OCEAN CONNECTIVITY &
reef Fish Abundance
Better Ocean Connectivity Boosts Reef Fish Populations
Research led by the University of Oxford and conducted in collaboration with CORDIO East Africa 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 were published in October 2024 in the ICES Journal of Marine Sciences.
Background
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. Climate change also impacts fish larvae dispersal and connectivity due to changes in ocean temperatures and currents, which could hinder reef resilience. Understanding these dynamics is essential for effective marine conservation, particularly in the Western Indian Ocean (WIO), where limited ecological knowledge exists. Connectivity among coral reef regions remains poorly studied, but it is vital for species persistence and can be influenced by oceanographic factors.
The Study
Between 2009 and 2015, reef fish data were collected from 51 sites across coral reefs in Comoros, Madagascar, Mozambique, and Tanzania using underwater visual census techniques. Surveys focused on four trophic groups critical for reef resilience, standardized within specific depth ranges. Environmental data, including sea surface temperature and chlorophyll a levels, were sourced from NOAA archives and summarized for each site. The study employed a “proportional oceanographic connectivity” metric to assess connectivity among reefs, using Lagrangian particle tracking to evaluate how many reefs were connected to each survey site over 30 days. The analysis utilized Generalized Additive Models (GAMs) to explore relationships between fish abundance and environmental variables, ensuring model robustness and addressing multicollinearity. Final models were selected based on Akaike’s Information Criterion, optimizing the fit for ecological insights.
Click the PDF below to read the full paper.
https://doi.org/10.1093/icesjms/fsae125
Findings and Recommendations
Connectivity particularly impacted 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 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 prioritisation 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.”
CORDIO Director Dr. Melita Samoilys, who co-authored the paper said: “The study shows that oceanographic connectivity between reefs in our region is a major driver of the abundance of reef fishes and this differed between different taxa. Higher connectivity did not always mean higher fish abundance. Interestingly, reefs with medium levels of connectivity were associated with the highest abundances of herbivores and detritivores suggesting replenishment of their populations is influenced by smaller scale factors. Clearly, this deeper understanding of ocean scale drivers of reef fish populations has local conservation relevance. For example, good local level management at sites can amplify herbivores populations at reefs that are moderately well connected. Since these fishes are key fishery species in the western Indian Ocean, this will build social resilience as well as ecological resilience.”
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 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.”
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.
This study was a collaboration between the University of Oxford, the National Oceanography Centre in Southampton, UK, CORDIO, the Institute of Zoology in London, UK, and the Bertarelli Foundation Marine Science Programme.