What is a recommended avoidance when pairing bitter beer with certain fish?

When pairing bitter beer with certain types of fish, it is generally advised to avoid oily fish. Oily fish, such as salmon or mackerel, have a rich, fatty quality that can clash with the intense bitterness found in certain beers. The bitterness can enhance the perception of fishiness, making the overall experience less balanced and enjoyable.

Conversely, lighter, leaner fish like whitefish or shellfish typically pair better with bitter beers, as their delicate flavors can complement the beer's profile without overwhelming it. The key in these pairings is to achieve harmony between the beer's bitterness and the fish's richness, making oily fish a less suitable match.