Mercury is a worry
Yellow tail tuna and many other fish (both freshwater and salt) can have varying amounts of mercury.
Limiting or not using most of these fish is smart. Raw tuna (sushi) is especially risky. See this site on amounts and frequency, depending on location, source, etc. Kids & pregnant women are especially vulnerable to toxic effects of mercury.http://www.nrdc.org/health/effects/mercury/protect.asp