Sometime in the 1970s (I think) I saw a really awful film set in a small town by a lake. The film opened with some people camping out in the woods. One of them was zipped up in a sleeping bag. Something (as yet unidentified) moved in between the sleeping camper and the camera, and we heard a "crunch." When the creature moved off, we saw that it had bitten the head off the camper, causing a fountain of blood to spout of of the neck. Far from being shocking or gory, it was so badly done as to be laughable.
The main story was that these huge and nasty mutated creatures were coming out of the lake and eating people. A local industrial company was under suspicion for poisoning the lake, but repeated water tests showed it to be clean.
In the end, someone had the idea of testing the water at the bottom of the lake, which revealed it to be heavily poisoned with mercury. As mercury is much heavier than water, it had sunk, so the previous tests (done from a small row boat) had missed it as they only sampled the water at the surface.
They got it all sorted out eventually, and the bad guys ended up in prison.
After the credits had rolled, and you thought it was all over, another monster reared up in front of the camera and roared. I think we were supposed to go "Oh no, it's not all over!" However, the few people stupid enough not to have left by now laughed at the ridiculous sight and left, shaking their heads wondering why they had bothered!
I have a morbid fascination to see it again. Anyone have any idea what it was called?