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.

But what is most spectacular is their complex nuptial behavior, as the male builds a nest with sticks and a special secretion.
The male then performs a dance to attract the female so that she will lay her eggs in the nest.
The male takes care of the laying, keeps the nest clean and oxygenates the eggs by creating a water current with his pectoral fins. He will also protect the fry until they can fend for themselves.

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

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