Influence des élevages extensifs en étang sur les rivièrese The influence of extensive pond breeding on streams

Résumé

Si de nombreux travaux ont porté sur l'influence des piscicultures intensives sur les rivières, on s'est peu intéressé aux conséquences des nombreuses créations de plan d'eau sur le milieu. Le but de cet article est de présenter en s'appuyant sur des exemples précis les différentes modifications apportées sur le milieu par une surface en eau d'élevage extensif.

Les aspects hydrologiques sont peu importants. Les changements physico-chimiques ont par contre de nombreuses conséquences, directes et indirectes :

- la température : le réchauffement estival agit sur les réactions chimiques, l'équilibre des biocénoses, la pathologie des poissons,

- le potentiel hydrogène, très variable en cas de faible dureté,

- l'oxygénation, liée à la température dont le déficit agit directement sur la faune pisciaire,

- la fertilisation par l'azote et les phosphates,

- les matières en suspension : action surtout manifeste lors des vidanges, directe sur le colmatage des frayères, indirecte sur la structure des communautés floristique et faunistique.

Le repeuplement des rivières par les poissons d'étang est fréquent. Les conséquences vont du déséquilibre faunistique à l'introduction d'espèces indésirables ou nuisibles.

Dans l'ensemble, on note un impact négatif sur les eaux courantes, particulièrement sur les rivières à salmonidés (1^ère catégorie). Sur les cours d'eau de 2^e catégorie, l'impact est toujours plus faible et on peut noter une action bénéfique due au lagunage.Une hiérarchisation des influences est donnée en tenant compte du type de plan d'eau et de son mode de gestion.

Mots-clés:

• Pisciculture,
• étangs,
• influence des eaux Courantes

Abstract

Although many studies relate to the influence of fish farming on rivers, little interest has been given to the environmental impact of artificial ponds. The aim of this article is to show through precise examples the various alterations in the environmental system caused by extensive fish breeding farms.

Hydrological aspects

Even in the absence of any breeding, the presence of a water surface brings about variations in the local water balance.

- losses due to infiltration may increase through the drilling of an impermeable substratum when digging the pond, but new springs are often observed, resulting in an inscreased stream flow.

- fosses due to evaporation vary according to local climate and environment.

Pond dams sometimes act as buffers against floods.

Influences can only be significant under particular conditions : regional temperature and humidity and the nature of the substratum.

Physico-chemical aspects

During the filling period, one of the greatest risks is the rise in temperature. Even though the latter only occasionally reaches 10 % of its initial value, the effects are numerous :

- reduction of dissolved oxygen content and acceleration of microbial processes of aerobic decomposition.

- effect on biocenosis : a 3 to 4 °C rise in average maximal temperature may result in a typological change in the streams normally hosting Salmonidae. The disappearance of benthic Invertebrata may be observec as well as their replacement by groups whose development is stimulated by temperature : e.g. Mollusca and Worms.

- effect on pollution : a 1 °C rise in temperature, from pH to basic values results in a growing concentration in ammoniacal nitrogen which turns out to be toxic. On the other band, a possible favourable affect upon second class fish breeding streams must be noted. Eutrophization and enhancement of the environment also occur in addition to an accelerated gametogenesis in some species. In the late summer, this results gametogenesis in some species. In the late summer, this results in larger fish an, consequently, increased chances of survival lacer. An EEC rule has set the tolerance limits of temperature rise according to the type of river (table 4).

If It remains sustined, the discharge of suspended matter may also have various influences. Except for accidents, the risk is slight, measured values are generally inferior to EEC standards. However, it is essential to consider that discharges minerals and organic compounds such as some phytosanitary products (Lindane, Atrazine) likely to cause immediate or delayed toxic effects.

The creation of a pond is clearly a cause of instability in the composition of water immediately downstream, especially regarding the pH and, indirectly, NH3 content. This risk is all the greater since restituted water is the result of overflow. As for oxygen, the problem may be assessed differently, according to the mode of restitution; overflow may improve oxygenation whereas overflow caused by an emptying device may cause deoxygenation. Whatever the mode of restitution, downstream water is enriched with soluble and particulate organic matter which can promote bacterial development and oxygen intake.

Finally, it is worth noting that pend phytoplankton is likely to culture the downstream river, all the more intensely since the sluggishness of water flow will be stopped up.

The impact will depend upon the kind of pond involved : a « ballast tank » unconnected with the stream will have a lesser influence than a derivated pond. Since maximal impact will occur in the discharging channel before reaching the stream's downstream zone. The influence will be much more harmful to Salmonidae streams which are more sensitive to a reduction in dissolved oxygen. The imbalance will be heightened by fish losses which are likely to happen, since all species must compete with the surrounding fauna and will be favored by environ mental imbalance.

During the emptying operation, besides the circulation of elements trapped by sediment, the addition of suspended matter will involve e high risk for fishbreeding downstream from the discharge. If, during the draining operation, discharge volume is lower than EEC standards (25 mg/l), the emptying operation causes content to reach more than 100 mg/l, with immediate or delayed side affects.

The most direct harm done is asphyxiation of most fish under certain circumstances. This is heightened is a real risk of bringing about a rise in temperature : passing from a Salmonidae to a Cyprinidae environment both through disruption of the inverterbrata fauna, and sealing of the spawining places dug in the streambed gravel at emptying periods. The risk of a reduction in fauna varies and invertebrata biomass should also be brought up.

Whenever fish are discharged, repopulation of the river usually results. In this case again, the affects will be negative for Salmonidae streams or intermediate-type water in which the replacement and introduction of new species can occur. Everything will depend upon the breeding carried out in the pond.

In 2nd-category fish-breeding rivers, only a reinforcement of population and possibly a diversification of species will be observed.

As a conclusion, all artificial ponds have a negative impact upon freshwater especially in 1st-category fish farming rivers : e.g. changes in physico-chemical characteristics, introduction of species. This impact occurs during the filling period of the pond (variation in temperature) and the emptying operation (various discharges). In some cases, favourable consequences can be observed (table 1) : natural creation of a lagoon with trapping of elements in solution (phosphates, nitrogen). Moreover, the presence of alcalinity is stronger because of water sediment transfers, as is pH, showing a better fish farming potential.

The impact on freswater depends upon the type of pond and the method used for emptying.

Keywords:

• Fish farming,
• ponds,
• impact on fresh waters