In situ estimation of active dispersal abilities in reef fish early life stages using tracking technologies

1. Most reef fishes possess an early pelagic stage that ensures the crucial role of maintaining connectivity between distant populations, as movements of older demersal stages are generally restricted. While classically considered passive, numerous studies show that most larvae largely influence dispersion scale and settlement rate by actively swimming horizontally/vertically in an oriented way during most of their pelagic phase.2. Laboratory measurements of active dispersal skills differ from natural behaviors of individuals observed by divers manually annotating depth and bearing every 30s, while carrying a low-speed flowmeter to estimate average speed. Here, we improved this protocol through the use of electronic measurement devices to achieve enhanced feasibility, replicability, efficiency, and safety. Bearing and depth could be precisely measured at high-frequencies using a logger fixed on an optimized diving tray, which allowed us to reduce tracking duration from 10 min to 5 min, to track more individuals. It also permitted studying in situ the temporal dynamic of vertical speed and direction changes. All further steps, including data entry, sensor calibration, circular statistics and 3D track reconstruction (Madwick filter), were automated within interactive pipelines, enabling to obtain results within 3h after dives during fieldwork.3. We conducted in situ trackings for a diversified set of species (32 per ocean) during developments in the Caribbean (Guadeloupe), before being routinely applied in the Indian Ocean (Maldives) with a majority of trackings successfully carried out (74%) despite offshore conditions. High individual orientation accuracy, combined with great swimming/sinking abilities possibly dependent on depth/current, suggests that pelagic larvae/juveniles can swim in a correlated random-walk (CRW). This occurs even when orientation cues are too scarce for a consistent orientation among species/zones to emerge (biased CRW), marking a difference with their behavior in coastal environment.4. Although biophysical models of dispersal ease the development of informed conservation strategies at large spatial scales, comparisons with genetic connectivity demonstrate that only models incorporating realistic active behaviors yield comparable outputs. Our methodological advances overcome various obstacles preventing measuring the parameters necessary for active models, not only for demersal fishes, but also for any small pelagic organism in any aquatic habitat.