The appearance of large trout in various sections of the Ego river has become a symbol of the river's environmental improvement. This transformation is the direct result of the completion of sanitation works that have eliminated the wastewater discharges that, for decades, severely affected water quality in Eibar and Ermua.

The infrastructure, which intercepts wastewater from Ermua and Mallabia to divert it to the Apraitz treatment plant in Elgoibar, was an engineering challenge due to the area's complex topography. This project was made possible through the collaboration of the Basque Water Agency (URA), the Gipuzkoa Water Consortium, and local municipalities.

Experts note that trout are a demanding species requiring clean, oxygenated water. The reduction of foam and odors, along with increased water transparency, confirms that the river ecosystem is entering a new phase of biodiversity after years of urban and industrial degradation.