5 days ago
Tracking Sharks Around A South Pacific Island
Tiger sharks were mostly seasonal visitors, especially mature females. Their peak residency was in ... More the Austral summer (November to April), and they largely disappeared during the cooler months. getty
Norfolk Island may be small (it measures as just 14 square miles or 35.7 km² of land in the middle of the South Pacific), but it's a busy place when it comes to sharks. A recent multi-year study tracked the movements of four species — tiger ( Galeocerdo cuvier ), sandbar ( Carcharhinus plumbeus ), dusky ( Carcharhinus obscurus ), and Galapagos sharks ( Carcharhinus galapagensis ) — using acoustic tags to figure out how they share space around this remote island. The findings show that even in relatively tight quarters, these predators use the island in remarkably different ways depending on age, season, and habitat preference, often shaped by both natural forces and human activity.
Located about 870 miles (1,400 km) east of mainland Australia and 460 miles (740 km) northwest of New Zealand, Norfolk Island is part of Australia's external territories and has a population of around 2,000 people. Sitting on the Norfolk Ridge, it is surrounded by deep ocean and influenced by both tropical and temperate currents. This mixture of temperature attracts an array of both migratory and resident marine species, and serves as a natural laboratory for studying remote island ecosystems. Its surrounding waters are considered ecologically significant, with parts overlapping marine parks and biodiversity areas.
The tags showed that tiger sharks were mostly seasonal visitors here, especially mature females. Their peak residency at Norfolk Island was in the Austral summer (November to April), and they largely disappeared during the cooler months. When these big sharks were around, they preferred the west side of the island, particularly an area near a long-time livestock waste dumping site, which provided an artificial but consistent food source. This same area is also home to a breeding colony of seabirds, offering another seasonal buffet for these big predators to dine on; these behaviors suggest that tiger sharks are not just following instinct, but are actively responding to what us humans do. Sandbar sharks showed similar timing as the tiger sharks, arriving mostly in the warmer months and spending time in deeper offshore waters. But unlike them, they didn't stick to one area, and while we don't know exactly where they went in the off-season, their disappearance suggests some sort of migration or habitat shift. All the sandbar sharks tracked in this study were likely immature, and their use of deep water aligns with what's been seen in other parts of the world, like Western Australia, where juveniles favor offshore nursery grounds. According to the scientists, it is still unclear whether Norfolk Island serves as a nursery or just a seasonal stopover.
Galapagos sharks were detected around the island year-round and often in the same nearshore areas, ... More especially around the Kingston and Cascade piers. getty
The Dusky and Galapagos sharks displayed different behaviors though. Both species were detected around the island year-round and often in the same nearshore areas, especially around the Kingston and Cascade piers. Why? Well, these spots are known for regular fish waste dumping by local fishers. The daily routine that may have trained the sharks to visit in search of an easy meal. Most of these individuals were immature, and their movements were tightly clustered around the tagging locations, which suggests a strong site attachment, possibly influenced by food availability or safety from predators. But what is especially interesting with these two species here is that dusky and Galapagos sharks rarely overlap in other parts of the world. Dusky sharks are usually coastal and associated with continental shelves, while Galapagos sharks prefer seamounts and remote islands. Around Norfolk? The story is different; they're not only overlapping in space, they're doing so in similar ways. They're returning to the same spots, day in and day out! And this co-residency raises questions about how they share resources and what kind of competition, if any, exists between them. Early data show dietary overlap too, which could mean these species play similar roles in the local ecosystem.
Not all tagged sharks were accounted for during the study period, however. In fact, a significant number were never detected again after tagging. That could be due to a few things: some sharks might have moved to deeper areas beyond the receiver range, others may have died due to predation (possibly from the very tiger sharks they shared space with), and some may simply have avoided the tagging locations afterwards. Still, the big picture here is that this study helps highlight the importance of seamounts and offshore islands as more than just isolated dots on a map. These places can clearly be essential marine habitats, especially for sharks, supporting different species and life stages in different ways. Whether it's a seasonal feeding stop, a year-round home, or a safe haven for younger individuals, Norfolk Island offers a snapshot of how sharks can partition space to coexist, even when their habitats overlap. Work like this is critical as conservation efforts increasingly focus on preserving whole ecosystem functions, not just individual species. Knowing who's using what, where, and when can guide protections to ensure these oceanic hotspots continue to support the diversity of life that relies on them.
