The Gemara says that Mashiach will come at a time when fish are extremely scarce, or perhaps completely unavailable. The Gemara quotes a verse in Yechezkel from which Gemara learns this. The verse refers to oil-like rivers. Rashi explains that rivers of oil are congealed, (perhaps he means that they are relatively thick compared with rivers of water), and therefore we would not find fish in them.

Dov Freedman