Food For Thought: Dietary change was a driving force in human evolution
(By William R. Leonard)
Scientific American, Dec. 2002.

Fish is a potentially more humane choice indeed... but overfishing is becoming a problem.
(See for example: Counting the Last Fish
Overfishing has slashed stocks--especially of large predator species--to an all-time low worldwide, according to new data. if we don't manage this resource, we will be left with a diet of jellyfish and plankton stew. By Daniel Pauly and Reg Watson, Scientific American, July 2003. This one is not available for free online, and I don't have time to look for other references)
For environmental ethics I'd chose fish from farms, or from species not yet overfished. Everytime I eat shrimp I get some background guilt knowing that the trawling nets typically used to fish them wreck havock on the seafloor (I've actually witnessed that myself).
Which goes to show that ethics can be involved in almost any food decision... there has to be a middle ground between providing the mass quantities of food needed, ethics and economics, which our societies have yet to find.