Better ocean connectivity boosts Western Indian Ocean reef fish populations

New research supported by National Oceanography Centre (NOC) scientists 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).

Connectivity particularly impact herbivorous reef fish groups, which are most critical to coral reef resilience, providing evidence that decision-makers should incorporate connectivity into how they prioritise conservation areas. The study, led by the University of Oxford, 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.

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.

Protecting coastal livelihoods

Dr Katya Popova, a co-author of the paper and a Principal Scientist in NOC’s Marine Systems Modelling Group, said: “It is rewarding to see our research on ocean connectivity and our cutting-edge models playing a crucial role in providing information relevant to establishing marine protected areas.

“This is particularly important for the Western Indian Ocean, where the livelihoods of coastal populations critically depend on coral reefs that are under immense pressure from 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 prioritisation should include connectivity for decision making for 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.”

Rising sea surface temperatures

While sea surface temperatures (SST) 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, providing several key services to reefs by 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 connectivity. 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 SST and chlorophyll levels also had a strong influence on the abundance of fish species at all levels of food chains. Senior author Professor Mike Bonsall added.

“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.”

Exploring the impacts of human activities

The researchers now plan to explore the impacts of human activities, including how human population density and market distance impacts on 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 NOC, the University of Oxford, 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.

The findings have been published in the ICES Journal of Marine Sciences.

Marine Systems Modelling

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