The spiny stickleback is a brownish fish between 5 and 8 cm in size that lives in the Chestnut River. Despite its small size, its life cycle is very interesting. In addition, its appearance is unusual because it lacks scales and instead has bony plates. Its name is due to the fact that it has three erectile spines at the will of the fish, with which it defends itself from enemies.
During reproduction (from March to July) the female has a rounded belly and a silvery color. The male acquires a very striking coloration, with a metallic blue back, red abdomen, golden gills and large green eyes.
In the Castaños River we also find the eel, which is listed as critically endangered worldwide. This fish is born in the Sargasso Sea. The larvae, transported by marine currents, travel to the eastern Atlantic coasts, where as juveniles (elvers) they swim up the rivers and complete their development. Dams, dykes and other river infrastructures without adequate passages become impassable barriers. Once they reach sexual maturity, they leave the rivers to return to the Sargasso Sea to spawn.
It has a management plan approved by the Provincial Council of Bizkaia in 2008. Its threats are: modifications in the riverbed, alterations in the flow and exotic predatory species such as gambusia, perch, sun perch and red crab. Their population trend is unclear.
Idea and design: Equinoccio Natura, S.C.
Photos: Jon Maguregi
Cartoonist: Oscar Domínguez