MEASURES - The journey of sturgeons restocking in the Danube


Restocking is a common method to increase the natural population size of intensively utilized fish species. But if it is conducted for conservation purposes it is not as easy as it seems. Let see the journey behind the fish, waiting to be released.

The first step of restocking is to collect a broodstock for artificial reproduction. The origin of mother and father fish is crucial to maintain the genetic integrity of the wild population. In other words, the genetic background of the parent specimens has to be analogous to the genetic traits of the natural population to be strengthen. It does not necessarily mean that the parent species have to be collected exactly from the waterbody where the younglings are about to be released. The genetic traits can be similar even between two river sections far from each other or between two tributaries. Generally, the genetic variability is increasing with distance, but it is more complex than that. The mobility of the species is another important factor determining genetic variability. In case of a species which spends its entire life within a small area, the distance is a more considerable factor on the genetic variability than in case of migratory species. Since the genetic background is crucial, it is usually checked after the broodstock is collected. This information contributes to proper restocking management plans.

After a specific time for acclimatisation, the collected broodstock goes through an artificial reproduction process. They are injected with hormones, which ensure the timed egg-maturity. Then the eggs and the milt are collected in a plastic container while the fish is sleeping. The life of the released fish starts at this point. During the rearing they are moving sequentially form a smaller tank to a larger one. In the last phase of the rearing, they live several months in rearing ponds. During the rearing, the tanks and ponds are filled with river water specific for the location where they will be released. This is crucial in the case of migratory species because they have to get used to the water to recognise it later when they are about to return for spawning. The phenomenon when the hatched fish first meets with a water type and become acclimatized to it, is called imprinting.

After the reproduction phase, the parents can be released or alternatively can become part of an ex-situ gene bank. This second kind of gene stocking is the cornerstone of genetic conservation. Rear species or representatives of populations characterised with unique genetic background, can be held in safe places, similarly like zoos. They can serve as a source for re-establishment efforts for endangered species or reduced populations.

The tale of the younglings is not finished with the release. They also get a small tag to recognise them if anybody catches them in the future. The recapture data is very important as it provides information about the success of the restocking effort, the condition of the wild population, the state of the natural environment, and last but not least, in case of migratory species about the continuity of eco-corridors.

Throughout the MEASURES project there will be a series of restocking actions with sturgeons that will take place in Romania and Hungary. Keep in touch! 

Programme co-funded by European Union funds (ERDF, IPA, ENI)