Thanks for the link!
This is a good rundown of the specific problems with fish: http://www.littlebigcat.com/nutrition/why-fish-is-dangerous-for-cats/
So, shrimp and shellfish are exposed to and ingest toxins and pollutants, the levels depending on where they were harvested. I think they still would not be good to feed to as a substantial portion of their food. However, the farther down the food chain an aquatic animal is, the lower the level of toxins, so presumably filter feeders would be better than whitefish, which are large and much higher up the food chain.