Some fish is fine to eat while pregnant. If you ask your doctor, s/he most likely has a chart that shows which fish has the highest levels of mercury. Some fish are quite healthy for you to eat while pregnant. I had fish at least twice a week during my most recent pregnancy. I just made sure it was cooked really well.
The human pollution component....well, that is a good point. I would think that fish from the fish farms are fine. I do think it's important to get some fish during your pregnancy, just the right kind. Again, talk with your doctor about this.