Oppian notes in Halieutica that the passionate desire of the normally wary mullet when about to spawn “renders it so unguarded” that if a male or female is caught, fasted to a line, allowed to swim to sea, and then gently drawn back to land, shoals of the opposite sex will follow the captive close to shore and into waiting nets.
This “Judas fish” technique worked well in those ancient times, and it still works well in modern Greece.
If a single Judas fish is effective, imagine a Judas school of fish. The ancient Greek Aelian writes of a place called Athena’s Isle, which contained a lagoon where schools of tame mackerel were fed.
Fishermen threw food to them but observed a “treaty of peace,” so the fish were immune from pursuit and attained a great age. After being fed, they were thought to repay the fishermen by leaving the harbour to meet “strange” mackerel.
The strange mackerel fish not flee from their brethren, and the tame mackerel encircled then and held them in place until the fishermen could net them. Meanwhile, the tame fish were said to return hastily to the lagoon to await their afternoon meal.
Although such extreme cooperation appears apocryphal, Aelian concludes his account: “And this happens everyday.“