Characterizing fish communities associated with drifting fish aggregating devices (FADs) in the Western Indian Ocean using underwater visual surveys

Abstract

We adapted a visual census method, mainly used in demersal and reef fish studies, to characterize fish communities associated to drifting fish aggregating devices (FADs) in the Western Indian Ocean. Drifting FAD associated fishes from both equatorial (Seychelles) and tropical waters (Reunion Island) were examined by divers. A total of 32 species (belonging to 16 families) were observed associated with drifting FADs in equatorial waters, and 24 species (14 families) were found around FADs in tropical waters. Twenty species were found in both regions. The highest number of species observed at a single FAD was 18 (12 ± 2, mean ± SD) in equatorial and 13 (10 ± 3) in tropical waters, not counting circumnatant species loosely associated with the FAD. Some species like Kyphosus vaigiensis, Canthidermis maculata, Elagatis bipinnulata, Acanthocybium solandri and Coryphaena hippurus were observed on all or most of the surveys. In this study, the contribution in biomass of the 18 common species associated with drifting FADs (but excluding circumnatant species), represents more than 98% of the biomass. The overall biomass values of closely associated species remains well below tuna biomass estimates for circumnatant tuna schools at FADs, estimated as high as 200 tons. The species that most significantly contribute to the by-catch in tuna purse-seines logically match those that showing the highest biomass values in our surveys (Carcharhinus spp., Elagatis bipinnulata, Coryphaena hippurus, Canthidermis maculata, and Acanthocybium solandri). One of the most abundant and ubiquitous species in our study was the spotted oceanic triggerfish Canthidermis maculata that sometimes formed massive schools of many thousands individuals around the drifting FADs. Future research is needed to explore the role of such non tuna species in the attraction and aggregation processes of tuna around drifting FADs.

Swimming depth of dolphinfish (Coryphaena hippurus) associated and unassociated with fish aggregating devices

“…However, at night, dolphinfish may forage on prey of the deep scattering layer that rise to occupy the uniform-temperature surface layer. Although we did not observe the dissociation of any fish from a FAD, Taquet et al (2007) suggested the need to forage may prompt dolphinfish to leave FADs or other floating objects.…”

Visual censuses around drifting fish aggregating devices (FADs): a new approach for assessing the diversity of fish in open-ocean waters

“…The depths of the divers (0 to 15 m) and the visibility of the waters defined the semi-spherical volume of water centered on the FADs (horizontal radius of 25 m and a vertical radius of 30 m). The duration of each underwater visual census was standardized at 30 min (Taquet et al 2007). Direct abundance estimates of individuals by species were conducted when fish schools consisted of less than 50 individuals, while more abundant species were estimated by assigning them to abundance classes.…”

“…Each underwater visual census was complemented by a recording with an underwater video camera. The diver with the video camera would first record each fish species found in close aggregation with the FAD and then, from a stationary point 15 m below the FAD, would slowly spin around 360°and film all the fishes swimming around that position (Taquet et al 2007). The video camera was equipped with a wide angle lens to better estimate the abundance of schooling species.…”

Use of video monitoring to quantify spatial and temporal patterns in fishing activity across sectors at moored fish aggregating devices off Puerto Rico

“…If similar instruments are deployed on FADs equipped with video cameras, the amount and change in biomass of fish aggregating around the FADs (Freon andDagorn 2000, Girard et al 2004) could be related to fishing activity. In addition, fishing activity could be compared with the diel movements of fish at FADs (see, Dagorn et al 2007, Taquet et al 2008, Whitney et al 2016) and compared with catch reports to allow estimates of when specific species may be more vulnerable to fishing. These strategies could provide fishers with information to adapt fishing strategies by TOD and managers to estimate fishing mortality and biomass depletion.…”