Gasterosteus Aculeatus
Gasterosteus aculeatus, popularly known as the three-spined stickleback.
It is a small freshwater and marine fish found in numerous rivers, streams, and coastal waterways worldwßide, including Croatia.
Gasterosteus aculeatus is a native species in Croatia, where it may be found in both freshwater and brackish settings.
Although the species is not designated as endangered in the country, habitat loss and degradation, pollution, and other environmental stressors may have an impact on populations.
The three-spined stickleback (Gasterosteus aculeatus) is a fish which shows great morphological variation throughout its range, ideal for questions about evolution and population genetics.
Many populations are anadromous (they live in seawater but breed in fresh or brackish water) and very tolerant of changes in salinity, a subject of interest to physiologists.
This species can occasionally reach lengths of 8 cm (3.1 in), but lengths of 3–4 centimetres (1.2–1.6 in) at maturity are more common.
Endangered species in Croatia, the reason for the decrease in numbers is due to water pollution.