Animal movement is a fundamental ecological process used to identify habitat use, community structure, biodiversity patterns, trophic interactions, reproductive behavior, and population distribution and abundance [1–4]. Knowledge of these outcomes can be beneficial for selection of ecologically relevant spatial and temporal scales for management . Movement and space-use in the aquatic environment is a multi-dimensional process [6–10]. Consequently, measuring changes in horizontal and vertical dimensions at different temporal scales (e.g., day, night, crepuscular periods, seasonal, etc.) can provide insights into habitat use, intra- or inter-specific interactions, behavioral responses to thermal or chemical stratification, and physiological impacts, among others [6, 9].
Fish utilize different temporal and spatial scales to find food, reproduce, avoid predators or seek favorable environmental conditions; with some larger species (e.g., sharks, marlin, tuna and tarpon) moving between winter and summer sites [2, 6, 10, 11]. Migratory species can also exhibit high site fidelity to nearshore areas, where an individual fish occupies a geographically discrete home range or activity space where they partake in routine activities such as feeding, resting and breeding [1, 12–14]. For many highly mobile tropical marine fish, activity spaces can encompass multiple habitat types such as coral reefs, seagrass beds and mangroves habitats, as well as lagoons and estuaries [5, 15, 16].
Atlantic tarpon (Megalops atlanticus) is a highly mobile pelagic species that supports important fisheries. Tarpon range across coastal areas, estuaries, and rivers of the western and eastern Atlantic Ocean, the Caribbean Islands and the Gulf of Mexico [6, 17, 18]. Tarpon spend their leptocephali larval stage in open ocean and as juveniles settle nearshore in warm estuarine, mangrove and lagoon habitats, where food resources are high and predator pressure is low [16, 19–21]. Adult tarpon typically range in size from 90–250 cm FL, with males reaching sexual maturity at 90 cm and females at 128 cm FL [17, 22–24]. Much of our knowledge of tarpon movements and behaviors come from satellite tracking and conventional anchor tag studies conducted in Florida, southeast Atlantic, Gulf of Mexico, and the northern Caribbean (e.g., Mexico, Belize, Cuba) [6, 17, 18, 25, 26]. These studies have focused on large-scale movements (> 500 km) of large adult tarpon (> 130 cm-FL). Little information exists on movements of tarpon within insular regions of the western Atlantic Ocean (i.e., the Lesser Antilles).
Acoustic telemetry is an effective technique for collecting direct and spatially explicit evidence of the small-scale three-dimensional movement patterns for fish. Target animals are implanted with a coded transmitter, typically in the body cavity, and released to be detected and recorded (location, depth, date, time) within an array of moored underwater acoustic receivers [5, 27–29]. Acoustic telemetry allows researchers to gather evidence on habitat utilization patterns, home range size, diel activity, site fidelity, migration pathways, ontogenetic habitat shifts, and vertical movements [27, 30]. In marine management, the data gained from acoustic telemetry studies have been used to design (size and placement) and evaluate effectiveness of marine protected areas [5, 28, 29], identify essential fish habitat  and assess the frequency of use within these areas [12, 13, 32].
This study applied acoustic telemetry to quantify the spatio-temporal movement patterns (i.e., activity space, depth gradients, rates of movement) of tarpon and the influence of changing environmental conditions in the U.S. Virgin Islands. This study addressed the following questions: a) what is the overall activity space of tarpon?; b) how does their activity space change during diel and crepuscular periods, as well as across months?; c) do tarpon show different rates of horizontal and vertical movement during different time scales (diel, monthly)?; d) how do changes in environmental conditions (i.e., water temperature, dissolve oxygen) influence activity space of tarpon?