Better ocean connectivity boosts reef fish populations, finds study

Research led by the University of Oxford has found that oceanographic connectivity (the movement and exchange of water between different parts of the ocean) is a key influence for fish abundance across the Western Indian Ocean (WIO). The findings are published in the ICES Journal of Marine Sciences.

Connectivity particularly impacts herbivorous reef fish groups, which are most critical to coral reef resilience, providing evidence that decisionmakers should incorporate connectivity into how they prioritize conservation areas.

The study also revealed that alongside oceanographic connectivity, sea surface temperature and levels of chlorophyll (the green pigment in plants that drives photosynthesis) strongly predict reef fish distribution and abundance in the WIO. Protecting reefs is essential in this area, particularly for rapidly growing local communities, which are highly dependent on reefs and vulnerable to the impacts of climate change.

Lead author Laura Warmuth (Department of Biology, University of Oxford) said, “It was striking that herbivorous fish—which are critical to reef resilience—were particularly strongly impacted by ocean connectivity. Efficient conservation area prioritization should include connectivity for decision making regarding marine protected area management across country borders. This is particularly relevant in the human-pressured WIO region, where annual bleaching is predicted on most coral reefs by mid-century, even under optimistic climate change scenarios.”

Coastal communities are highly dependent on reefs for food security, with small-scale fisheries providing up to 99% of protein intake and around 82% of household income in the WIO. Home to some of the world’s poorest communities and seeing rapid population growth, locals are at an ever-increasing risk of climate change, which has the potential to devastate reefs with successive coral bleaching.

While sea surface temperatures are rising around the world, temperatures in the Indian Ocean are increasing faster than other tropical oceans—and it is one of the most vulnerable ocean regions to thermal stress. Fish diversity is central to reef resilience, and varying fish provide several key services to reefs through their different feeding patterns, such as feeding on algae which can compete with corals.

The researchers developed a metric of proportional oceanographic connectivity to simplify complex oceanographic models, allowing them to incorporate this element into ecological models. Typically, across the study reef sites, medium connectivity levels were associated with higher fish abundances, rather than high levels. High connectivity may help with larvae dispersal but can come with side effects, such as stronger wave exposure or increased dispersal of pollutants or invasive species.

The study revealed that sea surface temperatures and chlorophyll levels also had a strong influence on the abundance of fish species at all levels of the food chain.

Senior author Professor Mike Bonsall (Department of Biology, University of Oxford) added, “It is really imperative that decisionmakers responsible for marine planning understand how ocean patterns and environmental factors affect reef fish across the food chain. Our work emphasizes how crucial this link is between ocean currents and fish ecology for understanding the broader impact of environmental change and fishing regulations on sensitive coral reef fish systems.”

The researchers now plan to explore the impacts of human activities, including how human population density and market distance affect reef fish abundance and biomass in the WIO. They will also investigate how environmental and oceanographic factors are predicted to change for different climate change scenarios, and how fish abundances and distributions will change with them.

The study was a collaboration between the University of Oxford, the National Oceanography Centre in Southampton, UK, the Coastal Oceans Research and Development in the Indian Ocean (CORDIO) NGO in Mombasa, Kenya, the Institute of Zoology in London, UK, and the Bertarelli Foundation Marine Science Programme.