Flock-living fish warn each other with so-called alarm substances. Pike know this very well.
If a pike or some other predator attacks and injures the river minnow, this fish immediately secretes an alarm substance into the water. This pheromone is a warning against imminent danger to all minks in the vicinity. As soon as this substance is in the water, all fish in the stock immediately hide, wherever it falls.
Canadian scientists studying the mechanisms of fish behavior recently established, that the pheromone secreted by minks does not lose its alarming properties even after passing through the stomach and intestines of the pike. Nay – Everything shows that, that the pike know it very well. The alarm substance was also separated from the excrements of predators and it turned out, that it has lost none of its properties. Three scientists from the University of Saskatchewan conducted close observations of pike and fish that feed them in huge aquariums. It follows from the studies published later, that all the gunshots immediately began to panic flee, as soon as some pike droppings were in the water, who had just eaten the minnows earlier. The pikes were kept in a separate aquarium, however, the droppings of one of the predators alone were enough, to create total panic in the herd.
If the pike had previously eaten other species of fish, no changes were observed in the behavior of minnows. After letting them into the aquarium with pike, the fish swam between the standing predators without fear.
It's amazing, but it seems, that the pikes instinctively sense, that their droppings contain an alarm substance to alert the fish, they hunt. Scientists have observed, that in the long, but a narrow aquarium, predators always defecated in one and the same corner. Pike usually clung to one end of the aquarium, defecation, on the other hand, always took place at the other end of the aquarium. Thanks to this, the alarm substance produced by the minnows was as far as possible from the "hunting positions" of hungry pikes.
Canadian scientists speculate, that it just pays off for pike to be "decent."”, because it is the only way they do not warn their potential gains – at least in the aquarium.
How an alarm substance was discovered
Karl von Frisch, the professor of zoology, discovered the existence of alarm substances in herd fish more than fifty years ago. Due to regular baiting in a certain bay of Lake Wolfgang quickly got used to his presence of a large herd of minnow. Originally, the zoologist only wanted to observe the different behavior of fish living in herds. For this purpose, however, it was necessary to mark individual minnows. Charles v. Frisch discovered earlier, that after cutting a certain nerve, the minnows' anal fin becomes much darker. The dark fins were a very good mark.
The slight cut was completely harmless to the fish, a minnow "after surgery” she joined the herd immediately. The zoologist, however, made an astonishing discovery - a slightly injured fish was returning to the water, the whole flock of minks ran to panic. There was no question of trust and attachment to a systematically enticed place. The above behavior of the fish led the researcher to the right track. Only later discovered, that alarm substances are produced in special cells of the epidermis. A fish releases an alarm pheromone when it is attacked by a predator. Due to the secretion of this substance, the caught fish has no chance of being saved, but for the rest of the fish in the herd, it is often a decisive signal of life or death.