Ontogenetic shifts in home range size of a top predatory reef-associated fish (Caranx ignobilis): implications for conservation

Defining the home range of vulnerable species is critical for designing effective spatial management strategies. However, animal home ranges often change with ontogeny, and quantifying the associated temporal and spatial changes is particularly challenging for mobile marine species. Here, we investigated how the space use of a top predatory reef-associated fish (giant trevally Caranx ignobilis) scales with body size. Fish were tagged with acoustic transmitters and passively tracked for > 3 yr at a tropical island and atoll in the Republic of Seychelles. A sheltered atoll environment was critical for juvenile fish (< 60 cm fork length, FL) that exhibited a shift in home range location and area as they matured into adults. Small (60-100 cm FL) and large (>100 cm FL) adult fish appeared to favour shallow coral reefs and associated reef drop offs whilst sharing a similar core home range location. Large adult fish utilized a greater diversity of habitat types and had significantly (p < 0.05) greater annual dispersal distances (mean = 35.29 km, max = 91.32 km) than small adults (mean = 13.72 km, max = 21.55 km). Additionally, the home range of large adults (mean = 209.74 km(2)) was significantly (p < 0.05) larger than that of juveniles (mean = 38.73 km(2)) and small adults (77.32 km(2)) and there was a significant (p = 0.02) relationship between fish length and home range size. Furthermore, tagged fish took up to 34 mo (mean = 18.54 mo) to utilize the full extent of their home range. The habitat shift and expansion in home range size throughout ontogeny should be taken into account when designing effective spatial management plans for C. ignobilis.

Automated summary

In this study conducted in the Republic of Seychelles, it was found that the space use and home range of giant trevally (Caranx ignobilis) changed with body size as they matured. Juvenile fish preferred sheltered atoll environments, while large adult fish utilized a greater diversity of habitat types and had significantly greater annual dispersal distances. The expansion of home range throughout ontogeny should be considered in spatial management plans for this species.