Physical and chemical temporal variations in two spawning areas with contrasted sedimentation dynamics and preliminary results on the early life survival of brown trout (Salmo trutta).

The effects of fines and water quality on incubating salmonid embryos were studied in two tributaries of the river Oir (Normandy, France) with differe nt fish populations. Brown trout are very abundant in la Roche stream and t heir density is low in le Moulinet stream. We collected suspended and depos ited sediments circulating within and sealing the substrate of two reproduc tive areas every 8 to 15 days in 1996 and 1997. Incubators were buried in t he streambed to estimate the survival from egg to fry in 1996, and from fer tilization to hatching and from hatching to emergence in 1997. The characte ristics of the interstitial water (oxygen, pH, nitrates, nitrites and ammon ia) were measured once a week in 1997. This was performed with sampling tub es buried in the substratum and with a sample collected in each incubator a t hatching time. Preliminary results indicated that the amount of sediment in the spawning a rea was higher in la Roche stream during the incubation period. The particl e size distribution was mainly sandy while it was silty in le Moulinet stre am. No survivors were collected in fry traps at emergence in 1996 and betwe en 0 and 11% (mean : 3.3%) were found in 1997 in le Moulinet stream. The su rvival in one fry trap was 20% in 1996 in la Roche stream, between 0 and 23 % in 1997 (mean: 7.7%). The hatching-to-emergence survival was high in the two rivers (83.6 and 82.3%), with no rain falling during this period. Disso lved oxygen concentration of the interstitial water was significantly lower in le Moulinet stream (P = 0.01) during the second part of the incubation period while it remained quite high in la Roche stream (minimum value of 5 mg/l). Nitrate concentration is significantly different from that in the su rface water (P = 0.01) in le Moulinet stream and equivalent in la Roche str eam. Concentrations of nitrites and ammonia were low in the interstitial wa ter of the two streams (maximum values : 12.9 mu g/l N-NO2 and 240 mu g/l N -NH3 in le Moulinet stream, 11.4 mu g/l N-NO2 and 1.4 mu g/l N-NH3 in la Ro che stream). The higher mortalities observed in le Moulinet stream could be caused by a synergism of unfavourable factors such as low concentrations o f oxygen and nitrites.