Coral reefs are known for their aesthetic beauty and rich diversity of fish, making them popular tourist destinations, as well as providing sustainable livelihoods for coastal communities worldwide. Unfortunately coral reefs, and the species they sustain, are declining across the world, due to climate change and human impacts on marine ecosystems.

Corals and Fish Diversity

Biodiversity is the variety of living species that can be found in a particular ecosystem. Coral reefs are called “rainforests of the ocean” due to the high levels of biodiversity depending on coral reefs. Occupying less than 1% of the ocean floor, coral reefs are home to more than 25% of all marine fauna and flora. This diverse ecosystem is important, as a more biodiverse ecosystem can better withstand significant disturbances. In addition, humans also receive benefits from coral reefs in the shape of ecosystem services, through the provision of food, medicine, storm and flood protection, and a revenue for fishing and eco-tourism. An estimated 6 million fishermen in 99 countries worldwide depend on coral reefs for their livelihoods – over a quarter of the world’s small scale fishermen. Reef fishes are a valuable part of marine biodiversity and play important roles in the marine food web – from predators to prey – but they also provide protein for millions of people.

Declining Corals

Coral reefs are sensitive habitats, and are falling victim to destructive human activities. Overfishing, unsustainable tourism practices, marine pollution and climate change are threatening coral reefs worldwide. Corals are physically destroyed by ghost fishing gear and plastic litter, and coral bleaching is occurring due to changes in ocean conditions. Coral bleaching is a coral’s response to stressful conditions. When a coral is stressed, it expels the symbiotic algae that lives within its skeleton, causing the corals to turn white. If bleached for prolonged periods, the corals can die. Bleaching is strongly associated with heat stress, however changes in acidity, salinity and light may also cause corals to bleach.

As coral reefs host a wide diversity of marine life by providing food, habitat and shelter, it is expected that a decline in coral reefs will have knock-on effects in the ecosystems. A new study published in Proceedings of the Royal Society B: Biological Sciences investigated this hypothesis, and suggested that in many areas fish biodiversity could be halved as a result of coral reef decline.

Linking Coral and Fish Diversity

An international team of researchers, led by Giovanni Strona from the University of Helsinki, assessed to what extent fish diversity depends on coral diversity. To determine this, the researchers took maps of the world’s biggest reef regions, and divided the maps into a grid of cells measuring one latitudinal per one longitudinal degree. Fish and coral diversity were mapped for each cell by collecting and collating data from online databases compiled by the International Union for the Conservation of Nature (IUCN). In total the team looked at 6964 coral-reef-fish species and 119 coral genera.

The researchers created a mathematical model to determine how fish diversity varies in relation to coral diversity. The researchers looked at factors such as salinity, temperature and reef isolation. Using this model, they investigated three scenarios described in the latest Intergovernmental Panel on Climate Change (IPCC) assessment to understand how the expected future loss of coral reefs due to climate change would affect fish biodiversity. In terms of coral bleaching, the scientists assumed a substantial coral mortality in a given reef locality at a given time if the scenario projected temperature increases two degrees higher than the historical average.

The Results of the Study

Using the most pessimistic climate scenario, reefs worldwide will be affected by at least one mass mortality event by 2060. Under the most optimistic one, more than 60% of coral reefs would still experience a mass mortality event. In the most pessimistic scenario, local fish diversity would decline by an estimated 40% by 2060. This represents more than the fish living on the reefs, indicating that other species would be indirectly affected by a decline of coral reefs.

Unfortunately the model has a limitation: it does not reveal much about which fish species will be more affected. The research team is now focusing on this, as determining this will be vital for preventing biodiversity loss.

On their own, fish species are projected to survive changes in oceanic conditions due to climate change, however corals are not so tolerant. Although not many fish depend directly on corals for food and shelter, many fish species will be secondarily affected, possibly leading to extinction. Consistent with other small-scale studies, the models in the study suggested that in a hypothetical world without coral, local fish richness across the globe would be around half of its current value. This will lead to more areas with low or intermediate fish species richness and fewer fish diversity hot spots.

Looking Closely at Papua New Guinea

An 8-year study done in Papua New Guinea and published in PNAS supports the findings of the modeling study. A devastating decline in coral cover caused a parallel decline in fish biodiversity, both inside marine protected areas and in areas open to fishing. The study found that over 75% of reef fish species declined in abundance, and 50% declined in less than half of their original numbers. The species that depended on coral reefs as juvenile recruitment sites declined the most in abundance. Rare coral specialists became locally extinct.

Many marine ecologists are expressing serious concern over the worldwide decline in coral cover due to global warming, coral bleaching, overfishing and pollution. Most attention to the protection of fish species has focused on marine protected areas, preventing fishing in areas. However, comprehensive strategies for protecting marine diversity should depend on an understanding of how species respond to degradation of their habitats. Although marine reserves are necessary for the top-down impact of human fishing, these conservation actions must be combined with management strategies that address bottom-up processes, as these processes are more likely to lead to extinction.