When investigating spatial protection for fish and their associated habitats, many studies have considered home range size and egg and larval dispersal of the species concerned (inter alia [31, 32]). However, few studies have considered the implications of changes in home range size that occur with ontogeny, especially for more mobile species. In this study we show that for giant trevally (Caranx ignobilis), a top reef predator and an iconic fishery species, habitat connectivity, dispersal distance and home range size increases significantly with fish size. Additionally, the type of habitat used by adults differs fundamentally from the more sheltered nursery habitats used by juveniles. This has important implications for spatial management of the species and the habitats that it frequents.
Seven out of the 17 tagged fish in this study were monitored for the full period of this study (3.5 years) representing some of the longest tracked and monitored giant trevally on record [10, 16, 33]. Additionally, tagged fish ranged in size from 45 to 135 cm (calculated size) representing the largest range in size of tracked fish as well as some of the largest giant trevally tracked on record [33, 34]. The overall detection frequency and residency index of tagged fish within the receiver array at the study site was relatively high (mean = 67.82) and similar to previous studies [10, 34] suggesting that the receiver array at the study site provided good coverage of the movements of this species (Fig. 2, Table 1).
Although we had a limited sample size of juveniles to thoroughly investigate the habitat use of this life stage, evidence from this study appears to support previous research, highlighting the importance of sheltered habitats for juveniles before they disperse and move between habitats over broader spatial scales when reaching approximately 50–60 cm in fork length [19, 20]. Sheltered bays, lagoons and estuaries often form a critical nursery environment for juvenile fishes as these habitats provide shelter, food and relatively low predation risk . Indeed, the sheltered habitat provided by the St. Joseph Atoll lagoon may play a key role for many fish species that exhibit ontogenetic migrations between a sheltered nursery habitats to more exposed coral reef dominated ecosystems . Such nursery habitats can play a critical role in the structure, diversity and biomass of associated fish communities and it appears that St. Joseph Atoll may be particularly important for giant trevally recruitment as it is one of only a few sheltered habitats within the broader region [37, 38].
The transition from juvenile to adult habitat use in giant trevally appeared to be characterized by a substantial shift in habitat type from an atoll lagoon to a coral reef environment. Many species undertake ontogenetic migrations and transition out of one habitat to another as their requirements for shelter, food and reproduction change [1, 5, 7, 39]. Indeed, the distinct change in habitat use recorded between juvenile and adult phases in this study suggests the occurrence of a shift in the functional phase of the fish. This is supported by the fact that there was a 4 to 5 fold increase in home range size (as measured by the core range, activity space and occupancy area) from the juvenile to small adult phase which coincided with the use of new habitat types (Table 2, Fig. 4). Such a rapid increase in the area occupied by fish and associated habitat shift may be linked to attaining a size threshold where predation risk decreases substantially [8, 40]. As giant trevally grow into adults they would have less risk of predation from the many top predatory sharks present at the study site known to prey on small reef fish including juvenile giant trevally [41–43]. Such a release of predation risk is probably an important factor determining the timing of the transition between juvenile and adult habitat for giant trevally [40, 44]. Additionally, the transition from the juvenile habitat to adult habitat occurs close to the size at sexual maturity (60 cm) and it is likely that reproduction is also a key driver of this shift. Such an ontogenetic step change in area use and habitat type, as observed in this study, is indicative of shift into maturity driven by reproductive demands .
Large adults appeared to connect a wider diversity of habitat types from shallow reefs, atoll sand flats and lagoons to broad ranging excursions throughout the Amirante Bank (Fig. 5). Both small and large adults appeared to share a common central area of use with similar core habitat types with a diverse and productive marginal coral reef around D’Arros and St. Joseph as well as the deep channel between these islands which has been observed to be an especially productive feature utilized by other large reef fish at the study site [24, 25, 45]. Thus, it appears that this core area meets the trophic demands of all small adult fish, but potentially not the large adult fish which range throughout the Amirante Bank . By the end of the monitroing period the largest adult fish would have weighed in excess of 40 kgs (maximum size fish ID 17 was calculated to measure 135.5 cm or 48 kg in year 3) and as such, represented a large-bodied top predator with substantial dietary demands. To fullfil their energetic requirements, these large adult fish likely need to range further than smaller adults to locate sufficient feeding opportunities. Similar wide ranging movements driven by foraging are observed in many other species and often occur as these animals need to increase their prey encounter rates [1, 46].
