Impacts of Reef Fisheries
Humans are as much a part of the coral reef ecosystem as are the corals and fishes themselves. When harvesting takes place at sustainable levels, fishery impacts on coral reefs are moderate and can be absorbed by the ecosystem. Prior to the development of technologically sophisticated or highly destructive fishing methods, and links to large-scale seafood markets, coastal communities had only limited capacity or incentive to over-exploit coral reef fisheries. Essentially, there was little point in harvesting more than the amount required for the community’s immediate needs, since, even using traditional preservation techniques (smoking and drying), the harvest was highly perishable. In addition, many communities regarded reef fishery resources as an ‘emergency larder’ to be used when natural disasters such as cyclones or droughts caused the loss of agricultural production. Many customary practices, such as closing off reef areas following the death of a clan leader or respected elder, also had beneficial impacts on marine resources by creating sanctuaries or protected areas.
Today, things are quite different. Apart from the most remote and isolated areas, most coastal communities in tropical countries of the Indo-Pacific are integrated to a greater or lesser degree into the cash economy. Even where the lifestyle is largely subsistence in character, there is still a need for cash to purchase fuel, staples and consumer items, and to pay for school fees, transportation and health services. Cash needs are greater still in more urbanised settings, and harvesting marine resources is an obvious way to meet this need. As a result commercial imperatives are placing ever-increasing pressures on coral reef fishery resources, even in far-flung areas. Where refrigeration is available a wide variety of species may be harvested for fresh or processed use. Where infrastructure is more limited, the species that can be harvested may be restricted to sea cucumbers, trochus, pearl shell and shark fin, which can be dried to produce a sufficiently high-value product without the need for refrigeration.
The impacts of fishing on coral reefs may thus vary from being quite limited (in a very few inaccessible locations, or where reefs are highly protected, such as in the USA and Australia), through the over-fishing of selected species, to wholesale over-exploitation accompanied by widespread habitat destruction. The results of such impacts are not always obvious, or well-understood. Where fishing and habitat destruction has been extreme, reefs may become barren and unproductive, a result seen in some parts of the Philippines. Removal of herbivorous fish from the reef community may result in algal blooms which smother corals and eventually replace them, in a process which may be irreversible. This may give rise to secondary effects such as outbreaks of ciguatera poisoning, which arises from a microscopic alga that grows on the surface of certain seaweeds which themselves grow on dead coral surfaces. Where over-fishing is more selective, such as the large-scale removal of holothurians which has taken place in many areas to supply the sea cucumber trade, impacts may be harder to see. Sea cucumbers are primarily sediment feeders and are responsible for turning over of bottom sediments, playing a similar role to earthworms on land. Removal of sea cucumbers is thought to lead to reduced oxygenation of seafloor sediments, altering the habitat and making it less favourable for burrowing bivalves, which may themselves have subsistence or economic value.
A further impact of coral reef fisheries, which is only now becoming widely recognised, is on the ability of the resources themselves to recover. It is the view of many fisheries managers that, if fishing pressure on a stock is reduced or eliminated, the stock will then recover to previous levels of abundance. However this model is too simplistic for many tropical species, because of their specialised reproductive strategies and other aspects of their life histories. In the case of sea cucumbers, pearl oysters, giant clams and other reef-associated invertebrates, as well as possibly some fish, reducing population densities below certain levels will severely impede fertilisation success, leading to reproductive failure. The result is that these populations may be unable to recover even if fishing is stopped completely. There are numerous examples of tropical lagoons in the Pacific Islands where previously abundant resources of pearl shell and sea cucumber have been wiped out and have not recovered even after several decades.
Source: Gary Preston (written for ReefBase)