Why is it a bad thing to eat marlin but a good thing to eat cows?

Another interesting story today is that Wegmans (a major east coast grocery chain) is urging supermarkets and restaurants to stop selling marlin as part of the "Take Marlin Off The Menu" campaign, organized by conservation groups. The grocer stopped selling marlin in September.

My question is, what about the cows? Why is it a bad thing to eat marlin but a good thing to eat cows?