The maximum annual step dispersal distance recorded by a juvenile giant trevally (ID 3) appeared to be representative of the single foray out of St. Joseph Atoll (Fig. 3). This excursion was likely the start of the fish’s transition to an adult environment as the fish attained over 60 cm FL within the first year of monitoring. Unfortunately, due to post release mortality, the annual maximum dispersal distances of the other juvenile fish tagged in this study could not be determined but observations confirmed that juveniles are likely confined to the atoll lagoon habitat. The observed significantly (p < 0.05) greater annual mean step dispersal distances exhibited by large adults was most likely driven by foraging demands as discussed previously. Interestingly, for those small adult fish that were tracked over multiple years, some fish increased their maximum dispersal substantially as they grew (fish ID 9 and 10) whilst some others appeared to maintain relatively similar maximum dispersal distances over the monitoring period (fish ID 12). This may reflect some individual differences in habitat use. For fish ID 17, the recorded annual maximum step dispersal distance remained constant (year 1 = 91.08, year 2 = 91.32, year 3 = 90.24) highlighting the mobility of this large adult fish and it suggests that the extent of the receiver array may not have been large enough to capture this fish’s true maximum dispersal distances. Indeed, giant trevally have been tracked undertaking return migrations to and from spawning aggregations of over 1200 km emphasizing the remarkable dispersal capabilities of this species . However, tagged fish remained on the Amirante Bank and did not cross the deep channel (oceanic trench) separating the bank from Desroches Island, approximately 40 km away. Hence, it is possible that the maximum dispersal distance is restricted to the extent of the Amirante Bank and that dispersal is limited by deep water (> 1000 m) as fish may not cross deep open ocean. It is however possible that early life history stages (eggs and larvae) are transported between the bank and other other island because evidence from genomic work indicate a panmitic population throughout the southwest Indian Ocean (JR Glass, pers comm).
The overall annual core home range (0.44–9.31 km2) and activity space (1.36–255.89 km2) calculated in this study was orders of magnitude greater than the monthly core range (0.0001–0.016 km2) and activity space (0.004–0.062 km2) of giant trevally monitored at a coral reef in Australia . Although, this may simply reflect the different time scales (annual vs monthly) and the different spatial extent of the respective receiver arrays. However, giant trevally in both studies did appear to exhibit a persistent core area of habitat use representing a key habitat and site attachment. The size of the core range and activity space between small adult and large adult was not significantly different (p > 0.05), however, there was a significant (p < 0.05) increase in the maximum occupancy areas between small (92.74 km2) and large (207.57 km2) adult fish. The similar core ranges of small and large adult fish most likely represent an overlapping key habitat whilst the increase in maximum occupancy areas between small and large adults appears to be driven by the increased dispersal distance undertaken by large adults.
In comparison to many other coral reef-associated teleosts, the home range size calculated for giant trevally in this study (represented by the total occupancy area, Table 2) was substantially larger  (see supplimentary Table S1 therein). In fact, the total occupancy area of large adult giant trevally was more similar to and or greater than several large bodied shark species (Negaprion acutidens, Carcharhinus melanopterus and Carcharhinus amblyrhyncos) monitored at the same study site . This highlights the ecological role played by adult giant trevally as a top predator linking ecological processes over large spatial scales. Indeed, the trophic position of giant trevally was recently found to be equivalent to many top predatory shark species confirming its role as an apex predator within their respective marine communities .
Home range scaling
As animals requirements for resources increase with body size, an increase in home range size typically occurs . However, this relationship is complicated by predation risks and reproductive demands. For some reef fish, the social status linked to reproduction limits the home range of mature adults and breaks down the relationship between fish size and home range area [8, 45]. However, giant trevally do not have such reproductively driven social and territorial constraints, other than the need to aggregate annually to spawn. Previously, giant trevally have been recorded travelling over 1200 km between annual spawning aggregation events . Therefore, the observed relationship between fish length and home range area (Fig. 8) is likely driven by foraging events as the metabolic demands of larger fish increase with size and they range further in search of prey . However, fish size only explained 33% of the home range size variance in the linear model, suggesting that other factors are also important determinants of home range size for this species. Besides the step change in home range location and size from juveniles to adults, likely driven by reduced predation risk and maturation, individual foraging strategies and learned behaviour may also play an important role determining the home range of these fish. Results from the linear mixed model also revealed a strong influence of individual ID (57% deviance explained) on reported maximum occupancy area in comparison to the effect of fish size (13% deviance explained). This is likely due to the relatively small cohort of tagged fishes available in this study, the tagging bias towards small and large adults (due largely to post-release mortality occurring in juveniles) and a reliance on original length measurements to estimate future fish growth and sizes throughout the monitoring period. In order to further refine the current understanding of spatio-temporal home range scaling in giant trevally relative to size, future studies should aim to increase the number of tagged fishes and ensure an even distribution of tags across ontogenetic stages throughout the entire monitoring period.
