The mercury from ocean products is not usually a problem because selenium protects against the effects of the mercury as long as there are more moles of selenium than moles of mercury. (Lol, high school chemistry used in real life!).
Anyway, there is enough selenium in ocean water and not so much mercury that people and chickens can eat ocean fish products without concern about the mercury.
I expect the comments are made by people who looked at only one thing. Or, more likely, copy pasted someone who looked at only one piece of one thing.