When big fish "pee"...


Jahson Alemu, Marine Biologist, talks about coral reef health, and the important balance that some species contribute in unimagined ways. This feature was first published in the Tobago Newsday on Thursday, 19th January 2017.

It’s relatively common knowledge that beautiful white sand beaches and reefs of tropical areas around the world exist largely thanks to parrotfish droppings. But fish urine is also important for maintaining a healthy reef.

Corals thrive in low nutrient environments and the conservation of these globally imperilled ecosystems is largely dependent on mitigating the effects of anthropogenic nutrient enrichment. A recent study published in Nature Communications provides a novel perspective on the connectivity between corals and reef fish, where reef fish act as suppliers of key nutrients (nitrogen and phosphorus) to corals. When fish urinate, they release phosphorus and nitrogen into the water (both of which are crucial to the survival and growth of coral reefs). Most of this is supplied by large, carnivorous fish such as groupers, snappers and barracudas. The authors argue that fish hold a large proportion, if not most, of the nutrients in a coral reef in their tissue, and they're also in charge of recycling them. If you take the big fish out, you're removing all of those nutrients from the ecosystem. The increasing scarcity of large, carnivorous fish on coral reefs today does not bode well for nutrient supply for our reefs. In fact large-bodied predatory fish have been estimated to account for less than 3% of total fish biomass in Tobago.




The removal of rare and endangered species of large fish like this Goliath grouper deprives our coral reefs of needed nutrients. This one was on sale at the Grange fish market, Tobago. Photo courtesy Jahson Alemu I

The study looked at links between fishing pressure and i) fish community structure; and ii) nutrient production over 143 species of fish at 110 sites across 43 Caribbean coral reefs ranging from no-fishing marine reserves to heavily fished reefs with few large predatory fish. While fishing did not have a significant effect on the number of species on the reefs, it did result in the significant reduction in the biomass of fish available on the reef, i.e. heavily fished sites supported very few large bodied predatory fish. Further, reefs with more large, predator fish had healthy levels of nutrients, while reefs depleted of large fish had nearly 50 percent fewer nutrients, including phosphorus and nitrogen, essential to coral survival. Simply put, the lead author of the paper states that “fish biomass in coral reefs is being reduced by fishing pressure. If biomass is shrinking, there are fewer fish to pee.”

Grouper and snapper are top predators that are important components of our reefs, our fisheries and part of our culinary heritage. Unfortunately, slow growth rates, late maturity, strong habitat dependence and susceptibility to coastal impacts make these fish particularly vulnerable to overfishing throughout the region. But restoring key fish populations (such as large bodied predators and parrotfish) and improving protection from overfishing and pollution could help the reefs recover and make them more resilient to the impacts of climate change. Already one possible approach for achieving this balance, called ecosystem based fisheries management (EBFM) is being encouraged throughout the region. Compared to traditional fisheries management that has focused on maximizing fisheries yields (MSY), EBFM focuses on dialogue among stakeholders and maintaining a healthy ecosystem to ensure long term productivity of the fisheries which are themselves an integral part of the ecosystem.

Talk to any fisher these days and you’ll hear the cry that fishing has become harder, and the fish they are catching are smaller and fewer. Basically, there are fewer and smaller large fish available (both to us as consumers and to the coral reefs as a source of nutrients and ecosystem balance) and the potential yield is decreasing. Moving forward, only by understanding and managing the effects of fishing as well as other impacts on the ecosystem can managers ensure sustainable use of coral reef fisheries.
A High-hat fish is placed in a plastic bag for half an hour during fieldwork in The Bahamas. Scientists measured the nutrient content in the water before and after to determine the fish’s nutrient output. Photo courtesy Jacob Allgeier/University of Washington

References:
Alemu, I., Jahson, B. (2014). Fish assemblages on fringing reefs in the southern Caribbean: biodiversity, biomass and feeding types. Revista de BiologĂ­a Tropical, 62, 418-431.
Allgeier, J. E., Valdivia, A., Cox, C., & Layman, C. A. (2016). Fishing down nutrients on coral reefs. Nature communications, 7. (http://www.nature.com/articles/ncomms12461#s1)
Mumby, P. J., Hastings, A., & Edwards, H. J. (2007). Thresholds and the resilience of Caribbean coral reefs. Nature, 450(7166), 98-101.

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