When humans remove fish - especially those higher on the food chain - from coral reefs, the key ecosystem-sustaining nutrients cycled by these fish are reduced by half. These results, published in Nature Communications this week, shed light on the costs of over-fishing not only for fish communities, but also for the ecosystem services that fish provide.

In coral reef ecosystems, fish act as an important ‘reservoir’ of nutrients, which could be lost when fish are removed from the system through fishing. Yet it has been unclear whether this effect might be driven by a loss of biomass, or instead by changes in fish community structure.

Jacob Allgeier and colleagues studied the capacity of fish communities to store nutrients across a sample of 43 coral reefs in the Caribbean. Some sites experienced intense fishing pressure (resulting in loss of large predatory fish and dominance of small-bodied fish) while other sites were located within marine reserves with prohibited fishing (resulting in relatively intact fish communities). The authors find that nutrient capacity (a combination of storage and excretion) is higher in communities of fish with a wider range of body sizes and with higher-level predators. Further, nutrient capacity was approximately 40-46% lower at heavily-fished reefs than those protected from fishing, and it declined with increasing human population density in the vicinity of the reef.

The results suggest that, in terms of their contribution to the flow of nutrients, not all species are equal; the loss of nutrient cycling after fishing is driven mostly by the selective removal of large, predatory fish species.