Groundbreaking research conducted by the University of Oxford has uncovered a crucial factor influencing fish abundance across the Western Indian Ocean (WIO): oceanographic connectivity. This refers to the movement and exchange of water between different parts of the ocean.
The study, recently published in the ICES Journal of Marine Sciences, highlights the profound impact of connectivity on herbivorous reef fish groups, which play a vital role in coral reef resilience. These findings provide compelling evidence for decision-makers to prioritize conservation areas based on oceanographic connectivity.
Furthermore, the research revealed that sea surface temperature and levels of chlorophyll, a green pigment in plants that drives photosynthesis, significantly influence reef fish distribution and abundance in the WIO. With local communities in this region heavily reliant on reefs and facing increasing vulnerability due to climate change, safeguarding these vital ecosystems is more critical than ever.
“It was striking that herbivorous fish – which are critical to reef resilience – were particularly strongly impacted by ocean connectivity,” lead author Laura Warmuth (Department of Biology, University of Oxford) said. “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.”
In the Western Indian Ocean (WIO), coastal communities rely heavily on reefs for food security, with small-scale fisheries contributing up to 99% of their protein intake and approximately 82% of household income. However, these communities, home to some of the world’s poorest populations, are facing escalating risks due to rapid population growth and the looming threat of climate change. The potential devastation of reefs from successive coral bleaching events poses a significant danger to their way of life.
Notably, the Indian Ocean is experiencing faster temperature increases compared to other tropical oceans, rendering it one of the most vulnerable regions to thermal stress. The resilience of these reefs hinges on the diversity of fish species, which play a crucial role in maintaining reef health through their varied feeding patterns, including the control of algae that can compete with corals.
The researchers have developed a groundbreaking metric of proportional oceanographic connectivity, simplifying complex oceanographic models and incorporating this crucial element into ecological models. Across the study reef sites, they found that medium connectivity levels were associated with higher fish abundances, outperforming high connectivity levels. While high connectivity may aid larvae dispersal, it can also bring about adverse effects such as stronger wave exposure and increased dispersal of pollutants or invasive species.
Furthermore, the study unveiled the significant influence of sea surface temperatures and chlorophyll levels on the abundance of fish species at all levels of the food chain.
“It is really imperative that decision-makers 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,” added senior author Professor Mike Bonsall from the Department of Biology at the University of Oxford.
The researchers are planning to investigate how human activities such as population density and market distance impact reef fish abundance and biomass in the Western Indian Ocean (WIO). Their goal is to uncover the potential changes in environmental and oceanographic factors under different climate change scenarios and how these changes will affect fish abundances and distributions.
This groundbreaking study is a collaborative effort involving 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.
Journal reference:
- Laura M Warmuth, Stephen Kelly, Melita A Samoilys, Ekaterina Popova, Catherine E I Head, Michael B Bonsall. Environmental change and connectivity drive coral reef fish abundance in the Western Indian Ocean. ICES Journal of Marine Science, 2024; DOI: 10.1093/icesjms/fsae125