The king pickerel is a large species of fish which apparently inhabits the filthier sections of the Pontar Delta, previously considered extinct. Geralt and Linus Pitt both witness one swallowing a dead rat while on the way between Foam and Novigrad on the Malatius and Grock barge.
"The white-finned bullhead has died off completely. Linus Pitt broke the silence. The mullet has died, as have the snakehead, the kithara, the striped loach, the red-belly dace, the long-barbel gudgeon, the king pickerel..."
- pg.175, Blood of Elves (UK edition)