Zebrafish are social, shoaling animals. Aggression between fish is expressed in the form of chasing each other and occasionally ‘biting’. Aggression can occur among females only, males only, and between males and females. Aggression is associated with both reproduction and dominanceDominance This term describes the ranking of animals. A dominant animal occupies the top position in the hierarchy of a group of animals..

Social isolation

Long-term social isolation has a negative impact on 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. and health. When fish must be housed in long-term isolation for experimental or other reasons, consider providing social stimuli by enabling visual contact with conspecifics or pictures/videos of conspecifics, or by housing the fish together with a ‘companion fish’ from a distinctly different line for easy recognition.

Dominance

As zebrafish display territorial and dominance-related aggression, it is important to keep a close eye on the composition and density of zebrafish groups. Dominant fish can monopolize enrichment materials, or ‘claim’ certain tank areas. For example, by swimming mostly in the upper water column or crisscrossing the entire tank. Submissive animals, in contrast, can develop signs of chronic stressStress An animal’s biologic response in trying to deal with a threat to its internal equilibrium (homeostasis) by internal and external stressors. A failure to do so may lead to ‘distress’ resulting in an inability to cope, leading to poor welfare..

Reproduction

The reproductive success of matings is affected by a number of factors:

• Aggression between males and females
• The ratio of males to females in the holding tank
• Any released female pheromonesPheromones Scented substances that have a signaling function for the animal species that secretes them.
• Partner preferences

Furthermore, breeding success may vary depending on the zebrafish line. Males and females are separated overnight and allowed to mate in the morning. This happens in special spawning tanks (see figure 1). The males fertilize eggs that have just been deposited by the female. The eggs sink to the bottom from where they can be harvested. There is no parental care.

Figure 1 shows spawning tanks, in which males and females (here seen in the foreground) can be separated overnight. Different ratios can be used, e.g. 1 male to 1 female, or 2 males to 1 female. On the female side, a plastic plant provides enrichment. In the morning, the tank divider is removed, the water replenished and the insert is placed in a slanted position; this provides a shallow area, which is what females prefer to deposit their eggs. The tank bottom is corrugated and the eggs sink into the deep grooves where they cannot be cannibalized. Factors that stimulate oviposition are changing the temperature or conductivity of the water and increasing the intensity of the light (‘dawn’). Figure 2 shows the top view of a spawning tank, figure 3 shows the side view.

Housing of eggs

Eggs are ‘housed; in petri dishes filled with E3 growth medium. At a temperature of 28–28.5 degrees Celsius and a day-night cycle of 14 hours of light to 10 hours of darkness (14L:10D), the eggs will hatch within 48 to 72 hours. The density guideline at this temperature is 50 eggs per petri dish. A 14L:10D day-night cycle is preferred to continuous light or darkness, because under unnatural lighting conditions, the risk of developmental abnormalities is increased.