Husbandry and breeding
Zebrafish are held at seven Max Planck Institutes (other fish species include the brown trout and rainbow trout, the three-spined stickleback, the turquoise killifish and the lamprey). They are cared for there by trained vets and animal attendants, as well as by scientists and technical assistants.
The fish facilities at the Max Planck Institutes consist of several thousand aquariums and the associated filter and water treatment systems. Activated carbon filters, biological filters and UV light radiation remove suspended solids, heavy metals and other pollutants and pathogens. The water in the aquariums passes through the filters four to six times an hour; between ten and thirty percent of the water is replaced by fresh water every day. Fully automated air conditioning and light technology keeps the temperature of the water constant at 26 degrees and also keeps a constant day/night rhythm.
The fish swim in family groups of a maximum of 70 fish in 13-litre aquariums. It is important that zebrafish are kept in large groups as in the wild they also live in schools and in this way are protected from predators. The tanks do not contain any plants or sand for hygienic reasons. The animals are fed twice (or if necessary three times) a day with the usual commercial flake food and with live or deep-frozen crustaceans (brine shrimp, water fleas).
Zebrafish are sexually mature at around two and a half months. To breed them, a pair of fish is placed in a special one-litre mating cage. As in the wild these animals prefer to spawn in the shallow waters of the shore, the mating cage contains a gently rising insert that simulates a shoreline. The parents would eat their own eggs immediately; therefore, the eggs can trickle through this sieve-like insert to the bottom of the aquarium. From there they are collected and placed in Petri dishes.
The fish larvae, which measure about one millimetre in size, spend the first five days of their lives in these dishes. During this time, they still feed from their egg yolk and do not need to be fed. On the sixth day, they are transferred to 1.5-litre tanks, where they live until they are sexually mature. As soon as they are fully grown, the young fish will be kept in the larger family tanks.
Hygiene
Infections pose a great risk to the fish, as pathogens can quickly spread in the fish facilities. In particular, bacteria belonging to the Mycobacterium genus can cause the dreaded fish tuberculosis, a disease that leads to ulcers and bleeding in zebrafish and ultimately to the death of the animal. Antibiotics cannot be used against the pathogens, as they would also kill the bacteria in the biological filter. However, a large aquarium cannot be operated without these bacteria, as they convert toxic nitrite into harmless nitrate.
Pathogens can also multiply on a massive scale among stressed fish. In addition to hygiene, infrequent restocking and calm conditions in the experiment sites are therefore high priorities at the Max Planck Society's fish farms. Regular monitoring of water quality, disinfection of the water, aquariums and equipment as well as strict quarantine rules are designed to prevent the animals from becoming ill.
