 2007); nevertheless we assume that they do not play a significant role in our case study as they are likely to change at a rate that is slow enough to not have had any effect. Thus, we propose that the school internal morphology, as described by the relationship between the school volume and the number of vacuoles, is a phenotypic expression therefore characteristic of a species (Brehmer et al 2007). Further investigations are planned, with a focus on ground truth protocols to establish the true composition of the schools.…”

“…Several studies (Coetzee 2000;Brehmer et al 2007) have shown that changes in school behavior depend on environmental conditions. The external school morphology can vary according to the variation of the environment; there is, however, a lack of information on the effect of the environment on the internal spatial structure of a school.…”

“…However, it is still often difficult to distinguish between the echoes of cooccurring fish species during these surveys. Several studies have been devoted to solving this problem (see Horne 2000), using a variety of methods, such as: i) echo-trace morphology extracted from the two dimensional (2D) echogram (Cushing 1963;Haralabous and Georgakarakos 1996;Moreno et al 2007); ii) multi -frequency information (Horne et al 2000;Fernandes 2009;Fernandes and Stewart 2004;Korneliussen et al 2009); and iii) broad-band echosounders (Reeder et al 2004). …”

Target strength of skipjack tuna (Katsuwanus pelamis) associated with fish aggregating devices (FADs)

“…Doray et al, 2006;Josse and Bertrand, 2000;Moreno et al, 2008), and there have been no published TS f ð Þ and TS L ð Þ models for in situ skipjack. This lack of knowledge prevents both fishers and scientists from interpreting data provided by the acoustic tools used when fishing or studying fish at FADs.…”