Coral reefs worldwide have been suffering from numerous effects of climate change, including rising ocean temperatures to increasing levels of acidity. Unsustainable fishing practices have also wreaked havoc on reef ecosystems by physically damaging the coral (such as in “dynamite fishing”) and drastically reducing the populations of species vital to the reefs’ food chain. Interestingly, a recent study from the journal Nature Communications exploring the effects of declining fish species on the health of coral reefs found that it was not a decrease in the number of species that most affected reef vitality, but rather a decrease in the amount of fish urine circulating in the water.

Fish and Coral 

It may be surprising to hear that fish urine plays such an important role in the survival of coral reefs. The scientific community has long known that the presence of fish allows coral reefs to grow significantly faster than in their absence; however, it was not until recently that researchers realized the critical factor to coral survival was the presence of water-borne nutrients excreted through fish urine, rather than biodiversity.

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Coral reefs have what is known as a “tight” cycling of nutrients – in other words, there is very little input of nutrients from external sources into the reef ecosystem. Instead, it is the fish living in the vicinity of the reef that constitute the main drivers of this nutrient cycle, accomplished mainly through their production of waste. Phosphorus, a key nutrient, is released through fish urine, while nitrogen is excreted in the form of ammonium through the gills. These nutrients are then assimilated into the coral and exchanged for energy from the coral’s symbiotic photosynthetic algae, which are then consumed by other reef organisms.

The higher up a species is in the coral food chain, the greater its excretion of nitrogen and phosphorus. Large, predatory fish tend to occupy this top spot. Unfortunately, it is these same larger fish (such as grouper, snapper, and barracuda) that are favored by the fishing industry for human consumption. Remove these species from the system, and a crucial element of the reef’s nutrient cycle is eliminated.

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Protecting Coral Reefs

Given the critical role that fish urine plays in coral reef ecosystems, it is vitally important that the main drivers of the reef’s nutrient cycling – large, predatory fish species – are protected. The proliferance of large-scale commercial fishing operations has pushed many species of fish to the brink of extinction, with some conservation experts predicting that our oceans could be empty by the year 2048 if fishing continues at its current rate. The FAO had previously stated that around 80 percent of global fish stocks were “fully- to over-exploited, depleted, or in a state of collapse.” In addition, around 90 percent of large predatory fish stocks, such as sharks, swordfish, and bluefin tuna, had witnessed a steep decline in their numbers since the 1950s.

The fewer predator fish species targeted, the more secure the exchange of nutrients between fish, corals, and algae. Consumers can do their part by reducing the amount of seafood in their diet (especially larger fishes), or by eliminating it altogether. By leaving fish off the menu, you can help keep over 225 fish and 125 shellfish in the oceans. Check out One Green Planet’s #EatForThePlanet campaign to learn more about how you can use your food choices to benefit the oceans.

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This is just a small example of how every single species plays such an integral role in the greater ecosystem. We like to pretend that our actions cause minimal harm the world around us, but often fail to see the minutia of how just one small alteration can change the environment for good. Share this article and encourage others to learn more about how they can save the oceans with their daily choices.