Tracking the movement of all individual group members in their natural environment remains a challenging task. Using advances in computer vision and Deep Learning, we developed and tested a semi-automated in situ tracking system to reconstruct simultaneous three-dimensional trajectories of marked individuals in social groups of a coral-reef fish. Our system has a temporal resolution of 10s of milliseconds, allowing for multiple 30-min tracking sessions that have been repeated over weeks to months. We present the technique and illustrate its application for Dascyllus marginatus, a planktivorous damselfish that lives in social groups associated with branching corals. Our technique identified all individuals 85-100% of the time, with a mean spatial error of $ 1.3 cm. It provides a cost-effective semi-automated tool for in situ research on movements and foraging of individuals within small site-attached groups of animals in their natural environment.

By bending their body to reduce drag forces, Garden Eels, fish that feed while being anchored to the bottom, effectively feed on drifting zooplankton even when currents are strong. AbstractA major challenge faced by sessile animals that feed in the flow is to maintain effective feeding postures while enduring hydrodynamic forces.Garden eels exhibit an exceptional lifestyle: feeding on drifting zooplankton while being "anchored" in a burrow they dig in the sand. Using underwater observations, sampling and 3-D video recording, we measured the feeding rates and characterized feeding postures of garden eels under a wide range of current speeds. We show that the eels behaviorally resolve the tradeoff between adverse biomechanical forces and beneficial fluxes of food by modulating their body postures according to current speeds. In doing so, the eels substantially reduce drag forces when currents are strong, yet keep their head well above bottom in order to effectively feed under conditions of high prey fluxes. Those abilities allowed garden eels to become one of the rare oceanic fishes that live in sandy, predation-rich habitats and feed on zooplankton while being attached to the bottom.