Siluro, the Megapredator in the Tagus River
The Siluro catfish (Silurus glanis) is a large fish species (it can reach 2.8 metres in length and weigh 120kg), predatory and extremely fertile, depositing up to 350000 oocytes to be fertilised. In Portugal, it is an invasive species that has proliferated in recent years in the River Tagus, contributing to the decline of native fish species. In the last decade, some Silurians have been found in the River Douro, in the Meimoa reservoir near the Serra da Malcata and in the Montargil reservoir.
Filipe Ribeiro, a researcher at MARE, is studying the ecology of the Siluros to find methodologies to help control its populations in habitats where this species is invasive. As part of the Megapredator project, led by MARE in collaboration with the Instituto Agrário de Santarém, 261 Silurians were analysed along a stretch of the River Tagus (from Vila Franca de Xira to Abrantes) to find out what these animals feed on.
In an interview with the Público newspaper, Filipe Ribeiro said that "of the 36 species of fish that exist in this stretch of river, only five species were not detected in the Siluros' stomachs. It's a very generalist predator".
At European level, Filipe Ribeiro is working on the Life-Predator project, with the aim of reducing Siluro populations in protected areas. "The Tagus River in Portugal is over 200 kilometres long. Doing population control in every single place is not financially or operationally possible. That's why we have to focus on the natural parks and the places where the silurian could have the most impact on fish with high commercial value," explains the MARE researcher.
The project aims to reduce around 90 per cent of the catfish biomass in five small isolated lakes, which are part of the Natura 2000 network, 50 per cent in the case of small bodies of water and 10 per cent in large lakes and reservoirs. "The number of siluro is already so high that there has to be a very large fishing effort to have an effect, at least to lower the population," Rui Pedro Rivaes, a researcher at Mare who is part of Life-Predator and is leading the study to test different fishing methods for catching siluro, explained to Público. "We've been implementing combinations of different techniques, with different mesh nets and the use of palanques, which are hooked lines that are fished with live or dead bait."
Over the last year, researchers have monitored the population at four sites: In the Belver and Fratel reservoirs (Tagus river), in the Cedillo reservoir (international Tagus) and in the Meimoa reservoir (Serra da Malcata), and it will be this summer that they will deploy fishing gear. This will allow them to see if it is possible to control the Siluro population using fishing gear, and at the same time contribute to the recovery of native fish species.
In addition, part of the work of the MARE researchers also involves alerting the population to the ecological danger posed by Siluros. According to the survey carried out by the team of researchers, there is a minority of sport fishermen who value this species as a trophy. "These are the people who need to be worked on. There should be a greater presence of enforcement agents on the ground, targeting this group of people, to show them that they can't transport fish from the River Tagus to another reservoir," says Filipe Ribeiro. "We know perfectly well that the siluro was taken from the River Tagus to the Meimoa reservoir. It's totally against Portuguese law. But it's not an environmental crime, unfortunately."
This proposal is also part of the Action Plan for Siluro Control in Portugal, drawn up by Mare for the ICNF: "The plan proposed different management solutions, which were discussed internally by all the ICNF technicians at national level," says Filipe Ribeiro, explaining that the idea is for it to become a Council of Ministers Resolution, which is currently awaiting approval by the government.
To read the Público newspaper article in its entirety, click HERE
Photographs by the LIFE-Predator project