Listening to the ABC RN they had fish scientists talking about research into fish intelligence including its implications for fish farming and fish stocks.
There are more fish species than the total of land vertebrates (animals birds..).
Fish learning has some cultural aspects so memory of the best places to feed and breed may be carried culturally in the memory of schools and the older fish. When these fish are killed the knowledge is lost making it very difficult for the species to recover. The implosion of cod around the UK were given as an example.