The USDA Economic Research Service has released its latest data set on domestically grown catfish and trout and U.S. imports and exports of fish and shellfish that may be products of aquaculture, such as salmon, shrimp, and oysters.
The July 16, 2012 update of the ERS Aquaculture Data is available at: http://1.usa.gov/OUzD5n
IMHO Overseas competition & likely the recession. Hopefully we can turn this around. Here is an article from Time magazine that will provide some of the background to your question: http://ti.me/qpRe10