Issues in Ecoloay Number 8 Winter 2001 ECOLOGICAL IMPACTS OF AQUACULTURE Conversion of coastal habitats into shrimp farms car lead to large losses in wild fisheries stocks.In Thailand where The use of wild fish to feed farmed fish directly im shrimn farms have been carved out of manarove forests w pacts ocean fisheries.But aquaculture can also diminish wild estimate that a total of 400 grams of wild fish and shrimp fisheries indirectly by habitat modification,collection of wild are lost from nearshore catches for every kilogram of shrim seedstock,changes in ocean farmed.n addition,if other fish food webs,introduction of non- and shellfish species caught native fish species and diseases from waterways adjoining that harm wild fish popula- mangrove areas are considered tions,and nutrient pollutior the total reduction increases to (Figure 6).The magnitude of 447 grams of wild fish biom such impacts varies consider ass per kilogram of shrim ably among different types of raised.If the full range of ecc aquaculture systems,but it can logical effects associated with be severe. mangrove conversion is taker into account,includ Habitat Modification mollusk productivity in mar Hundreds of thou coral ree ne mp fa the ands been trans ering the or s s(Figure 7). as tr of essential ecological service that mangroves fo cluding nursery habitat for ju venile fish and shellfish nrote including subsistence fish tion of the coast from batter ermen-into a privatized farn ing storms and typhoons,flood resource that benefits a small control,trapping of sediments numher of investors and filtering and cleansing of Figure 5-Flow chart of capture (wild)and farmed nutrients from the water fisheries products from aquatic primary production.Num- Use of Wild-Caught Seedstock Mangrove forests pro bers refer to 1997 data and are in megatons (million vide food and shelter to many metric tons)of fish.Thicker lines refer to direct flows of Many aquaculture uvenile finfish and shellfish tha aquatic primary production through capture fisheries operations,especially extensive are later caught as adults in and aquaculture to humans.Thin lines refer to indirect ponds.stock wild-caugh coastal and offshore fisheries and minor flows.Red lines indicate negative feedbacks rather than hatchery-reared In Southeast Asia,mangrove on the aquatic production base.(Modified from Naylor finfish or shellfish fry.Examples dependent species saccount for etal.2000) nclude farming of milkfish in roughly one-third of yearly the Philippines and Indonesia wild tuna in South Aust tras in South Asi and parts of L atin am ca, pan s not a true native wild fis cora 901 to mark capt by re ing the high morta mor ity a The seed-stock q re ope caught 10 for wild fisk re on y.and the prop 75 nt of total finfish f collected inshore net -the remaining85 percent of fry are discarded and lef6 Issues in Ecology Number 8 Winter 2001 Figure 5 — Flow chart of capture (wild) and farmed fisheries products from aquatic primary production. Num￾bers refer to 1997 data and are in megatons (million metric tons) of fish. Thicker lines refer to direct flows of aquatic primary production through capture fisheries and aquaculture to humans. Thin lines refer to indirect and minor flows. Red lines indicate negative feedbacks on the aquatic production base. (Modified from Naylor et al. 2000) ECOLOGICAL IMPACTS OF AQUACULTURE The use of wild fish to feed farmed fish directly im￾pacts 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 popula￾tions, and nutrient pollution (Figure 6). The magnitude of such impacts varies consider￾ably among different types of aquaculture systems, but it can be severe. Habitat Modification Hundreds of thou￾sands of hectares of mangroves and coastal wetlands around the world have been trans￾formed into milkfish and shrimp ponds (Figure 7). This trans￾formation results in direct loss of essential ecological services that mangroves provide, in￾cluding nursery habitat for ju￾venile fish and shellfish, protec￾tion of the coast from batter￾ing storms and typhoons, flood control, trapping of sediments, and filtering and cleansing of nutrients from the water. Mangrove forests pro￾vide 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 down￾stream 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 biom￾ass per kilogram of shrimp raised. If the full range of eco￾logical effects associated with mangrove conversion is taken into account, including reduced mollusk productivity in man￾groves and losses to seagrass beds and coral reefs, the net yield from these shrimp farms is low — even without consid￾ering the use of fish meal in aquaculture feeds for shrimp. Moreover, building aquaculture ponds in mangrove areas trans￾forms fisheries from a common property resource available for use by numerous local people — including subsistence fish￾ermen— 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 alter￾native to wild harvests, but rather a means to raise wild fish to marketable size in captivity by reducing the high mortal￾ity rates characteristic of wild populations. Collection of seed-stock for aquaculture operations can have very large consequences for wild fisheries if it re￾sults 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