Kindai, an aquacultural tuna being raised by a Japanese University, may provide some hope. But, aquaculture has its own problems and the demand will surely outstrip supply of this new, pricey product. Megu, in NYC, is one sushi bar that uses Kindai.
Kindai tuna, developed at Kinki University in Japan is shipped with an “authenticity declaration” covering its life history, from its 20 days of gestation to its diet in an open-water facility to the material on netting surrounding it. It is said to contain far lower levels of mercury.
Remember the "zero toxin" tilapia? I think we've got to wait and see about some of these efforts.
Wegmans supermarkets and Legal Sea Foods (both East Coast, US) reject fish that exceeds the FDA limits.
Don't forget that the Seafood Watch guide is now downloadable to your phone or PDA.