Giant trevally (Caranx ignobilis) is a large (up to ~170 cm fork length, ~80 kg) top predator found broadly on coral reefs (Sudekum et al. 1991) but also enters estuaries and atolls as both juveniles and adults (Lédée et al. 2015; Meyer et al. 2007). It is sometimes referred to as a semi-pelagic species and is distributed in tropical and sub-tropical waters throughout Indo-Pacific (Glass et al. 2020). Due to their phenology, giant trevally have the ability to translocate nutrients over varying spatial scales (Papastamatiou et al. 2015; Sackett et al. 2017) and thus, this species provides an important ecological role.
Although captive specimens living more than 30 years may provide evidence of potential longevity (Sudekum et al. 1991), age and growth estimates using otoliths and bomb 14C dating indicated giant trevally showed rapid growth and maximum ages of 25 years old in Hawaii (Andrews 2020). Giant trevally display high site fidelity with varying migratory habits which is thought to be predicated on foraging success (Daly et al. 2018; Lédée et al. 2015; Meyer et al. 2007; Sudekum et al. 1991). Giant trevally is primarily viewed as a piscivorous predator that has a wide range of dietary preferences, including benthic invertebrates, and can occupy several trophic positions (Farmer and Wilson 2010, Meyer et al. 2001, Sudekum et al. 1991). Spawning has been documented in Hawaii to occur in the summer months during full moons (Sudekum et al. 1991). Spawning aggregations of ~1000 individuals (~80-100 cm TL) have been reported in the daytime at 15-20 m (da Silva et al. 2014), but there is no information on how far giant trevally will travel to participate in aggregations in Hawaii (Johannes 1978).
The South China Sea is the largest marginal sea of the western Pacific Ocean, which extends across tropical and subtropical areas (Morton and Blockmore 2001). Donghsha Atoll (DA, also called the “Pratas Island”), is one of the biggest islands located in South China Sea and is approximately 340 km southeast of Hong Kong, 260 km south of Shantou on the Chinese mainland, and 445 km southwest of Kaohsiung, Taiwan (Neo et al. 2018). Dongsha Atoll is a major reef with over 646 species of fishes (Chen et al. 2011) and plays an important role as the main breeding and nursery grounds for numerous marine organisms (Chou 2016; Dai 2004). Moreover, due to a well-developed coral reef structure with high coral coverage, DA has been utilized as an important commercial fishing ground for several countries including China, Hongkong, Vietnam and Taiwan (Dai 2004). Because of its significance as an important conservation area, since 2007, DA has been designated a Taiwanese National Marine Park (McManus et al. 2010). The effectiveness of Marine Protection Areas (MPA) resorting populations of marine predators is complicated when their range extends across different countries, where they may be subject to different fisheries and management regulations (Daly et al. 2018). However, it is important to implement conservation efforts that target key areas and biological processes which are critical for species conservation (Meyer et al. 2007).
Multiple techniques have been employed to describe the geospatial movements of marine animals and these advances have provided ecological insights by connecting animal movements to measures of their physiology and responses to the environment (Block et al. 2011; Hussey et al. 2015). Short-term acoustic monitoring has revealed diel and seasonal shifts in habitat use of giant trevally (Lowe et al. 2006) and experiments conducted in Hawaii and the Great Barrier Reef showed environmental parameters played a key role in movement patterns (Lédée et al. 2015; Meyer et al. 2007).
Pop-up satellite archival tag (PSAT) technology is an established fisheries-independent tool to monitor ambient depth (pressure), temperature, and light levels on tags attached to animals to chronicle both horizontal and vertical movement patterns (Block et al. 1998; Lin et al. 2020; Schlenker et al. 2021). PSATs can offer many benefits to study vertical dive behaviors, as well as providing information on migration routes, possible spawning areas, thermal habitat, exchange rates between areas and post-release mortality (Chang et al. 2020; Griffiths 2020; Madigan et al. 2020; Musyl and Gilman 2019). Despite regional faunal surveys, there is little information on giant trevally movement patterns at and around DA. Without effective management, there is concern the coral reef ecosystem of the Dongsha Atoll National Park may not persist due to typhoons, flood plumes, and climate change (Cheng et al. 2020). The objective of this research was to use PSATs to gain fundamental ecological information on movement patterns, habitat preferences and home range to inform decision makers on protecting resources of Dongsha Atoll National Marine Park.