New Invasive Fish Spreads Through The Ebro Delta

The spread of the dojo loach (Misgurnus anguillicaudatus) or weather loach, as this oriental fish from the same family as the colmilleja (Cobitis Paludica) of Spain and Portugal is commonly known, contrasts starkly with the decline of native fish in the rivers and wetlands throughout the Iberian Peninsula, where 80% of species are threatened. The introduction of species from different areas is one of the most serious threats to the preservation of biodiversity, as explained in this study published in the latest issue of Biological Invasions.

“Aquatic ecosystems on the continent are the most seriously affected by invasive species, with fish populations being particularly devastated,” Miguel Clavero, the report’s lead author and researcher at the Forest Technology Centre of Catalonia, tells SINC. Nowadays, the majority of river basins throughout the Iberian Peninsula contain more foreign species than native ones.

The dojo loach is a small yellowish fish measuring around 20cm, which was first discovered in 2001 in the Sèquia Mare, a channel draining the rice fields of the Ebro delta, located on the northern hemidelta (left side of the river), although individuals were also seen in the river itself. Within a few years, the dojo loach has come to occupy most of the hemidelta and has moved towards the southern part, where it was first found in 2005.

Clavero explains that “today, the species occupies a minimum of 31 1x1km UTM quadrants within the interior of the delta, and several thousand individuals have been caught during operations to monitor ichtyofauna carried out by the Ebro Delta National Park”. There is a high probability of the species expanding towards new areas. In fact, the researchers say that in the spring of 2007 the dojo loach was detected in the basin of the River Onyar, which is a tributary of the River Ter, in an area very close to the urban centre of Girona.

Salinity is a barrier

Despite its rapid expansion, Clavero and his team have shown that the distribution of the dojo loach “seems to be limited by salinity”, as it only occupies waters with a low salt concentration. This is one of the reasons why the fish has not invaded lagoons, swamps and other natural wetland areas in the delta.

“The presence of the dojo loach, like that of many other invasive fish species in the Ebro delta, is linked to the continuous flow of river water channelled from the Azud de Xerta reservoir to irrigate the rice fields, which cover 60% of the surface area of the delta,” the researcher points out.

Introduction of species is a threat

The researchers believe that the dojo loach population first appeared in the Ebro delta “after some individuals escaped from tropical fish maintenance and distribution facilities”. Centres that breed and maintain exotic fish pose a serious danger and are “an epicentre of the biological invasion process”.

This fish could be responsible for parasitic platelminths becoming established in the area, and could indirectly compete for resources with native species. “Each addition of a new species to an ecosystem has an impact on its other inhabitants which, in the case of fish at any rate, are always negative,” stresses the scientist. In the lower part of the Ebro, the presence of the dojo loach could threaten the survival of the freshwater blenny (Salaria fluviatilis) and the colmilleja (Cobitis paludica).

The dojo loach or weather loach is, according to Clavero, one of the most recent introductions of an invasive species in the Iberian Peninsula. With a long, somewhat flattened body, it is a fish designed to live in waters with weak currents, where it feeds on snails and insect larvae. It can survive temperatures close to freezing, and tolerates very low levels of dissolved oxygen.

Although it is not so common in Europe, it is spreading in other parts of the world such as the Philippines, United States, Central Asia, Australia and various Pacific islands, where it is used as an aquarium fish, as well as for live bait and as a food source.