Lopez et al : A model for estimating biomass of fish species associated with fish aggregating devices 
167 
schools (Fonteneau et al., 2013). In addition, while 
encircling tunas associated with a DFAD, purse sein- 
ers also enclose nontarget species that are associated 
with the object and thus increase the bycatch-to-catch 
ratio in comparison with ratios obtained from setting 
on free-swimming schools. 
Today, many DFADs are equipped with satellite- 
linked echosounder buoys (Lopez et al., 2014), which 
provide fishermen with remotely collected rough esti- 
mates of the DFAD-associated tuna biomass, as well 
as accurate geolocation information. Fishing crews 
use the biomass estimates from the buoys, along with 
other information (e.g., environmental conditions from 
remote sensing data, catches by other vessels in the 
same area), to decide on the best DFAD to visit next. 
A biomass estimate provided by an echosounder buoy 
is represented by a single value and does not comprise 
information on species composition (tuna and nontuna) 
nor the size distribution of the aggregation. Fishermen 
obtain information only on the size and species compo- 
sition of the aggregations after setting on the floating 
device and the catch is hauled on deck. Therefore, fish- 
ing crews cannot currently use echosounder buoys to 
remotely assess the catch composition (biomass of ev- 
ery species and size category) before deciding on which 
DFAD they will set a course. 
On the other hand, scientists have been studying 
fish aggregations at DFADs using various observation 
techniques such as active acoustic tracking (Matsumo- 
to et al., 2014), passive acoustic tracking (Taquet et 
al., 2007a; Filmalter et al., 2011; Schaefer and Fuller, 
2013; Matsumoto et al., 2014), scientific acoustic sur- 
veys (Moreno et al., 2007), and underwater visual cen- 
sus (Taquet et al., 2007b). Clearly, scientific knowledge 
on the behavior of DFAD-associated aggregations is 
currently limited by the cost of accessing and working 
on remote DFADs. Developing autonomous observa- 
tion tools that can be attached to DFADs for continu- 
ous direct long-term monitoring of fish aggregations 
has been a challenge in recent years (Dagorn et al., 
2007a). Therefore, echosounder buoys used extensively 
by fishermen appear to be a powerful tool for observ- 
ing fish aggregations at DFADs remotely in a cost-ef- 
fective manner. However, there is currently no satis- 
factory buoy that can provide reliable information on 
the abundance and sizes of the different species that 
compose these fish aggregations. The lack of reliable 
information precludes gaining a better understanding 
of the behavioral processes involved in the dynamics of 
these aggregations. 
The objective of this study was to develop a new pro- 
cessing method for data collected by one of the brands 
of echosounder buoys used by tropical tuna purse sein- 
ers, by using data on the vertical distribution of the 
different fish species and fish sizes at FADs, their cor- 
responding target strength (TS, dB re 1 m 2 ; MacLen- 
nan et al., 2002) and weight values, and further param- 
eter optimization against the data from a set of fishing 
operations on DFADs. 
Materials and methods 
The buoy 
At the start of this study, three companies were manu- 
facturing echosounder buoys for DFAD fishing. Echo- 
sounder specifications (frequency, beam width, depth 
range, etc.), and the algorithms used to convert acous- 
tic backscatter into biomass values differ between 
companies. Twenty-four Spanish tropical tuna purse 
seine skippers were interviewed to collect data on echo- 
sounder buoy usage, display quality, and reliability of 
different brands. Manufacturers were also contacted 
to obtain technical information on their products. On 
the basis of all available information, the Satlink buoy 
(SATLINK, 3 Madrid, Spain, website) was selected for 
the purpose of the present study. 
The buoy contains a Simrad ES12 echosounder, 
which operates at a frequency of 190.5 kHz with a 
power of 140 W (beam angle at -3dB: 20°). The sound- 
er is programmed to operate for 40 seconds. During 
this period, 32 pings are sent from the transducer 
and an average of the backscattered acoustic response 
is computed and stored in the memory of the buoy’s 
software program (hereafter called “acoustic sample”). 
Volume backscattering strength (S v , dB re 1 m _1 ; Mac- 
Lennan et al., 2002) values smaller than -45 dB are 
automatically removed by the internal module of the 
buoy, as a precautionary measure to eliminate signals 
that likely correspond to organisms smaller than tuna 
(e.g., organisms of the sound scattering layers; Josse et 
al., 1998; Josse and Bertrand, 2000). The observation 
depth range extends from 3 to 115 m (with a blank- 
ing zone [a data exclusion zone to eliminate the near- 
field effect of the transducer; Simmonds and MacLen- 
nan, 2005] from 0 to 3 m depth) and is composed of 10 
homogeneous layers, each with a resolution of 11.2 m 
(Fig. 1). Because echosounder buoys belonged to fish- 
ing crews, they were not calibrated at the deployment 
site; instead, they were calibrated by manufacturers in 
tanks before delivery. 
Raw acoustic data are provided for each depth layer 
and were originally converted to biomass (in metric 
tons, [t] ) by using an experimental algorithm devel- 
oped by the manufacturer, which is based on the TS 
of skipjack tuna, the main target species of the fleet 
fishing around DFADs. This conversion is automati- 
cally executed in the internal module of the buoy for 
each integrated layer (n= 1, 2, ..., 10) by means of a 
depth layer echo-integration procedure (Simmonds and 
MacLennan, 2005), with the assumption that there 
was the presence of only individuals of skipjack tuna 
of identical weight (for confidentiality reasons, authors 
of this article are not allowed to describe some of the 
proprietary technical information regarding the echo- 
sounder buoy. Readers interested in more technical de- 
3 Mention of trade names or commercial companies is for iden- 
tification purposes only and does not imply endorsement by 
the National Marine Fisheries Service, NOAA. 
