La sauvegarde des intérêts piscicoles au cours des travaux d'aménagement des cours d'eau

Abstract

INTRODUCTION The aim of river development is to counteract adverse effects on the river's role as a "conveyor" or "vector". Though river engineering work may also help to protect the aquatic ecology, it also frequently harms the fish population. Appropriate protective measures, therefore, are an important factor to consider in any river development or conservation project.RIVER ENGINEERING River engineering projects with the aim of improving flow conditions include both conservation or maintenance measures and the implementation of new river works.MAINTENANCE Most "private" rivers (i.e. not under State management) generally lack adequate maintenance, being subject to increasing pollution, silting, obstruction and bank degradation (Fig. 1), and also because riparian populations are unable to meet their individual obligations. "Public" (i.e. State-controlled) waterways, on the other hand, are generally well-maintened, except if down-grated. The latter become eutrophic and obstructed (Fig. 2). In attempting to remedy critical situations arising from a lack of maintenance, inconsiderate action has frequently been taken, e.g. in the form of excessive, ill-timed and ill-placed silt and weed-clearance operations which have caused warping and destroyed various fish species spawning grounds and shelter spots (Fig. 3). A total lack of maintenance has very serious consequences, as it results in a kind of "chain-reaction" of physical, chemical and phyto-zoological factors including obstruction, silting, flow retardation, rising water temperature and nutrient content, decreasing daylight, destruction of water plants, substitution of the periphyton and bentos, disorder and upsets in the fish population. Inconsiderate flow-control operations are apt to result in disastrous conditions, e.g. entrainment of toxic sediment, spawning grounds being left high and dry for prolonged periods, etc. NEW PROJECT IMPLEMENTATION Improvement of a river's capacity as a collecting and exchange medium usually involves resizing, profile and bed correction work affecting the physical, chemical and phyto-zoological conditions and the fish population's environment. Typical "trouble-factors" include sudden increases in light intensity, temperature and O2/CO2 variations, and replacement of underwater aquatic vegetation by standing vegetation or fibrous water-weed beds, which completely upsets the system in equilibrium within the biological "hierarchy" of the aquatic medium. Resizing of rivers to standard dimensions tends to dull the diversity of the natural aquatic and "subaquatic" landscape in which fish thrive. Safe hiding places and spawning grounds both become increasingly scarcer. Silt and deposit clearance does, however, have a number of beneficial effects. LOCKS AND RIVER BARRAGES River barrages are erected on the normal river bed. They have little effect on fish ecology, except in the case of sudden releases of water, which may cause mechanical or organic pollution. As most barrages are comparatively small, they are seldom equipped with fish-ladders (Fig. 5). PROTECTIVE MEASURES - MAINTENANCE Protective measures under this heading are simply a matter of complying with all water management regulations and issuing suitable regulations for such new situations as may arise. Weed-clearance in strips down the river and silt clearance outside spawning grounds when the fish are not spawning are examples of sensible measures (Fig. 6). NEW PROJECT IMPLEMENTATION Important requirements under this heading include minimal straight river sections (i.e. conservation of bends) and preservation of an adequate depth of water at low-water conditions by means of a channel in the normal river bed (Fig. 7 and 8). On no account should project implementation work be allowed to affect the biogenic grounds, specific trees or shrubs or string, of bushes along the river banks (Fig. 6). Temporary (removable) barrages (Fig. 9) and new fish habitats and spawning grounds (Figs. 10-14) are requirements serving the fish population which are well worth considering in any river engineering scheme. CONCLUSION Carefully planned maintenance at regular intervals, sensible action and simple techniques "tailored" to suit individual river requirements are vital in preserving the more vulnerable fish species.