Yes and this has been a major problem with farmed shrimp. However, in the case of mollusks (oysters, clams, scallops, etc.) we have not heard of this being the case, although theoretically it is possible. Alternatively, some farming of some shellfish can take pressure off of wild populations, thus helping them. The three ways that farming can potentially hurt wild populations are via:
1. Habitat Loss. If all suitable habitat for the shellfish are converted to farms, there would be no place left for wild populations to exist. Oysters, for example, have very specific habitat requirements. Oyster fisheries have disappeared from many estuaries due to habitat loss (but this loss has not been attributed specifically to shellfish farming to date). In the case of shrimp farming, massive amounts of tropical mangroves have been converted into shrimp farms. These mangroves were essential nursery areas for juvenile shrimps as well as many other species of crustacean and fish.
2. Spread of Disease. Intensive monoculture (the farming of one species) almost always presents the risk of disease. When you place so many similar organisms into a tight place, stress can lower immune systems. Any disease or pest that comes along can then spread like wildfire due to both the close proximity of the organisms and their similar genetic make-up. If the farm is an open system (exchanging water with the natural environment), this disease can then spread into wild populations. This has been a major problem with both farmed shrimp and farmed salmon.
3. Introduction of invasive species. If the species being farmed is not native to the region of the farm, a risk is presented for that organism to escape into the environment, establish itself and potentially out-compete native species. We’ve seen this happen with salmon (Atlantic salmon escaping into British Colombia and threatening the native stock). I cannot think of a particular example where this has happened with shellfish farming. However, shellfish introduced via other means (e.g. ballast water of ships) have wrecked havoc in the places where they have been introduced (e.g. zebra mussels into the Great Lakes). So, there is considerable risk and thus a good rule of thumb is to stick with the farming of native species.
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