Fishing is fundamentally altering the food chain in coral reefs and putting dual pressures on the valuable top-level predatory fish, according to new research by the Wildlife Conservation Society, Lancaster University, and other organizations.

Fish that sit at the top of the food chain, such as such as snapper and groupers, are highly sought-after in restaurants the world over, commanding a high price in fish markets and supporting fishing communities across the tropics, but maintaining them may be challenged by the complexity of the coral reef food web, according to a newly published study titled "Human disruption of coral reef trophic structure" in the journal Current Biology.

"Given the fragile state of the world's coral reefs it is important to understand how human activity such as fishing impacts upon coral reef ecology," said lead author Nick Graham of the Lancaster Environment Centre, Lancaster University. "Our study has shown these top-level predatory fish are only likely to be viable in overall lightly fished reefs, for example the Great Barrier Reef. To both conserve these top-of-the-food-chain fish, and to maintain fisheries for them, overall fish biomass on the coral reef needs to remain high.

Studying a large array of reefs in the Indian Ocean, the researchers found that reef food webs are altered in ways that can undermine valued species by bottom-up losses of available food as much as the top-down forces of fishing. These predators feed on other moderately sought fisheries species, such as parrotfish and surgeonfish, which appear both slow to recover from fishing but are also replaced by sea urchins as grazers, which are not the prey of these valued predators.

Consequently, replacing fish at the bottom of the food web with sea urchins may bolster the mid-tier species of fish but at a cost to the most prized predators. Fisheries in these situations struggle to maintain their preferred catch and pricey yields. The ecosystem is fundamentally altered in ways that may undermine the potential to recover their value.

The team also found that an hourglass food web shape emerges in what is frequently predicted to be an ecosystem pyramid, implying that energy in the ecosystem may accumulate at the top of the food chain by high productivity but low biomass of the mid-tier predators.

This suggests lightly fished systems support these valuable top-level fish if lightly fished but fishing lower in the food chain can cause a collapse of the top tier predators. This is also most likely to occur when these mid-tier predators are fished and when herbivorous fish are replaced by sea urchins at the bottom of the food chain. Sea urchins proliferate when their mid-tier predators are fished even lightly.

These ecological insights cast a new light on how to manage tropical fisheries and policies for maintaining intact food webs, filling an important gap in our understanding of fisheries targets on coral reefs.

"Previous research by our team has identified target levels of biomass which sustain fisheries for a diverse array of species, while maintaining ecosystem structure. This current work identifies a higher target for fisheries that aim to target predatory fish and focus on high value fisheries," said Dr. Tim McClanahan, Senior Conservation Zoologist of WCS and a study co-author. "Key to these targets is the objective of maintaining the ecosystem at the same time as supporting fisheries and livelihoods."

"Understanding how humans alter energy flows within coral reefs gives us another tool for deciding how much fish we can safely take for ourselves," said Dr. Aaron MacNeil of the Australian Institute of Marine Science, and Dalhousie University. "And by accounting for the energy stored in the system, we can choose to allocate effort to different parts of the food web and maximize overall catch and function."

McClanahan added: "Millions of people in coastal communities around the world rely on natural resources from coral reefs and other marine ecosystems. Studies such as this one that determine how much fishing these ecosystems can sustain are more important than ever if we are to keep coral reefs fully functional for sustainable use."