las comunidades de peces del río guadiamar y el accidente minero ...

Las comunidades de peces del río Guadiamar
y el accidente minero de Aznalcóllar
In addition, and in agreement with Ferreira et al. (2007), the analysis shows how
at the site scale, native species prefer coarser substrates and fast-flowing water. This
preference reflects the typical conditions of natural areas where, first, a coarser substrate
indicates absence of fine material overload from agricultural erosion, reservoir deposits
upstream or urban pollution (Doadrio, 2001); and secondly, calm waters are found in
higher proportion in the middle and lower sections of the river, where the increase of
both habitat degradation and exotic species has been cumulative, creating an unsuitable
environment for native species (Ferreira et al., 2007).
Regarding exotic species in the Guadiamar River basin, results confirm that the
number of exotic species increases from the source to the mouth of the river (Moyle and
Light, 1996a, 1996b; Gido and Brown, 1999; Kopp et al., 2009). This is probably due to
the accumulation of pernicious effects as the river reaches its lower section (Sheldon,
1988; Corbacho and Sánchez, 2001), accumulating exotic individuals from upstream
reservoirs and those going upstream from the mouth (Ruiz, 1998; Clavero et al., 2004).
Moreover, in the Guadiamar River basin, initial habitat degradation after the spill
favoured the rapid colonization of exotic species (Olias et al., 2005; Fernández et al.,
2007). Toxic mud removal works inevitably caused the elimination of important natural
elements for native species such as riparian vegetation or rocky shelters, leaving an
altered area where exotic species, generalists and better adapted to degraded zones
(Corbacho and Sánchez, 2001), have established more successfully than natives.
Exotic species establishment in the Guadiamar River basin is a consequence of
fishermen and government introductions for sport fishing (Fernández-Delgado, 2003).
The reservoir therefore becomes a source of exotic species, but their dispersal is not
homogeneous along the river course. Downstream colonization is more effective than
upstream, since individuals barely go upriver towards the source streams. This
asymmetrical movement may have a twofold explanation. First, the exotic species in the
Guadiamar River basin posses either a flattened body adapted to lentic ecosystems, such
as centrarchids and cyprinids, or a small size, such as the eastern mosquitofish. Both
body shapes have not evolved to be efficient in dealing with upstream colonisation of
the turbulent streams that fill the reservoir (Bernardo et al., 2003), while the fusiform
native species find no problems to overcome these currents and even use upstream areas
as spawning sites (Hubbs, 1940; Nikolsky, 1963; Herrera and Fernández-Delgado,
1992). The second cause may also be related to the adaptation process of exotic species
to the stable conditions of the water bodies where they originally inhabit (Elvira and
70