If fish do not have a memory, are classed as not being aware and cannot think, then, with all the millions of fish in the sea, how do they know that they are swimming with their own kind and if they don't know what they are doing how does it happen?
There is nothing new with the 'discovery' that fish have an excellent memory. Aquarist have known for many decades that fish are very adept with learned behaviour, something which requires a good memory. Natural behaviour such as salmon remembering the taste/smell of the river where they were born in years ago needs a good memory too.
The fish have a '3 second memory' myth has no logical foundation.
somebody once said (think it was on here actually) that we attached the 3 second memory myth to fish to stop us feeling guilty for cramming goldfish in a tiny bowl. Now THAT'S a theory I'd buy into!