Posted by: The ocean update | September 29, 2016

Study suggests commercial fishing depletes key nutrients in coral reefs

Soft coral under Goff’s Caye, a small island near Caye Caulker in Belize. Photo via Wikimedia Commons.

Soft coral under Goff’s Caye, a small island near Caye Caulker in Belize. Photo via Wikimedia Commons.

September 29th, 2016 (Mike Gaworecki). In coral reef ecosystems, fish typically constitute a substantial portion of living biomass and thus represent an important reservoir of nutrients. So it makes sense that the removal of biomass via fishing impacts the nutrient capacity of coral reefs. A new study examines how drastic those impacts are.

Many of the ecosystems most impacted by human activities, such as tropical forests and coral reefs, are also among the most biodiverse in the world. But a recent study determined that conserving biodiversity may not be enough to ensure coral reefs rebound from the impacts of exploitation by mankind.

In coral reef ecosystems, fish typically constitute a substantial portion of living biomass and thus represent an important reservoir of nutrients. So it makes sense that the removal of biomass via fishing impacts the nutrient capacity of coral reefs.

And because nutrient inputs from outside reef systems are scarce, the replacement of nutrients removed by fishing occurs at a slow rate — “a dynamic that is analogous to the disruption of nutrient cycles in tropical rainforests following intensive timber harvest,” according to an article published in the journal Nature Communications last month.

But it is unknown exactly how much fishing — and, in particular, the selective exploitation of certain species — impacts the fish nutrient capacity of coral reef ecosystems, the authors of the article, a team led by Jacob Allgeier of the University of Washington, Seattle’s School of Aquatic and Fisheries Science, write in the article.

Maintaining and, where necessary, rebuilding coral reef fish communities is key to the food security and livelihoods of billions of people around the world, the authors note. A 2015 study found that reef fish biomass can take 35 to 60 years to recover from heavily depleted levels.

Allgeier and team estimated the impacts of fishing on coral reef fish nutrient capacity in 110 fish communities on 43 coral reefs in the Caribbean. Pressures on the reefs the team studied ranged from minimal fishing in well-protected marine reserves to heavy fishing activities on reefs with no protection.

The researchers found that nutrient capacity is crucial to the recovery of coral reef fish communities, and write that the results of their study suggest that “in addition to well-acknowledged conservation targets such as biodiversity protection, a broader perspective that incorporates predictable impacts of fishing pressure on nutrient dynamics is imperative for effective coral reef conservation and management.”

These results support commonly held assumptions about the theoretical relationships between species richness and ecosystem function, the team writes in the article — but they add that this relationship is nonexistent when accounting for the biomass of the communities.

“Fishing reduced fish-mediated nutrient processes by nearly half, but, contrary to expectations, reduction in nutrient processes was not due to species loss,” the team found. Instead, changes in feeding and size structure of the fish community was the main driver of reduced ecosystem function on the coral reefs they studied.

Species extinction is rightly a critical conservation concern, but the team concluded from their analysis that efforts to preserve feeding groups and community size structure are also needed to maintain nutrient capacity in coral reef ecosystems.

“Ecosystem function cannot be maintained simply by the presence of high numbers of species, as many other changes occur in the structure of the communities with fishing pressure,” the team writes.

The team determined that targeted fishing of species higher on the food chain is reducing the capacity of coral reef fish communities to store and recycle nutrients by nearly half, and that can slow the recovery of coral reef systems altogether, as nutrients enhance coral growth and primary production and may even regulate nutrient ratios at the ecosystem scale.

“Our findings underscore the growing need to incorporate animal-mediated nutrient dynamics in models of ecosystem function, particularly in light of the rapid rate of exploitation of animal biomass throughout the world,” they write. “Improved models of nutrient dynamics on coral reefs will also enhance our understanding of the negative impacts of anthropogenic nutrients for coral reefs that is drastically needed for conservation and management.”

Citations : Allgeier, J. E., Valdivia, A., Cox, C., & Layman, C. A. (2016). Fishing down nutrients on coral reefs. Nature Communications, 7. doi :10.1038/ncomms12461

MacNeil, M. A. et al. Recovery potential of the world’s coral reef fishes. Nature 520, 341–344 (2015). doi :10.1038/nature14358

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