Coral reefs are one of the wonders of the natural world but unfortunately they, like much of our oceans, are under threat from human beings, how you may ask? Well mainly from overfishing. There are two major threats to coral reefs, the first is coral bleaching which most of the time leads to death, and the second is trawling where ships scrape nets along the seafloor to catch more fish.
Corals are incredibly complex creatures, made up of one main animal, the coral polyp (which is related to jellyfish and anemones). These live on a self made hard skeleton with many hundreds of microscopic organisms living within it, but the main one is a tiny algae called a zooxanthellae (pronounced zoo-zan-thell-ay). These organisms can produce energy from light like plants can but they give most of this to the polyp so it can grow the coral. These algae are what give corals their bright colours but without them they become white as their hard skeleton shows through the transparent polyps, this is what is known as bleaching; coral bleaching happens when ocean temperatures increase and the coral ejects its algae. Without the algae supplying them with energy the corals become more prone to bacteria and viruses in the water which most of the time kills them.
Now, how is this linked to overfishing? Well it has to do with two things. First a different kind of algae called fleshy algae that grows in mats on reefs, and secondly many species of fish which eat this algae called grazers. On a healthy coral reef this fleshy algae is kept trimmed back by these fish, but overfishing leads to a removal of sharks (for their fins). The overfishing of sharks causes their prey (the predators of the grazers) to increase in number, thus reducing the number of grazers and so the algae grows out of control. As it grows it dies rapidly releasing massive amounts of nutrients into the water, this causes the bacteria in the water to increase in number greatly which causes a huge increase in the chance for corals to get sick and die even causing some of the bacteria that live inside the coral to turn against them.
As for trawling, that explanation is much more simple – these massive nets (up to 100m wide) scrape along the seafloor destroying everything in their path all in the name of getting slightly more fish. These nets can destroy entire coral reefs in a matter of days, destroying coral that have lived in some deep regions for over 3000 years. This can cause the whole ecosystem of the coral reef to disappear and for all the benefits it gives to the ocean to vanish with it.
So, in short fishing of large predators causes a decrease in grazing fish which cause unrestrained growth of algae and thus disease proliferation and coral death. While trawling demolishes coral reefs in the blink of an eye with huge reef sized nets.
While key steps are being taken it is not enough, only around 3% of marine protected areas (MPAs) are actually no take zones meaning some form of fishing is allowed in MPAs even trawling! These protected areas need to increase in size and they need to be no take zones or only allow very minimal fishing. As for trawling I believe it should be banned, this is because it is destructive and is not worth the trade out of more fish for total destruction of coral reefs. But things are getting better, the number of MPAs increases every year and overfishing is being reduced as more people are becoming aware of the problem and raising awareness. So is the future bright for coral reefs? That remains to be seen, but things are definitely getting brighter.
Coral Reefs In The Microbial Seas, Forest Rohwer, Merry Youle, Chapter 6