in Scotland
Shoaling Shoaling and schooling behavior of common minnows occur early in their development, as soon as they become capable of swimming. Shoaling behavior then increases and becomes dominant by three to four weeks after its emergence. This behavior generally benefits individual minnows by improving predator avoidance and foraging. However, there are also costs of living in groups such as increased competition for food and risk of infection. Shoaling behavior is modified depending on the situation such as presence of predators or resource availability.
Predator avoidance The group formation of common minnows can be explained by the selfish herd effect proposed by W.D. Hamilton. According to the
selfish herd theory, a group forms as individuals try to reduce their domain of danger by approaching others and continuously moving toward the center of the group where the risk of predation is the lowest. As the theory predicts, common minnows increase their shoaling behavior in response to increased predation pressure.
Alarm substance Common minnows can detect the predators' presence and communicate with their shoalmates by a chemical signal that is detected by
olfactory nerves. The chemical, named
Schreckstoff after a German word meaning "fear substance" by Karl von Frisch who first described it, is contained in specialized skin cells called alarm substance cells and is released from an injured or killed minnow. The shoalmates can detect the chemical and respond to the increased risk of predation. The production and release of this alarm substance are
altruistic because the sender of the signal, who does not directly benefit from the signal released upon its injury, has to pay the cost for the production and release of the chemical. In fact, the alarm substance cells decrease in number when the common minnows are in poor physical condition due to scarce food, indicating that there is metabolic cost for producing and maintaining the specialized cells. The apparent altruistic behavior is not clearly understood, because the likely explanation of kin selection is not supported by the shoal structure of common minnows in which shoalmates are not necessarily closely related.
Shoaling adjustment in response to predation risk When common minnows sense the alarm substance, they form tighter shoals as individuals move to be in the central position in their shoaling group. However, in an experiment where common minnows were habituated to the chemical by continuous exposure, common minnows did not react to the signal. Only the naïve common minnows reacted to the signal by relocating themselves to the central position in the group. In another experiment, researchers observed common minnows in semi-natural setting and found that common minnows' shoaling behavior varies depending on the habitat's complexity. Minnows tend to respond to increased predation risk by forming larger shoals in structurally simple habitats and by reducing their rate of movement in complex habitats.
Predator inspection When potential predators come near the shoal, some common minnows take the risk of approaching the predators in order to inspect the predator and assess the danger. Predator inspection behavior increases the risk of being attacked and eaten by the predator, but the behavior is beneficial to the inspectors as more alert minnows react more quickly to the attack of the predator. Common minnows are expected to recognize predators by their appearance. In an experiment, common minnows inspected a realistic-looking model of a pike, one of the major predators of minnows, and a simple cylinder model. Common minnows showed high level of
alertness, such as low feeding rate and frequent skittering after their visit to the realistic model, but they became easily habituated to the simple model and resumed foraging even in proximity to the model. In addition to identifying predators by their appearance, common minnows can respond to the predators' motivation to attack. In an experiment, common minnows inspected a
northern pike behind a clear partition at regular intervals until the pike tried to attack the minnows. Their responses differed depending on when their visit was made. Minnows that inspected the pike just before the pike attacked were more alarmed than those who inspected the pike long before the attack. The observation shows that common minnows can detect the predator's impending aggressiveness and motivation to attack.
Variations in anti-predator activities Different populations of common minnows show varying degrees of anti-predatory activities. Common minnows from populations in high-predation areas usually show more intense predator inspection than those from low-predation areas. They tend to commence inspection sooner, form larger group of inspectors, inspect more frequently, and approach less to the predator. Some components of anti-predator activities are inherited, as indicated in the early emergence of shoaling behavior in laboratory-raised immature minnows. The varying levels of predator inspection and shoaling behavior in response to predator's presence can arise in laboratory-raised minnows even though they do not have any experience of predators. Their anti-predatory behaviors are qualitatively and quantitatively similar to their wild-caught counterparts. Anti-predatory behaviors are modified by early experience of predators. Early exposure to predators increases the inspection rate and shoaling tendency.
Foraging Shoaling behavior improves foraging success, because the demand for anti-predatory activities per individual is reduced and because more individuals scanning for food leads to quicker detection. In general, a larger shoal of fish locates food faster, which was confirmed to be true in common minnows.
Individual recognition and shoal choice Common minnows do not randomly choose shoalmates to forage with. They tend to associate with familiar shoalmates and prefer to form shoals with poor competitors for food, which indicates that they can recognize individual conspecifics. It is more beneficial to shoal with poor competitors because while group foraging helps the search for food, it also leads to competition for food among the shoalmates. Common minnows tend to associate with familiar shoalmates, but new alliances can form when different groups encounter. In an experiment in which common minnows from different groups were introduced to a common environment and monitored, they associated significantly more frequently with familiar individuals than unfamiliar individuals. The preference lasted up to two weeks, but by the third week, new association patterns were observed. ==Breeding in captivity==