Conserving fish biodiversity helps protect coral reef health

Parrotfish. Image: Acquarius Sea Tours

Research from the Dominican Republic shows how greater fish biodiversity makes for healthier coral reefs:

The health of coral reefs can be impacted as much by the diversity of fish that graze on them as by the amount of fish that do so, according to a new study by scientists at Bigelow Laboratory for Ocean Sciences.

The findings of the research, recently published in Science Advances, build on existing knowledge about how healthy fish populations contribute to healthy reefs. Herbivorous fish, like parrotfish and surgeonfish, graze on coral reefs and help keep reef surfaces clear of algae. Well-grazed reefs are known to be better for coral reproduction, growth, and survival. However, in the Caribbean, overfishing of parrotfish has been identified as one of the leading causes of the decline in coral reef health. A shortage of grazers means that Caribbean reefs are being smothered by algae; in fact Caribbean reefs seem to be more susceptible to algal growth than reefs in other parts of the world. In response to this overgrowth of algae, campaigns have been launched to encourage fishers and consumers to “pass on parrotfish” so that grazer populations can recover. The more grazers there are on a reef, the more likely that reef is to be healthy.

What this new research shows is that it’s not just the number of grazers that matters, but also the diversity of types of grazers:

“Scientists have long known that reefs are healthier when a large number, or a ‘high biomass,’ of plant-eating fish graze their surfaces,” said Doug Rasher, senior author of the study and a senior research scientist at Bigelow Laboratory. “However, this study revealed that having a diverse portfolio of those fish species on the reef is equally important to keeping reefs well grazed and hospitable to baby corals.”

Why is this? Simply put, different species of herbivorous fish consume different kinds of algae on different parts of the reef:

The algal turf community considered here, while superficially homogenous, is actually a consortium of filamentous algae, crustose coralline algae, seaweed germlings, microorganisms, detritus, and a variety of endolithic resources. … Different grazer species consume these resources in a complementary fashion, through targeting different taxa and spatially partitioning their feeding across microtopographic features [the millimeter to centimeter scale and among vertical versus horizontal surfaces [the centimeter to meter scaleThese, too, are the small scales at which algal-coral competition and coral settlement occur on the reef.

So increased fish diversity means increased efficiency in keeping reefs clear of algal growth:

Our findings imply a diversity-mediated cascade, wherein diverse herbivore assemblages more effectively crop the reef, in turn creating a more hospitable environment for coral settlement and survival, ultimately enhancing reef integrity.

The findings suggest that we may need new approaches to fisheries management for coral reef health in the Caribbean:

The findings have significant implications for the management of tropical fisheries. Many herbivores, such as parrotfish and surgeonfish, are eaten by people across the Caribbean. Management plans often focus on sustaining a given number of herbivores on a reef, and not necessarily promoting species diversity within the herbivore community.

“Fisheries are typically managed at the level of country or region, not the reef,” Rasher said. “This research shows that fostering fish diversity — both at the local and regional scale — is more important than we realized to the health of coral reefs.”

The full research article is available online: “Tropical fish diversity enhances coral reef functioning across multiple scales”.

[Image: Acquarius Sea Tours]

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