Re fish and mercury levels, it is only the long lived carnivores that are a problem as mercury is a metal that bioaccumulates, as in it stays within the body and doesnt get expelled with other waste products. Being a deep sea fish does not necessarily increase the amount of mercury it carries, but then many of the deep sea ones are carnivores anyway because of the lack of vegetation down there.

If you have to eat shark, salmon and tuna I would keep it to less than once a week. Port Phillip Bay is very high in mercury and the authority there reccommends that adults eat bay fish less than twice a week for that reason.