I enjoy having my sims fish, and it's a fairly good money maker for vampire sims, or in fact any sim with enough vampire lore (I think it's level 4) to create plasma packs. Since you can make a plasma pack out of any fish and sell it for 50 simoleans, I have them convert any fish that's worth up to 50 and sell any that are worth more than that (thank god the inland streams and ponds of Willow Creek and Oasis Springs were somehow stocked with...tuna). If the vampire has a good stockpile of plasma packs I start selling off the extra. If the fisher sim is not a vampire I just sell them all. :p