Danube Watch 1/2019 - Karlovac Freshwater Aquarium AQUATIKA

Aquatika is intended for everyone, from children and young people, to scientists, aquarium enthusiasts, nature lovers, and the local population, and aims to raise awareness of the exceptional biodiversity of our freshwater ecosystems. Its primary purpose is to educate visitors as to how the exceptional natural wealth of Croatia can be used in the function of sustainable tourism. The aquarium displays Croatian freshwater fish fauna and plants and their habitats, with more than 80 species of fish, including 30 endemic species, which are plants or animal species that are only found in one specific geographic area. If the area is very small (a cave, lake, river or mountain), this species is then classified as stenoendemic. It is important to note that among the 150 fish species found in Croatia’s rivers and lakes, 52 are endemic. This wealth of species is the consequence of Croatia’s geographical position, which includes both the Danube (Black Sea) and Adriatic Basin.

The aquarium has been designed as a two-story building with a ground floor and underground level and includes three buildings with an open area between them. Thanks to the green garden roof concept, the aquarium has a minimal impact on the landscape, and the green vistas of the surrounding areas have been preserved.

River continuity
The aquarium complex is designed to display the course of a karst river and its various habitats, such as the Korana where the aquarium is situated. The design is based on the idea of dividing the river course into its upper, middle, and lower sections.

The upper course flows at higher elevations which are nearer to the source and where the water is fast, cold, and high in oxygen. These reaches are inhabited by fish species that have adapted to such environmental conditions, such as the brown trout Salmo trutta, and the minnow Phoxinus phoxinus. Underwater plants that can tolerate the physical and chemical conditions in these upper reaches are mosses and diatoms. Invertebrates from the caddisflies genus, Trichoptera, live on the riverbed and build shelters of stones and twigs to increase their mass and prevent the water from washing them away.

The middle course of the river is characterised by a higher average temperature and lower flow speed with a somewhat lower oxygen content. The riverbed is gravelly and sandy, and invertebrates present here include the larvae of insects Insecta, snails Gastropoda, and crustaceans Crustacea. The aquatic vegetation includes mosses and diatoms, as well as green algae, filamentous bacteria, and fungi. Due to the better living conditions, there is a higher diversity of life, and this is where the most fish species are to be found, including the barbel Barbus barbus, chub Squalius cephalus, asp Leuciscus aspius, nase Chondrostoma nasus, bleak Alburnus alburnus, Danubian roach Rutilus virgo, and Balkan loach Cobitis elongata.

In the lower course, the riverbed is wide and deep, the water flows slowly, and average temperatures are higher than upstream in the middle course. This zone has rich aquatic vegetation, including water lilies. The bottom is sandy and silty, enabling the development of special communities of invertebrates, such as the molluscs Bivalvia and oligochaetes Oligochaeta. The fish species that are characteristic to the lower course are bream Abramis brama, carp Cyprinus carpio, catfish Silurus glanis, and zander Sander lucioperca.

Fish endemic to Croatia
The cave system habitat shows the endemic species of fish that are predominantly found in the karst landscapes and underground habitats. Endemics found in underground habitats are highly sensitive and are at risk of extinction. Their survival depends directly on human activities, specifically on the active protection and conservation of surface and underground waters. Some of the endemic fish, such as the Croatian dace Telestes polylepis, karst dace Telestes karsticus, Krbava minnow Delminichthys krbavensis, Jadova loach Cobitis jadovaensis, and Zrmanja chub Squalius zrmanjae are found only in Croatia and nowhere else in the world.

A home for sturgeons
The largest habitat in the aquarium includes the migratory fish species from the sturgeon family that migrate from the sea into the river to spawn. The majority of these species: the stellate sturgeon Acipenser stellatus, Russian sturgeon Acipenser gueldenstaedtii, ship sturgeon Acipenser nudiventris, Atlantic sturgeon Acipenser sturio, and beluga sturgeon Huso huso are considered to be regionally extinct in Croatia due to regulatory measures, the damming of watercourses and overfishing. It is interesting that these fish species date back to the time of the dinosaurs and that the beluga sturgeon is one of the largest fish ever to exist (a specimen eight metres long and weighing 3,200 kilograms was caught in the Volga River in 1884). Unlike the sturgeons, other species such as the European eel Anguilla anguilla migrate from the river into the Atlantic Ocean to spawn.

Standing waters
The standing waters are represented by the ponds and wetlands and abound with diverse plant and animal life. These areas serve as a natural filter and retains water during flooding. These waterbodies have a series of positive effects on the environment and are considered to be the protectors of biological diversity. The fish inhabiting the standing waters, such as tench Tinca tinca, Crucian carp Carassius carassius, and weatherloach Misgurnus fossilis, are all capable of adapting to the great changes that occur in the physical and chemical parameters of the water.

Alien fish species
the next habitat contains alien fish species that are not native to Croatia. Most of these are invasive and pose a threat to the native fauna. Alien species are those which do not naturally inhabit an area but were instead intentionally or unintentionally brought to that habitat through human activity. When an alien species creates changes in a given area and threatens the native biodiversity it becomes ‘invasive’. These invasive species compete with the native species both for space and food, with some crossbreeding with native species. They can also introduce new diseases and pests into the environment. Invasive fish species, such as rainbow trout Oncorhynchus mykiss, Prussian carp Carassius gibelio, and the grass carp Ctenopharyngodon idella, are among the most significant reasons for the extinction of native species, along with pollution, watercourse regulation, the construction of dams, and amelioration works.

Travertine
The final habitat shows the karst phenomenon of the creation of travertine waterfalls, and the importance of conserving them. In favourable conditions, carbon dioxide is liberated from the water and mineral calcite is separated out. Travertine building plants, such as algae, secrete sugars that the calcite sticks to, and caddisflies form netting that the calcite attaches itself to. Calcite crystals settle out of the water with the help of these travertine builders, and at certain places in the river’s course, travertine begins to form as a porous and soft sedimentary rock. Fish and crustaceans can find places to shelter within its hollows and channels. Stunning examples of travertine waterfalls can be seen at the Plitvice Lakes, on the Sljunčica River in Rastoke, and on the Mrežnica and Korana Rivers.

The natural water cycle & aquarium technology
Due to the natural water cycle and its geographical position, Karlovac is rich in underground waters. Near to the aquarium is a deep well that supplies the aquarium tanks with chlorine-free water. This natural water is pumped into the aquarium’s system, with particles such as algae and sand being removed from the water through a mechanical filter. Harmful compounds such as ammoniac are removed with a biological filter. The water then enters into a cooling chamber where the temperature is regulated. Oxygen is added to the water, which is then sterilised to remove viruses and bacteria. As it passes through the aquarium system, 5%, or 14 m3, of the water evaporates, and the same amount is returned to the Korana River via the surface runoff drainage system. As in nature, this hydrological cycle is repeated ad infinitum.

Aquatika is a unique freshwater aquarium in Europe and provides visitors with the unique experience of the underwater world that we seem to know well, but still have so much more to learn.