The scariest thing when it comes to eating fish, particularly wild fish, are environmental toxins like mercury, PCBs, and pesticides.
In general, the bigger and older the fish, the higher the likelihood that it’s contaminated. This is because these dangerous environmental toxins accumulate in the food chain: plankton absorb the contaminants from the water, a little fish eats the plankton, a bigger fish eats the little fish, and so on. These pollutants stay in the body once consumed, so every step up on the food chain, they’re magnified about 10 times. And this is why the big fish that we love to eat (like swordfish and tuna), who have spent 20 or 30 years eating contaminated little fish, are highest at risk for mercury contamination and could have levels millions of times higher than the water in which they swim. So, an easy thing to do is eat smaller and younger fish or ones from lower trophic levels!
However, it can be safe to eat the the big fish that we all love. A young albacore tuna fished from waters with historically low contamination levels and harvested around 3 to 5 years old, before significant toxins have accumulated, is a great and sustainable choice. Of course, there are lots of delicious smaller species further down the food chain like mackerel, snapper, trout, rockfish, and most salmons, which are all safe to eat. Regardless of the species, if the whole fish would fit in your picnic cooler… it’s probably at low risk for contamination.
The lowest risk of all, however, is a well-farmed fish! The best aquaculture operations carefully and continuously monitor water quality and have advanced filtration systems that protect both the fish and the environment. In addition, these fish are fed a clean and controlled diet that prevents contamination from entering their bodies in the first place. That’s one big reason we love farmed fish!