Storage density is more important than tank size.

Rebecca König of the Max Planck Institute for Biological Cybernetics on lab animal day keeping zebrafish

The zebrafish is an important model organism for the study of diseases. More than 80 percent of the genes known to date that can cause diseases in humans are found in a similar form in fish. The zebrafish is the second most widely used laboratory animal after the rat in the Max Planck Society. But how are the fish known as zebrafish kept at the Max Planck Institutes? Marking Laboratory Animal Day on April 24, Rebecca König, a veterinarian at the Max Planck Institute for Biological Cybernetics in Tübingen, describes what she and her staff are doing for the welfare of their fish.

Dr. Rebecca König, a veterinarian at the Max Planck Institute for Biological Cybernetics in Tübingen.

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What does the life of a zebrafish look like in your establishment?

The zebrafish at our institute were raised in petri dishes during their early days of life. After about five days – the time when the larvae begin to eat – we move them to larger aquariums. At the age of three to four months, they then enter the enclosure of adult fish. About five fish per liter live in aquariums from 3.5 to 12 liters.

In the wild, animals usually live for about a year, or in captivity for two years or more.

Aren’t aquariums too small?

Zebrafish teach fish. Stocking density plays a major role in maintaining it: if the density is too low, animals begin to form and defend territories. In addition, at lower densities, female fish develop more often than males.

If more fish live in an aquarium, you can keep them in larger tanks…

For research, we need different color mutations and genetic lines. We need separate cabinets for each of them, since all the lines are indistinguishable from each other with the naked eye. If the aquariums are larger, more fish will have to live in them to prevent them from forming areas. However, these additional fish are not required for breeding or experiments. In other words, animals that are not actually needed will be kept. And we want to avoid that.

In research laboratories, zebrafish are usually kept in aquariums from one liter to ten liters. Most aquariums that an aquarist can buy at a specialty store are much larger. Why is there this difference?

Standards for hobby and breeding lab animals may vary. The Animal Care Veterinarians Association recommends a volume of at least 54 liters for aquarium fish. On the other hand, the European Federation of Laboratory Animal Sciences does not set any minimum tank sizes. Instead, she recommends a stocking density of four to ten sexually mature animals per liter. This information is based on empirical values, in part also on scientific studies. So far, however, there are no legal regulations on this.

It should also not be forgotten that the fish in a hobby aquarium should share space with plants, sand at the bottom, decorative objects and technical equipment such as filters and heating. The space that zebrafish can actually use in such cabinets is actually smaller.

As a school fish, zebrafish feel more comfortable in larger groups.

© MPI for Developmental Biology (now: MPI for Biology Tübingen) / B. Schuller

Do the fish in small tanks have enough clean water?

Most laboratory animal facilities work with large water treatment systems. The water in ponds is completely replaced five to eight times an hour. This ensures that contaminants are constantly removed. Important parameters such as temperature, nitrogen decomposition products as well as fish health are checked regularly. New fish do not enter the facility until after quarantine or via eggs that have previously been surface disinfected.

Private home aquariums usually have a much lower exchange rate for water. The water is circulated and treated in a small circle, and usually only fresh water is added weekly or biweekly. Rarely can hobby aquarium owners provide their owners with such high standards of hygiene as our professional laboratory animal facilities.

Is there a difference between feeding lab animals and special zebrafish?

We offer high quality feeds tailored to the needs of this species. It also takes into account the different stages of development from the larva to the sexually mature fish. The feed offered in pet stores is often compound feed, that is, a compromise in order to please various aquarium animals.

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