Environmental degradation
The fish, like other organisms that live in river ecosystems, are influenced by four main variables: water quality, flow regime, nutrients and structure of the habitat. Changes caused by human action in any of these variables will affect fish communities.
Major changes can produce immediate quantitative effects, such as fish mortality from a polluting spill. The consequences of other changes can be slower and more difficult to measure. If the changes produce a less suitable environment, fish grow more slowly, they increase mortality and reproduction is less successful. All this can cause a cumulative decline that, finally, may lead to the extinction of one or more species. In the Mediterranean areas, these impacts are exacerbated by the increase in human population, along with the scarcity and irregularity of rainfall. Reducing river flows has repercussions, also, in an increase in the concentration of pollutants in the water.
The most important impacts our fish suffer are as follows:
Alteration of flows
Reservoirs act as water reservoirs and alter the natural hydrological regime of rivers downstream of the dam.. The regularization of the flows and the attenuation of floods and droughts of our rivers harms the native species and at the same time favors the introduced ones., better adapted to living in more stable environments. Another impact is caused by hydroelectric power plants. On the one hand, they divert the flow through canals and leave the river sometimes almost dry, and on the other hand, some of them when they return the water to the river, after passing through the turbines, they produce a sudden increase in flow, what harms the aquatic fauna.
Barriers
Dams and dams block the free movement of fish along rivers. The magnitude of this problem is obvious when we consider that in the rivers of Catalonia there are more than 900 barriers. Migratory species such as shads, sturgeon or eel are the most affected. For example, the eel used to go up to the headwaters of some rivers, but at present in almost all the rivers the passage to short distance of the mouth is barred. The shad or sturgeon need to go up the rivers to reach the breeding grounds.
To mitigate this problem, fishways can be built that allow fish to move through the obstacle. However, often the designs are not suitable and the efficiency is very low.
Modifications of the channels and banks
The distribution of fish is related to the availability of suitable habitats for each species, consisting of certain combinations of current velocity, depth, substrate and objects that provide shelters, like big rocks, fallen trees or aquatic vegetation. The modification or channeling of riverbeds causes the partial or total elimination of some of these essential natural elements for fish as a refuge or spawning sites.. Aggregate extraction also has a major impact, as well as having serious effects on fish habitat, they modify the slope of the rivers and favor the warming of the water.
Pollution
Pollution of urban and industrial origin was a serious problem in the rivers of Catalonia for a long time, but the situation has greatly improved thanks to the construction of wastewater treatment plants. Currently the main problem that persists is heavy metal pollution, microplastic, pesticides, herbicides, hormones and drugs, which can cause sublethal effects in fish. Some of these compounds accumulate in aquatic organisms and alter vital functions such as growth or reproduction.. You also need to pay attention to the so-called “silent pollution”, which is the increase in suspended solids in water due to erosion of the river basin, caused by the construction of infrastructure, agriculture or deforestation. Permanently turbid rivers that accumulate sediment and mud in the substrate make life difficult for many native species..
Introduction of exotic species
Currently, more than half of all continental fish in Catalonia are exotic species. Introduced fish have clearly negative effects on native species. Impacts include predation, competition for space and food, hybridization and transmission of pathogens.
