Since late summer, specialists from the Consorci de l'Estany have been carrying out the work of capturing invasive, non-native species of fish. The introduction of exotic species, originating from other parts of the world, is the main threat to the conservation of the biodiversity. In the case of Banyoles lake, the introduction of exotic fish has been identified as the main cause of the sharp decline suffered in the native fish populations and other aquatic organisms that originally populated this lake, such as mussels and crayfish. Today, over 99% of the fish in the lake, whether measured in terms of biomass or the number of specimens, are exotic species.
Through the Projecte Estany and the programme LIFE + NATURE, which is to be carried out between 2010 to 2013, the Consorci de l'Estany seeks to control the populations of invasive exotic species and enhance the populations of some native species, now in danger of extinction. The capture work that has begun is laid out in C.1. Campaigns to control exotice river fish of the Projecte Estany.
The main objective of this work is to reduce and control the group of introduced species, except species of a small size (Gambusia and the Italian river fish llopet) which are clearly outside the scope of any widespread control programme. This will offer the possibility of a spontaneous recovery of the populations of native river fish, a recovery which will be helped further by a restocking with specimens of these species from the same river basin.
These catches to control fish are carried out through intensive campaigns that combine different fishing techniques:
- Electrofishing: The basis of this technique is the use of electricity to "numb" the fish allowing them to be caught with ease. It enables copious samples of the natural environment to be obtained and for the fish to be released back into the environment whenever appropriate, because they recover well. In the case of Banyoles lake two forms are used.
- A fishing boat is used to make catches over the vegetated shoreline margins of the lake and larger ponds. A tally to allow the tracking of the activities of the fish has just been carried out.
- Fishing on foot (with backpack equipment or extendable cable) will be used only in areas with canals and streams to carry out precise inspections of these environments which, due to their characteristics, do not permit work with a boat.
- Large nets with three meshes, including a smaller one with a light in the centre.The fish are trapped and cannot escape. Nets are placed at different depths and areas of the lake (both at the edge and in open areas). It is not very selective, resulting in a greater variety of catches, with regards to both species and size. For this reason and because it causes serious injury or the death of most of the catch, it is a technique that is only used in the first phase, when the native fish populations have not yet begun to recover themselves. So far, four campaigns have already been carried out.
- Traps: in different parts of the lake and other areas of the environment, large traps are placed. In the Spring they are located where small streams and canals flow into the lake to carry out precise and effective controls on populations of certain species such as carp, which have a strong tendency to enter. This technique does not involve the death of the catch, so native species can be returned to their natural environment.
For this reason, this technique can be used throughout the project, and also once this is finished, it can be used to carry out a permanent, long-term, low intensity control.
The execution of these campaigns is coordinated by an expert specialising in fish from the Technical Office of LIFE which is responsible for the campaign along with the expert and two field workers.
To carry out the campaigns an action document had to be drawn up, technical staff hired and trained, material bought and the necessary equipment acquired.
Up until the time when more than 4000 catches were made, the most abundant species in terms of the individual numbers were sunfish, blackbass, perch, carp and zander (pike-perch), in this order, all non-native.The non-native fish caught are killed with the exception of those captured in the campaign's initial tally which have been returned to the lake, as well as a proportion of carp and blackbass that are being transported by the Forestry Consortium of Catalonia to controlled fishing zones for the practice of sport fishing, as established by the licence for catches issued by the Departament of Environment and Housing of the Generalitat of Catalonia.
The fate of the dead fish is different depending on their size: small fish are used as food for white storks that are part of a programme to reintroduce the bird to the region. They are also brought to the centre for the recuperation of endangered wildlife. The larger specimens are deposited in composting plants of organic waste for compost.