The fish fauna is one of the groups of indicators measuring the biological quality whose study is required by the Water Framework Directive. And, how can we know through fish the physico-chemical changes experienced in aquatic environments?
Physically, the fish community of an aquatic environment responds quickly to changes in their habitat. Depending on the species, an increase or descent in the numbers are followed by indepth changes (increase or decrease in levels) or water velocity. Changes in the morphology of the bed and riparian vegetation can induce changes in the species (feeding and nesting habits), or changes in the structure of the population (younger individuals in nesting areas or more adults in moments of pre-reproduction).
Of course, being a faunal group which maintains strong and constant contact with the water, any physico-chemical change in this condition affects the fish. For example, the presence of contaminants or toxic elements in the water (for example produced by algae), an increase or decrease of eutrophication, changes in the pH or conductivity, reduction of oxygen in the water, and so forth cause rapid changes in the fish community, mainly due to the disappearance of species that are able to live in a more narrow range of these parameters, which are more sensitive to these changes.
The benefits of fish as biological indicators are manyfold:
- They include different trophic levels (fitolancton feed, zooplankton, aquatic macroinvertebrates and other fish);
- They are located near the apex of the food pyramid of aquatic environments;
- They are present in most aquatic environments, even the most eutrophic or contaminated ones;
- They are very long-lived animals (some live over 20 years), so that may indicate historical impacts;
- Because of its size and mobility, they have a clear influence on the flow of energy and matter;
- There is an advanced knowledge of their biology and ecology and their relationship with the environment;
- sampling techniques, processing and species identification are simple.
While their role as indicator is studied and widespread in rivers and lakes, its application in wetlands cannot be performed analogously because of the hydrological and structural differences in the systems. However, the characteristics of fish as indicators prevail, so that the changes in their populations and communities give information of great interest to evaluate management practices for water quality and vegetation.
At present, the fish community of the Albufera is mainly made up of exotic species such as mosquitofish (Gambusia holbrooki), carp (Cyprinus carpio), goldfish (goldfish) and perch-sol (pumpkinseed). For competition, habitat loss and reduced quality of water, these species have shifted other natives like the mullet (Liza ramada), blenny (Salaria fluviatilis), and endemic species listed in the Habitats Directive and the fartet (Aphanius iberus) and samaruc (Valencia hispanica). Therefore, through the evaluation of management measures in the dynamics of both the native species as well as of alien species, the volume of information and the number of criteria will be increased to define water management priorities.
Given the fact that the objective is not to obtain information on the water quality but to assess the similarities and differences between constructed wetlands and the type of water management developed, indexes of qualitative and quantitative similarity will be used to compare different environments and the masses of water studied.
