The Cholesterol is in the Yolk. The Yolk has other things like vitamins and minerals in it. It is an excellent food for you. Id suggest you to eat the yolk once in a while. You can make an omelet with a lot of egg whites and only 1 yolk. The liver produces cholesterol on its own, if you eat some you are helping it, if you overeat and do not exercise and have a bad nutrition/habits, then you get the problems.
About tuna, yep theres a lot of contamination in the oceans. I personally do not take it for my sources of protein anymore. I would suggest that if you like tuna not to eat it everyday. Eat it occasionally.
thanks loads =]