After we restored the arm in 2013 and dug another inflow channel last year, it is necessary to check the species composition of the fish. And that is happening right now – experts, ichthyologists from the Faculty of Natural Sciences of the Comenius University are conducting such monitoring these days. They catch, identify, measure and release fish back into the water to see how local populations are developing.
We are curious about the results ourselves, although we can say for sure that the opening of the side arms is good for the fish populations, there is calmer water for them to spawn in. Another, open inflow in the upper part will help to wash away the fine sediments, and in some places it may be possible to expose the gravel bottom, which is very necessary for rheophilic (current-affinity) fish, whose populations in the Danube declined most drastically when the current in the arms was shortened due to navigational improvements and they gradually were carried by fine sediment.
The results of the monitoring have not yet been officially published, but we have the following information from a member of the expedition, ichthyologist Daniel Grula:
“The important thing is that juveniles (young individuals) of species of European importance such as pintails, scallops, shiny roach were found in the arm, which means that they reproduce in the arm. Other species of European significance were, for example, the predatory bream, the northern barbel, and the white-finned grouper. Juveniles of rheophilic – northern barbel, northern pike – mainly in flowing sections – in the inlet part, where they have better conditions and flowing water, were also found. But invasive species were also found, all kinds of non-native Ponto-Caspian bulls, or black catfish.
So we are happy, apart from the invasive species, which are unfortunately difficult to prevent, this is a nice result that shows that the restoration of river side arms is necessary.