Eel: Freshwater

Freshwater eel, River eel, Common eel

Unagi, Kabayaki

All eel species have a unique life cycle, where adults spawn in salt-water far from the habitat in which they will grow to maturity. It is assumed that European and American eels spawn in the Sargasso Sea, and that Japanese eels spawn near the Marianas Islands. The majority of eel available in markets, approximately 90%, is produced through aquaculture production. Wild populations of all three eel species are in severe decline from a variety of sources, most notably habitat loss and alteration but also factors such as pollution, disease, natural and anthropogenic climate change, and fishing.

Eels are carnivorous and require extensive use of wild-caught marine resources for their culture. It is estimated that aquaculture facilities use, on average, 2.5 mt of wild-caught fish to produce 1 mt of eel. Aquaculture operations are completely dependent on wild eel populations for seed and consequently growing demand and production of eels in aquaculture has increased pressure on wild stocks. Use of marine resources is a critical concern for eel aquaculture.

Escape from aquaculture facilities and subsequent survival has been documented. Escaped eels are ecologically similar to native eel species, and there is no evidence of ecosystem disruption by introduced eels, although this topic has not been investigated. All three northern-temperate eel species pose a high risk of escape from aquaculture facilities except for those raised in recirculating tank systems that have adequate anti-escape measures such as secondary containment. The latter are deemed as a low conservation concern.

Anguillid eels are vulnerable to a wide range of fish diseases and their susceptibility to disease is compounded by aquaculture techniques that raise eels at high densities, promoting the spread of disease. Concern has also been raised over the rise of viral infections in eels, and the spread of these infections through the global eel trade. Risk of disease is a high conservation concern except for eels raised in recirculating tank systems with sterilized effluent. The latter are deemed as a low conservation concern.

Eels are raised using a wide variety of aquaculture techniques, across regions and even within countries.
RECIRCULATING SYSTEMS can be located away from ecologically sensitive areas and those with advanced treatment,including sterilization of effluent, have the lowest risk of habitat effects.
OUTDOOR STILL WATER PONDS and GREENHOUSE PONDS with sedimentation tanks with Infrequent flushing systems have moderate effluent effects.
MODIFIED POLYCULTURE FACILITIES cultivate eel at low densities in wetlands with natural effluent filtration abilities; however, these operations require alteration of sensitive wetland habitat and as a result are considered a moderate risk for pollution and habitat effects.
OPEN NET PENS culture eels at high densities with no effluent treatment and these pens are located in areas of moderate ecological sensitivity, and thus are considered a moderate risk for pollution and habitat effects.
BASIC GREENHOUSE SYSTEMS and FLOW-THROUGH OUTDOOR PONDS have the highest risk of habitat and pollution effects because they release large amounts of untreated or inadequately treated effluent and these facilities may cover large areas at high densities. Eel raised in the system are considered a high risk.

China, Taiwan, Vietnam, and Japan have not been effective in managing the environmental concerns associated with eel aquaculture. Since eels have an inherently high disease load and risk of escape, precautionary management must enact regulation to minimize these risks. All these countries import foreign eels into aquaculture facilities such as outdoor and greenhouse ponds that have a high risk of escape and release of pathogens into the surrounding environment. Additionally, clustering of flow-through facilities that use large amount of water has caused widespread subsidence of land in Taiwan.

No.