I stopped eating any type of fish or taking any type of fish oil supplements when I read about how much we've devastated the oceans. We've reduced the fish population by something like over 90% since the 1950s and we continue to do very destructive things like bottom trawling. Just sickening.
Then there's factory farming and how destructive that is: tremendous water usage, ozone harming methane production, deforestation of the rain forests for cattle production, overuse of antibiotics and hormones, the utter cruelty of it all, and on and on.
Then there is the fact that animal products wreak absolute havoc on the human body (read The China Study if any of you don't believe me). Even if we ignore the preponderance of evidence showing how unhealthy eating animal products is, we still have the massive environmental impacts and everything else to deal with.
I'm not a total vegan yet, but I've been working to replace all animal products in my diet. I think I'm about 80-90% there. I feel like it's the responsible and sane thing to do just from an environmental standpoint alone.