Time to maximum occupancy
The time it took individual fish to reach their total area of occupancy varied between 7 and 34 months suggesting that individual fish may expand their home range size over variable and long periods of time (Fig. 9). For example, large adult fish ID 12 rapidly (6 moths) utilized its total area of occupancy whilst large adult fish ID 10 took 28 months to utilize its total area of occupancy. Although both these fish were large adults, presumably with similar metabolic requirements, it suggests that individuals may adopt new foraging strategies based on learned behaviour or changing prey availability over time . Further research is required to better understand how important learned behavior and individual foraging strategies are in determining fish habitat use and home range size. Nevertheless, this study confirmed that the maximum extent of a fish’s home range size may only be recorded after long periods (at least 18 to 34 months). This highlights the need to carefully consider the duration of the monitoring period and the spatial extent of receiver arrays for acoustic telemetry studies that aim to define the full extent of space use and home ranges of mobile aquatic species.
Implications for conservation
This study highlighted the importance of sheltered habitat within the St. Joseph Atoll as a nursery area for giant trevally in the region and confirmed that the juvenile and small adult core areas of habitat were spatially separate. The sheltered habitat of St. Joseph Atoll likely plays a critical role in recruitment success of giant trevally, as well as many other nursery dependent reef fish species and should thus be prioritised for conservation [37, 38]. For example, the protection of St. Joseph Atoll lagoon, reef flats and associated coral reef crests and drop offs will also protect critical habitat for Endangered humphead wrasse and other vulnerable species that frequent this unique habitat [41, 42, 45]. However, as highlighted in this study, prioritising the protection of St. Joseph Atoll should be done together with the surrounding marine environment to promote ecological processes and facilitate demographic and geographic linkages [9, 31]. This study has shown that giant trevally are capable of wider ranging movements than previously reported [33, 34, 49, 50] and to effectively protect the core range and activity space of adults, a protected area of at least 72.65 km2 of suitable habitat would be required. Additionally, to account for the dispersal distances and total area of occupancy of large adults, such a protected area would need to encompass at least 168.36 km2, but up to 678.71 km2, representing the maximum recorded occupancy area of a tagged large adult giant trevally. Protecting such vast areas represents significant challenges for management and enforcement but the information garnered in this study can be used to help prioritise effective conservation measures . The newly proclaimed Marine National Parks in Seychelles are a positive step towards large protected areas which may incorporate large portions of habitat of wide ranging species (Fig. 1, Seychelles Official Gazette No. 5). Additionally, we recommend that St. Joseph is prioritised for conservation with a ban on consumptive fishing and that further research is conducted into identifying the potential spawning aggregation sites within the recorded activity space of tagged adult fish. In order to maximise the effectiveness of such a protected area, we suggest expanding it from St. Joseph Atoll to include the associated habitat of co-occuring endangered and vulnarable species to encompass an area of at least 72.65 km2 around St. Joseph Atoll.
This study has confirmed that giant trevally exhibit an increase in space use area with increasing body size supporting our primary hypothesis. An important driver of a shift in home range location and area / habitat type from the juvenile to adult phase appeared to be a release in predation risk and a shift to sexual maturity. The core activity space of small adult and large adult fish was largely similar but the maximum dispersal distance and occupancy area of large adult fish was substantially greater than that of small adult fish, probably driven by foraging excursions as the metabolic needs of such large fish increase. The study also found that the home range of giant trevally at a tropical island and atoll marine ecosystem was larger than previously reported  and that individually tracked fish took long periods of time (mean = 18.54 months) to utilise the full extent of their activity space. These findings indicate that in order to effectively conserve this iconic and ecologically important top predatory fish, the conservation of sheltered nursery areas should be prioritised and that the extent of small and large adult home ranges and dispersal distances need to be taken into consideration.