A zebrafish that is alarmed by environmental stimuli will display a series of behaviours: diving to the bottom of the tank, erratic swimming and freezingFreezing An autonomic reaction that causes the animal to stiffen briefly when faced with sudden danger or when frightened.. These behaviours will disappear once the zebrafish feels 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 swim to the spot 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—for example, to other tanks for reproduction—their reaction can be observed. A healthy fish will quickly swim away from the fishing net zebrafish do not do this, or do so excessively, this is an indication that something may be wrong. Keep in mind that normal behaviour may vary between different fish lines. 

Interactions with conspecifics

Zebrafish are naturally social animals that require interaction with conspecifics to function optimally, both at rest and during activity. Their social behaviourSocial behaviour The gamut of behaviours displayed by an animal in the presence of other animals that are either not displayed when solitary, or are not displayed in the same manner. encompasses a wide range of activities. One of these activities is the display of aggression to determine the social hierarchy in the group. Abnormal behaviour directed towards tank mates is a reason for increased vigilance.

Zebrafish that are housed in prolonged isolation may gradually develop abnormal behaviour such as lethargyLethargy 1. somnolence; 2. state of disinterest and inactivity., or an altered appearance such as loss of colour. SolitarySolitary Alone, separated. In the context of mice and rats, the term pertains to housing the animal in a cage by itself. Social isolation is often an adverse welfare situation. housing must therefore be avoided. If it is nevertheless necessary, it is important to house the fish in a way that allows visual contact with conspecifics. Other possible solutions include introducing a companion fish from a distinctly different line (to avoid confusion). Tank or cage enrichment is also important; these can be floating plants as well as live food.

Interaction with the environment

When a zebrafish is placed in an unfamiliar environment, it dives to the bottom and from there gradually begins to explore and swim upward. In this way, the fish learns to understand its surroundings. The environment then becomes predictable and controllable. Predictable means that an animal can anticipate what happens in its environment. Controllable means that the animal can influence events through its behaviour. Predictability and controllability of the environment contribute significantly to the animal’s welfareWelfare A complex and dynamic internal state that includes: successful biologic functioning, positive experiences, and absence of negative conditions. The animal adapts effectively to internal needs and external stimuli and maintains a good mental state..