Schreckstoff--the fear hormone.
As a result, von Frisch made his first report on the existence of a pheromone, which is known as Schreckstoff (startle/ shock matter). An alarm signal is a response produced by an individual, the "sender", reacting to a hazard that warns other animals (the "receivers"), of danger. This chemical alarm signal is only released when the sender incurs physical or mechanical injury, such as when it has been attacked by a predator, and is detected by the olfactory system. When this signal reaches the receivers, they perceive a greater predation threat and exhibit an anti-predator response. Since populations of fish exhibiting this trait survive more successfully, the trait is maintained via natural selection. Interestingly, some research suggests that young fish, or fry, do not make the association between Schreckstoff and the potential presence of a predator as readily as adults, though it is unclear whether this association strengthens over time as a result of learning or physiological development. Also, the degree to which Schreckstoff is produced has been noted to vary in some species during the breeding season, though they do still exhibit anti-predator behaviour in response to Schreckstoff during this time.
Pheromones are probably the most ancient form of animal communication, and aside from the flight response noted, fish commonly use pheromone cues to navigate and coordinate intraspecific activities in waters that are often turbid, vast, or relatively featureless.
Many animals respond to the threat of predation by producing alarm signals that warn other individuals of the presence of danger. While alarm signals may be visual or auditory as well as chemical, alarm pheromones are common, especially among insects and aquatic organisms. Plants, too, emit chemical signals in response to attack. When facing alarm signals, some fish skitter, dart or dash, while others freeze; shoals disperse, huddle together or go into hiding, while others rise to the surface, even jumping out of the water. So what is lumped together under the label fright or alarm reaction is actually a whole suite of behaviours that vary within and between species.
This phenomenon has long been the topic of much discussion among anglers, especially with reference to catch and release angling. If an angler is fishing a spot or area, and a fish is either lost during the fight, or released after being landed, do the other fish nearby or in the shoal suddenly go off the bite? Many anglers believe yes, and especially during tournaments will not release fish over an active spot. Specifically, tiger fishermen have noticed a tendency for fish to go off the bite if "scrap" fish are released during an event. That these flight pheromones may even contaminate line, lures or indeed one's hands while catching fish, is thought by some to be possible, warning those un-caught fish of impending danger. Though tigerfish seem to be one species in our waters which have a reaction to Schreckstoff, it is not clear if each or all of our fish react the same way to being caught and released.
Some manufactured fish scents or attractants specifically employ fear pheromones from prey fish to trigger feeding instincts in predators. This may be the invisible attractant when fishing with live bait. If predators are triggered by Schreckstoff of their prey species, drifting or trolling a live bait (which by virtue of being hooked is injured) may account for more and better strikes. Conversely, scientists have studied and developed pheromones with the aim of controlling certain fish species in any given area. They are attempting to harness the capacity of pheromones to help manage fish populations, as pheromones are an environmentally-benign, highly-specific, and an inexpensive way to manage populations of nuisance fish. By introducing Schreckstoff specific to the nuisance species, they can drive them out of an area.