Overfishing

Coral reefs support highly productive, diverse and economically important fisheries. However, human population growth and growing market demands for seafood have depleted many targeted stocks. On the GBR, earlier export fisheries that flourished following European colonisation (e.g. for sea cucumbers, pearl shell, Trochus snails, and turtles) have collapsed or are no longer commercially viable. Harvested tropical megafauna (e.g. whales, dugongs, crocodiles, turtles, and sharks) are severely depleted worldwide. For example, the number of dugongs on the GBR has declined by more than 90% in the past 30 years. Larger species of carnivorous fishes (e.g. groupers and snappers), especially those with vulnerable spawning aggregations, have also been heavily overfished. Even on the GBR, where fishing pressure is relatively modest compared to most coral reefs, the biomass of carnivorous fishes has been reduced by 4-5 fold on fished reefs compared to adjacent reefs that are zoned as 'No-Take Area'.

Top-down effects. In a classic example of 'fishing down the food chain' many reef fisheries today rely heavily on herbivores and planktivores. Australia is unusual because it lacks a large human population reliant on subsistence fishing, and there is virtually no recreational or commercial fishing of tropical reef herbivorous fishes. Over-harvesting of herbivorous fishes and addition of nutrients both promote blooms of fleshy algae that outcompete adult corals and impede new recruitment. The depletion of herbivorous fishes on Caribbean reefs has caused ecosystem collapse, with unchecked algal blooms replacing corals. On some reefs, overfishing has reduced levels of predation and competition from fishes, triggering unsus-tainably high populations of grazing sea urchins. This top-down effect is unstable because of emergent diseases that cause mass mortalities of super-abundant sea urchins, and because bioerosion of the substrate by huge numbers of sea urchins can exceed the accretion rate of the reef (see Chapter 8).

The relative importance of herbivory and nutrient supply ('bottom-up versus top-down control') in regulating the biomass and composition of seaweed is an ongoing, sometimes contentious debate. Algal biomass is often highest where herbivores are naturally scarce, for example, on intertidal reef flats, in turbulent shallow water, or within the defended territories of pomacentrid damselfish. Reef substrates that are protected experimentally from grazing rapidly become colonised by mac-roalgae, which in turn inhibit recruitment and growth of corals. Previous experimental approaches to explore herbivore-algae-coral interactions have used a variety of

BOX 9.2 NO-TAKE AREAS

No-Take Areas (NTAs) are an important tool for protecting fish stocks and for maintaining the critical ecological functions of fishes. No-Take Areas also provide a refuge from harvesting and destructive fishing practices (e.g. damage from poisons, dynamite and fishing gear), and of course they allow targeted species to grow older and larger. These big individuals typically have disproportionately higher reproductive success, and their offspring spill over into adjoining areas that have lower levels of protection, influencing the seascape as a whole. Many tourist operations are located at NTAs, so that snorkelers and divers can view larger fish.

Once new NTAs are established, the recovery of severely depleted fish stocks continues for many years. In the Philippines, a long term study revealed that the biomass of predatory fishes increased exponentially (by 17-fold), and showed no sign of slowing after 18 years of continuous protection. On the GBR, researchers are also working on new NTAs (green zones) established in 2004 (see Chapter 12) where the size and number of coral trout and other targeted species is already increasing. NTAs are seen as important tools for managing the resilience of reefs, that is, their capacity to cope with natural disturbances and human impacts (Box 9.3). Grazing by herbivorous fish that are protected within NTAs can facilitate recruitment by corals after a disturbance, by preventing blooms of seaweed.

techniques, including algal transplantation (e.g. from areas of low to high herbivore density), herbivore enclosure and exclusion experiments, herbivore and algae removals, and so-called 'natural experiments' such as the die-off of the Caribbean herbivorous sea urchin, Diadema antillarum. All of these approaches have their strengths and weaknesses. It is clear that both top-down and bottom-up effects are important, and they are difficult to separate since overfishing of herbivorous fishes and declining water quality often go hand in hand. No-Take Areas (NTAs), where fishing is prohibited, are an increasingly common management tool for sustaining targeted species and maintaining their ecological roles (Box 9.2).

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