University of Oxford Research Highlights Ocean Connectivity as Key to Fish Abundance in the Western Indian Ocean

According to new research led by the University of Oxford, oceanographic connectivity- how different regions of the ocean move and exchange water-is a dominant influence for fish abundance across the Western Indian Ocean.

The findings are published today in the ICES Journal of Marine Sciences. Accordingly, connectivity most strongly influenced groups of herbivorous reef fish that are most critical to coral reef resilience, thus suggesting that connectivity should be integrated into how decision-makers prioritize conservation areas. Results also revealed that, in concert with oceanographic connectivity, sea surface temperature and levels of chlorophyll-the green pigment in plants responsible for photosynthesis-strongly predict the distribution and abundance of reef fish in the WIO.

Poor reefs protection in this region is notably important for the rapidly growing local communities that depend so much on the reefs and are highly vulnerable to the impacts brought about by climate change. The lead author, Laura Warmuth of the Department of Biology at the University of Oxford, said: "It was striking that herbivorous fish-which are critical to reef resilience-were particularly strongly impacted by ocean connectivity.". Furthermore, there should be efficiency in conservation area prioritisation and also apply connectivity in decision making for marine protected area management across country borders.

This is of especial pertinence in the human-stressed WIO region, where annual bleaching is forecasted by mid-century on most coral reefs under even the most optimistic eventualities of climate change'. Reefs provide food security for communities, with small-scale fisheries furnishing as much as 99% of protein intake and about 82% of household income in the WIO. Due to the fact that it hosts part of the poorest communities in the world and its population is rising rapidly, locals are increasingly at risk of the curse of climate change that could decimate reefs through successive coral bleaching. While the surface temperatures of the sea are rising globally, temperatures of the Indian Ocean are rising a bit faster compared to other tropical oceans, making this one of the most vulnerable ocean regions to thermal stress. Fish diversity provides certain key services to reefs through different feeding patterns.

For example, feeding on algae reduces a competitor for space with corals. The researchers developed a metric of proportional oceanographic connectivity that simplifies complex oceanographic models, allowing the researchers to incorporate this element into ecological models. Over the study reef sites, however, medium connectivity levels were more closely associated with higher abundances of fish, instead of high levels. High connectivity may favor larvae dispersal but is also similarly associated with negative side effects due to stronger wave exposure and increased levels of dispersal of pollutants or invasive species. In this study, sea surface temperatures and chlorophyll also had a strong effect on the abundance of all fish at every level of the food chain. Co-author Professor Mike Bonsall, Department of Biology, University of Oxford added: "It is eminent that the decision-makers responsible for marine planning understand how ocean patterns and environmental factors affect reef fish across the food chain.".

Our work underlines the importance of this linkage between ocean currents and fish ecology for understanding the wider role 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. Another aspect it will touch on is the manner in which oceanographic and environmental factors are forecast, with respect to the various climate change scenarios, and how fish distributions and abundance will change on that vector. The team collaborated with the University of Oxford, National Oceanography Centre in Southampton, UK; NGO Coastal Oceans Research and Development in the Indian Ocean, CORDIO, located in Mombasa, Kenya; Institute of Zoology in London, UK; and the Bertarelli Foundation Marine Science Programme.