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The mullet is a species of fish said to have inhabited the Pontar Delta, but now considered extinct. Geralt and Linus Pitt have quite a detailed discussion of the various fish said to be native to the area 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(s). 175, Blood of Elves (UK edition)