Overfishing is one of the major stresses on ocean health. According to the FAO, 70 percent of the world's marine fish stocks are fully fished, overfished, depleted or recovering. A study published in Nature in 2003 estimated that 90 percent of the large predatory fish such as marlin, large cod, large sharks, tuna and swordfish have disappeared from the world's oceans.
Overfishing affects not just those fish that are being caught, but the coral ecosystem as a whole. When large predatory fish are overfished, fisheries then turn to smaller herbivorous species such as parrotfish and surgeonfish affects the reef as whole. These herbivorous fishes play an important role in keeping algal growth in check. If their numbers decline, areas of reef may become overgrown with algae. This also affects coral settlement as the coral larvae need rocky, bare substrate to begin to start a new coral colony. The process of smaller and smaller fish being targeted is known as fishing down the food chain.
A study conducted by the ARC Centre of Excellence for Coral Reef Studies at James Cook University compiled a set of surveys over 300 reef sites to investigate the relationship between fish stocks and reef health. The results they found are worrying.
The reef is healthy. This amount of fish was most often found in reef areas where there were fishing rules, such as protected reefs and no-fishing zones.
When the amount of fish fell to 850kg per hectare, there was an increase in the amount of algae and a decrease in the amount of coral.
There was also a large decline in the number of herbivorous fish, meaning that less algae is eaten, and the algae starts to overtake the reef.
Below 150kg of fish per hectare, there was a collapse of the coral reef, and coral health rapidly descends to zero.