The crucian carp is a fish of two faces – or more precisely shapes. It can be either slim, so it could be compared to a small fish penknife, or tall, when a comparison with a plate would not be a completely inappropriate overstatement. These two forms of crucian carp have been known for a long time, but it was not until the early 1990s that their secrets were revealed. Crucian carp begin to change from lean to high-bodied fish when in the presence of fish predators, typically pike. This is an induced change in morphology, which is caused by chemical signals in the water. The tall body of the crucian carp brings some disadvantages, but also one major advantage: it is difficult or almost impossible for smaller pike to grasp and swallow the carp.

The crucian carp is also unique in that it can survive in water that is overgrown with plants with a lack or even a temporary absence of oxygen. It tackles this seemingly unsolvable problem by switching to an anaerobic metabolism. It has been experimentally proven that it can go without oxygen for up to 140 days.

The crucian carp’s remarkable adaptations lead to the question of what may threaten it. All sorts – but most of all different, non-native carp species. Of these, mainly the Prussian carp. Not only does it crossbreed with crucian carp, but it also sexually parasitizes it: the sperm of male crucian carp triggers the development of embryos in females of Prussian carp without fertilizing them, thus resulting in only Prussian carp offspring. Over time, the invasive Prussian carp almost completely displaced the original crucian carp from our waters. Obtaining crucian carp that are “uncontaminated” by genetic material from non-native species is also one of the great challenges of our project.

Ensuring that Prussian carp or other non-native carp or carp hybrids do not get into our waters presents another challenge. We can prevent their eggs getting into the cutting from the Vltava’s water, but, for instance, they could be carried there by wild ducks on their feathers or in the digestive tract.

Despite these and other difficulties, this year we may well be able to create a backup population of crucian carp in the former mill race, as well as some of our other fish, such as the sunbleak or the weatherfish. One of the “fish” projects taking place directly in Prague Zoo’s grounds thus joins the long line of our in situ activities, which mostly take place around the world.