One problem is that deformities actually occur fairly often in nature. From what I've read, reptiles are particularly prone, possibly because of the unpredictability of an embryo developing with lots of random variations in temperature.

There's a classic case around Chernobyl. Folks doing testing downwind of the reactor found a fair number of animals born with deformities. Disaster! Radiation! Mutants! And then they checked *upwind*, and found just as many...

Bottom line is that unless you know 100% what the standard rate of deformities is for a species, you can't say. It's quite possible that these fish may naturally do this fairly often, and the only difference is that humans have started paying attention. It's even possible that other groups like fishermen may already know about this, or individuals may have noticed it but not commented on it, so it never got as far as the boffins. This isn't to discount the effect of pollutants in the environment, which sadly is all too real (mercury, dioxins, etc), but until you know what's "normal" then you're not in a position to even take a educated guess.

Graham.