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There is a growing fear that genetically modified fish escaping from cage farms could seriously impact the surrounding environment. As fish from aquaculture come from broodstock, this broodstock is often selected to produce the best fish for aquaculture. Sadly, these traits are often less useful, if not harmful, in the natural environment. As such, there is a fear the escaped fish from farms may displace natural species, either through interbreeding or putting too much pressure on the local ecosystem (http://www.nature.com/embor/journal/v5/n7/full/7400197.html). 7).[] However, there is little evidence to support these claims. Though some farmed fish may interbreed with wild fish, there is little evidence that these news genes are harmful (C. Goudey, Personal Communication, November 20, 2007). However, the risk still exists, so we recommend mixing randomly selected wild fish with the broodstock, to minimize the divergence between the two groups.
On the issue of feed, there is great work being done on the subject of replacements for wild caught fish in farm feed. According to ( http://soyaqua.org/quickfacts.html 8) "Soybean meal can replace all or most animal meals in the feeds for the majority of cultured omnivorous freshwater fish." We encourage such efforts, as they may prove key to separating aquaculture from wild fisheries and allowing near indefinite scalability.
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There still remains the question of scale. Farms have produced anywhere from 63,000 to over two million pounds of tilapia per year(http://www.ag.ndsu.edu/pubs/alt-ag/tilapia.htm). 9). At an average per capita per year consumption of about 35 pounds, a single farm could feed as many as 50,000 people . (http://www.who.int/nutrition/topics/3_foodconsumption/en/index5.html). (10). Using the Maldives as an example, assuming an average per capita per year fish consumption, their population of 369,031 (http://www.aneki.com/Maldives.html11) could be fed with as few as eight large fish farms.
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6-http://aquaculture.noaa.gov/pdf/finalnoaa10yrrweb.pdf
7-http://www.nature.com/embor/journal/v5/n7/full/7400197.html). 
8-http://soyaqua.org/quickfacts.html
9-http://www.ag.ndsu.edu/pubs/alt-ag/tilapia.htm). 
10-http://www.who.int/nutrition/topics/3_foodconsumption/en/index5.html). 
11-http://www.aneki.com/Maldives.html
Low trophic fish:
catfish
tilapia
grey mullets
carp
trout
anchoveta
herring
mackerel
sardinella
anchovy
http://www.esa.org/science_resources/issues/FileEnglish/issue8.pdf