More diving activity, fewer reef sharks on Caribbean reefs

The study is based on 995 underwater video recordings made between 2012 and 2017 using so-called Baited Remote Underwater Video systems (BRUVs). The research covered seven reef systems across the Caribbean. Cameras were deployed around six inhabited islands in the Dutch Caribbean and on the Saba Bank, a large and remote reef area with minimal human disturbance.

“What stands out is that we observe this pattern even on reefs that are otherwise in good ecological condition,” says Twan Stoffers, fish ecologist at Wageningen University & Research and lead author of the study. “This suggests that human presence plays an important role in shaping where these animals occur.”

Around the inhabited islands, reef shark presence was strongly associated with diving activity and coastal development. A different pattern was found on the Saba Bank. On these remote reefs, shark occurrence was primarily linked to habitat quality, while along populated coasts sharks were more often found in areas with lower human pressure.

“On the Saba Bank, where human disturbance is minimal, natural factors such as water depth and reef structure mainly determine where reef sharks occur,” Twan Stoffers explains. “On populated reefs, these natural factors play a smaller role, and patterns are more closely linked to human activity.”

This means that the same species responds to different drivers depending on local conditions.

The influence of human activity and environmental factors differed clearly between species. For Caribbean reef sharks, lower numbers were associated with both intensive diving activity and higher levels of coastal development. These sharks were more frequently observed farther offshore and on less disturbed reefs.

For nurse sharks and southern stingrays, bottom-dwelling species, natural reef characteristics were more important. Their presence was more strongly linked to water depth and reef structure than to human activity. Nurse sharks were mainly found on structurally complex reefs, while stingrays were more often observed on flatter, less structured sea beds.

Sensitivity to human presence therefore varies among species: reef sharks appear more responsive to disturbance, while other species are primarily influenced by natural environmental factors and therefore reef quality.

To map human activity around the reefs, the researchers combined ecological field data with open data sources. Geolocations of underwater photographs shared on social media were used as a proxy for diving activity and shoreline activity by tourists, while publicly available spatial data provided information on the extent of coastal development.

“By combining underwater video data with spatial indicators of diving activity and coastal development, we were able to analyse patterns across a large region,” says Twan Stoffers. “This approach is especially valuable in areas where detailed data on recreational use and human-made infrastructure are not available.”

The findings show that, alongside natural habitat characteristics, human presence around reefs is associated with reduced reef shark occurrence. While previous research has largely focused on fishing and habitat destruction as the primary driver of shark declines, this study indicates that tourism and coastal development also influence where reef sharks are found.

According to the researchers, this underscores the importance of explicitly considering non-extractive human activities in ecological research and reef management, alongside traditional factors such as habitat quality and water depth.