In recent years, the fear of high mercury levels in fish has been instilled in us. Pregnant mothers and children are urged to limit their tuna consumption, and there are always new reports on which fish are safe to eat and which are not. The Environmental Defense Fund has a very handy seafood selector on its website http://www.edf.org/page.cfm that lists the safest and most harmful fish, sushi choices, health guides, and even printable pocket guides.
According to the EDF, the safest seafoods to eat are pink shrimp from Oregon, wild Alaskan salmon, and farmed rainbow trout. The worst offenders are Chilean sea bass and various kinds of tuna.
The easiest method for selecting the right kind of seafood is that the smaller the type of fish, the less amount of mercury it contains. Do not think that just because something is farm raised, that it makes it safer from being contaminated by mercury or antibiotics.
There are many toxins also that contain estrogen-like substances which are much harder to determine than the most common toxins like mercury. And those can be the most dangerous. These other toxins can cause female and male hormonal imbalance. It is better to be safe than sorry. Stick to the safer fish, do your research, and always eat any fish in moderation.