Response to external stimuli

A zebrafish that is alarmed by environmental stimuli will display a series of behaviours: diving to the bottom of the tank, erratic swimming and freezing. These behaviours will disappear when the zebrafish is at ease again.

Response to stimuli

The presence of abnormal responses to a stimulus can be determined in several ways.

• During feeding fish will immediately approach the area where the food is dispensed; the absence of this behaviour is an indication that something is wrong.
• Tapping on the tank or loud noises will cause the fish to dart away (startle response). An overactive or an underactive response may indicate a problem.
• When fish are caught to be transferred to another tank in preparation for breeding, their responses can be observed. Healthy animals will quickly swim away from the fishing net. If zebrafish fail to avoid the net, or if they do so excessively, it is an indication that something is wrong. Keep in mind that normal behaviour may vary between different fish lines. 

Interactions with conspecifics

By nature, zebrafish are social animals who need to interact with conspecifics to be able to function optimally, both at rest and while active. Their social behaviour covers a wide range of activities. One of these activities is the display of aggression to determine the social hierarchy in the group. Abnormal behaviour towards tank mates is a reason for heightened vigilance.

Zebrafish housed in prolonged isolation may gradually develop abnormal behaviour such as lethargy, or an altered appearance such as loss of color. Solitary housing must therefore be avoided, but if necessary, it is important to house the fish in a way that allows visual contact with conspecifics. Other options, if possible, are to introduce a companion fish of a distinctly different line (to avoid confusion). Tank or cage enrichment is also important; these can be floating plants of live food.

Interaction with the environment

As soon as a zebrafish is placed in an unfamiliar environment, it dives to the bottom, then slowly begins to display explorative behaviour and swim upward. The fish thus gets to know its surroundings, which become predictable and controllable. Predictable means that an animal can anticipate what happens in its environment. Controllable means that the animal, by its behaviour, can influence what happens. Predictability and controllability of the surroundings contribute significantly to animal welfare.