I was just reading about the best fish to buy, since my kids eat alot of shrimp I was worried about Mercury and toxins in fish. I found out the younger fish has less toxins since they build up in fish as they get older and bigger. We limit our tuna to twice a month, but other smaller fish like salmon you can eat a couple times a week, Im not sure how big yellowtail are. Our Sushi usually has tuna, yellowtail, eel, crab, shrimp and salmon, I only worry about the tuna so far.
If your trying to loose weight the easiest way is to have a protein, a starch and big helping of veggies three times a day. Your protein serving should be about the size of a deck of cards add a little if your bigger person. Onions are really good about carrying fats out of the body also. All those things work togethor in a meal and it is a good long term plan to keep your weight in check, if you don't get enough protein you will be hungry all day, and the veggies help carry out the fats. Sushi is a good source of protein and kelp is good at stimulating the thyroid along with shrimp. Try and stay away from too much fried stuff. Now if I could just follow that myself, easier said than done.