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

Abstract

-Several pelagic fish species are known to associate with floating objects. However, quantitative information on the main factors that drive this associative behaviour is still lacking. Small pelagic fish offer a particularly interesting case study for this phenomenon, since the small spatial scales involved in their association with floating objects allow experimental data to be collected at a fine scale. Here, we monitored twelve acoustically-tagged bigeye scads, Selar crumenophthalmus (Carangidae), (mean fork length 16.4 cm, SD 2.1 cm) around a floating object moored in shallow water (15 m depth) playing the role of a coastal fish aggregating device (FAD). To quantify the role played by variations in current and daylight, we calculated the speed distribution, pair correlation function and group polarization for the tagged fish hourly, from midday to dusk (13:00−18:00), for varying current strengths and daylight conditions. We found that the current induced a shift in the position of the aggregation upstream of the FAD, at distances that increased with the current strength. We found evidence of an expansion and a higher coordination in the aggregation at dusk, with increasing speed, distance among conspecifics and alignment. We discuss possible scenarios in which group polarization increases at dusk and suggest complementary measurements for future experiments that could confirm our findings.

Drifting baited stereo‐videography: a novel sampling tool for surveying pelagic wildlife in offshore marine reserves

“…When this happens, the metallic frame of the instruments could easily increase drag and act as a ''sea anchor'', allowing some degree of bait release to occur. The lack of clear species accumulation through time coupled with the high proportion of carnivores captured on camera both appear to confirm this, although it is noteworthy that sharks are also sensitive, and possibly attracted, to low-frequency acoustic cues (Myrberg 2001), and that thigmotactic tendencies (i.e., associations with floating objects) have been reported in numerous fishes (e.g., Capello et al 2013). Both processes may contribute to the observed sighting rates and partly compensate for a reduced odor plume (to an unknown level).…”

Movement behaviour of skipjack (Katsuwonus pelamis) and yellowfin (Thunnus albacares) tuna at anchored fish aggregating devices (FADs) in the Maldives, investigated by acoustic telemetry

“…Nevertheless, our results do highlight the need for further studies to specifically address the finescale behaviour of several fish simultaneously in the future. Studies of this nature could be conducted using alternative acoustic telemetry techniques (Espinoza et al 2011;Capello et al 2012), although performing such experiments around offshore anchored FADs represents a significant technological challenge.…”

Density and group size influence shoal cohesion, but not coordination in zebrafish (Danio rerio).

“…The parallel alignment and consistent spacing of large, synchronized groups may have a genetic basis, and thus be relatively fixed (e.g., Greenwood, Wark, Yoshida, & Peichel, 2013). In contrast, the physical properties of smaller, less-coordinated groups may be shaped more directly by physical factors that elicit taxes (light: Bode et al, 2010; Imada et al, 2010; water currents: Capello, Soria, Potin, Cotel, & Dagorn, 2013; Genin, Jaffe, Reef, Richter, & Franks, 2005), or constrain group motion (enclosure shape: Bazazi et al, 2008; Buhl et al, 2006; nest configuration: Shelton & Alberts, 2013).…”