FISH MIGRATION SOLUTIONS

One of the most harmful effects of human activity on aquatic and riparian ecosystems is that related to the modification of a river’s hydrological and hydraulic system. An example of such modifications are transverse constructions (dams and weirs) that limit or prevent the free movement of species, particularly fish, which are especially sensitive to changes in hydraulic conditions.

The diversity of fish in the Iberian Peninsula is not very high if we compare it to our neighbouring countries in central and northern Europe. However, the number of endemic species in Spain is very high due to the particular hydrological characteristics of out rivers. For this reason it is particularly important to look after these assets.

Two species of fish that are found in abundance in the waters of the Segura River basin are endemic: the Andalusian barbel (Luciobarbus sclateri) and the brown trout (Salmo trutta). Both of these species are migratory species that move along the river in search of suitable spawning grounds, always within the same river basin (potamodromous migrators).>

When there is a transverse infrastructure in the river, which is impenetrable for the species that need to travel upstream to reproduce, these species disappear from the inaccessible upper section of the river. In extreme cases, this can lead to the temporary or total extinction of the species, such as the case of the European eel (Anguilla anguilla) in the Segura River basin. For those species that are unable to pass the obstacle, this limitation of movement means it is impossible for them to colonise new territories, upstream habitats are lost and populations become isolated.

According to current legislation (River Fishing Act 1942 and the Redrafted Text of the Water Act 1/2001 and its later revisions, the latest of the 12th December 2013), the movement of fish along the rivers must be guaranteed, and any structure impeding or limiting it must be adapted to ensure this.

The solution to this problem ranges from the demolition of the obstacle in cases where it is in disuse, to techniques that allow fish to pass this type of infrastructures, known as fish ladders. There are several different types of bypass structures, including fish lifts, artificial rivers and pool-and-weir ladders. Whatever the type of solution adopted in each case, they should all meet a series of Basin requirements: the entrance should be easy to find, it should be easy for the fish to travel through them, the exit point should be safe, all the native fish species should be able to use them, and they should be able to be used when water levels are both high and low.