The use of wild fish to feed farmed fish directly impacts ocean fisheries. But aquaculture can also diminish wild fisheries indirectly by habitat modification, collection of wild seedstock, changes in ocean food webs, introduction of non native fish species and diseases that harm wild fish populations, and nutrient pollution.
The magnitude of such impacts varies considerably among different types of aquaculture systems, but it can be severe.
Habitat Modification:
Hundreds of thousands of hectares of mangroves and coastal wetlands around the world have been transformed into milkfish and shrimp ponds.
This transformation results in direct loss of essential ecological services that mangroves provide, including nursery habitat for juvenile fish and shellfish, protection of the coast from battering storms and typhoons, flood control, trapping of sediments, and filtering and cleansing of nutrients from the water.
Mangrove forests provide food and shelter to many juvenile finfish and shellfish that are later caught as adults in coastal and offshore fisheries. In Southeast Asia, mangrove dependent species account for roughly one-third of yearly wild fish landings, excluding trash fish. In Indonesia, Malaysia, and the Philippines, catches of finfish and shrimp increase with mangrove forest area.
Healthy mangroves are also closely linked to the condition of coral reefs and seagrass beds. As mangrove forests are lost, more sediment runoff is carried onto and can smother downstream coral reefs and seagrass beds. The degradation of these biologically rich systems, in turn, affects fish harvest: fish caught from reefs contribute about 10 percent of fish humans consume globally, and the proportion is much higher in developing countries.
Conversion of coastal habitats into shrimp farms can lead to large losses in wild fisheries stocks. In Thailand, where shrimp farms have been carved out of mangrove forests, we estimate that a total of 400 grams of wild fish and shrimp are lost from nearshore catches for every kilogram of shrimp farmed. In addition, if other fish and shellfish species caught from waterways adjoining mangrove areas are considered, the total reduction increases to 447 grams of wild fish biomass per kilogram of shrimp raised.
If the full range of ecological effects associated with mangrove conversion is taken into account, including reduced mollusk productivity in mangroves and losses to seagrass beds and coral reefs, the net yield from these shrimp farms is low — even without considering the use of fish meal in aquaculture feeds for shrimp.
Moreover, building aquaculture ponds in mangrove areas transforms fisheries from a common property resource available for use by numerous local people — including subsistence fishermen — into a privatized farm resource that benefits a small number of investors.
Use of Wild-Caught Seedstock:
Many aquaculture operations, especially extensive ponds, stock wild-caught rather than hatchery-reared finfish or shellfish fry. Examples include farming of milkfish in the Philippines and Indonesia, tuna in South Australia, shrimp in South Asia and parts of Latin America, and eels in Europe and Japan. In these systems, aquaculture is not a true alternative to wild harvests, but rather a means to raise wild fish to marketable size in captivity by reducing the high mortality rates characteristic of wild populations.
Collection of seed-stock for aquaculture operations can have very large consequences for wild fisheries if it results in high bycatch rates. For example, milkfish constitute only 15 percent of total finfish fry collected inshore by seine net — the remaining 85 percent of fry are discarded and left to die on the beach. Thus the capture of the 1.7 billion wild fry stocked annually in Philippine milkfish ponds results in destruction of more than 10 billion fry of other finfish species.
In India and Bangladesh, up to 160 fish and shrimp fry are discarded for every fry of giant tiger shrimp collected to stock shrimp ponds. The magnitude of annual fry bycatch has been estimated at somewhere between 62 million and 2.6 billion in three collecting centers in West Bengal, India.
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