A model based on data from echosounder buoys to estimate biomass of fish species associated with fish aggregating devices

The views and opinions expressed or implied in this article are those of the author (or authors) and do not necessarily reflect the position of the National Marine Fisheries Service, NOAA. Abstract—Most of the drifting fish aggregating devices (DFADs) used in industrial tropical tuna purseseine fisheries are equipped with satellite linked echosounder buoys, which provide fishing crews with remote, accurate geolocation information and rough estimates of FADassociated tuna biomass. One of the most common brands of echosounder buoys (SATLINK, Madrid, Spain) is currently calibrated for the target strength of skipjack tuna (Katsuwonus pelamis) and provides biomass data on that species. Using that brand of echosounder buoy, we developed a new behavior-based approach to provide relative biomass estimates and a remote target classification of fish aggregations at FADs. The model is based on current knowledge of the vertical distribution of the main fish species associated with FADs, as well as on appropriate TS and weight values for different species and sizes, and is further based on parameter optimization against a set of fishing operations on DFADs. This model reduced the error variability in biomass estimates by about 60% and also reduced the ranges of underestimation and overestimation by 55% and 75%, respectively. Similarly, the original coefficients of correlation and determination were also considerably improved from 0.50 and 0.25 to 0.90 and 0.82, respectively. We discuss how this new method opens new opportunities for scientific studies and has implications for sustainable fishing. Objects floating on the surface of the tropical and subtropical oceans attract a number of marine species, including tropical tunas (Castro et al., 2002; Taquet et al., 2007b). Taking advantage of this associative behavior, tropical tuna purse seiners regularly deploy drifting, manmade floating objects, also called drifting fish aggregating devices (DFADs), to facilitate their catch of skipjack (Katsuwonus pelamis), yellowfin (Thunnus albacares) and bigeye (Thunnus obesus) tunas. This fishing method is essential for the current operation of purse-seine fleets in all oceans; more than 50% of purse-seine sets (and greater than 70% of purse-seine sets during some years in the Indian and eastern Pacific Oceans) are made on floating objects. This method accounts for nearly half of the world’s tropical tuna catch (Dagorn et al., 2013). Indeed, it has been estimated that 50,000–100,000 DFADs are deployed annually worldwide (Baske et al.1; Scott and Lopez2), which drift at sea on average for periods of over one or two months depending on the ocean (Maufroy et al., 2015). However, the increasing use of DFADs has led to concerns. Setting on floating objects contributes to the catch of small and undesirable sizes of bigeye and yellowfin tuna that are usually not caught by sets on free-swimming