Die Ablaich- und Interstitialphase der Äsche (Thymallus thymallus L.) - Grundlagen und Auswirkungen anthropogener Belastungen -

The present study deals with the spawning and intragravel period of the grayling (Thymallus thymallus L.) -basics and anthropogenic influences- in a river with high nutient load. In the River Lahn, a right-bank tributary of the River Rhine in Germany, the number and structure of spawning sites in three different river sections were noticed, the spawning behaviour was observed and the number of swim-up-fry was counted from 1995 to 1999. In two consecutive years with different discharge (1996 and 1997) substrate samples were taken at each spawning ground, oven-dried and sorted through sieves from > 63 mm to < 0.063 mm according to standard procedures. For the assessment of interstitial water conditions, a multilevel tube-sample-system was buried at natural spawning grounds. The pore water was extracted simultaneously from three sediments depth (10, 20 and 30 cm) and analysed for O2, CO2, alkalinity, NO3-N, NO2-N, NH4-N, NH3-N, o-PO4-P, pH-value, temperature and conductivity.The influence of different nutrient loads to the developing eggs and larvae of the grayling was studied in the reach of an waste-water-treatment plant (wwtp) effluent. Exposition boxes with grayling eggs were buried into the sediment at three study sites with different nutrient load. Simultaneously a multilevel-tube-system was placed in the sediment.Daily variations of temperature, oxygen concentration, pH-value, turbidity and conductivity were registered every 5-minutes in the surface water and recorded on computer. These data were used to model the daily variations of the corresponding parameters in the interstitial water. The exposition boxes were retrieved at three different developing stages of the grayling (eyed-eggs, hatching and emergence). The mortality of the eggs and larvae was noticed and morphometric parameters were measured using a dissecting microscope. In the River Lahn, the spawning migration of the grayling took place at the beginning of April. Male grayling defended rectangular-shaped territories of 4.5 to 8 m². The spawning act started in the afternoon when water temerature reached 8°C. The grayling used shallow areas (mean water depth 41 cm) with a mean current velocity of 61 cm/s at the surface and 34 cm/s at the bottom of spawning. Between spawning acts grayling rested in pools at the river banks that were covered by overhanging branches and roots. The Spawning areas were characterized by a gravely substrate. It consisted mainly of coarse pebble (particle size 20-63 mm: 30-50%), some bigger stones (> 63 mm: 3-35 %) and coarse gravel (6.3-20 mm: 14-30%). The percentage of fines (< 2 mm) was low at all three spawning sites (5.3-12.3%). The duration of the embryonic incubation period depended on water temperature. The average incubation period lasted 156.3 day degrees (16.9 days at 9.25 °C). The limiting factors of the egg and larval period of the grayling in the River Lahn were ammonia concentration and the percentage of fines in the interstitial. At the obsevation field with the highest nutrient load ammonia concentration reached a maximum value of 108 µg/l NH3-N in the pre-hatch period and 52 µg/l NH3-N in the post-hatch period. Not a single larvae survived in this observation field until the end of the study. A toxic threshold of 25 µg/l NH3-N is discussed for the post-hatch period. The amount of alevins was significantly correlated with the percentage of fines (particle size < 2 mm) at the natural spawning grounds. The maximum percentage of fines at a natural spawning ground was 23.7%. This high amount of fines was caused by missing scouring of the interstitial due to the absence of a flood in winter 1996. Consecutively the ammount of alevins was reduced to the half compared to the corresponding value of the year with winter flood. The water pH at natural spawning grounds (sediment depth of 10 cm) was in the range of 8 to 8.9. At observation fields a maximum pH of 9.9 was modelled for a sediment depth of 10 cm. The pH of the surface water showed great daily variations from 7.3 to 9.9 with maxima in the afternoon. These maxima were caused by the increased photosythetic activity of the algae, the growth of which was strongly increased by the high nutrient load of the River Lahn. Another result of the increased photosynthetic activity was the high oxygen content of the surface water and interstitial water at day time which ranged from 11.2 mg/l to 16.5 mg/l O2 and 11.7 mg/l to 15 mg/l O2 (sediment depth: 10 cm), respectively. The dissolved oxygen levels always exeeded the saturation curve at day time. At night time the respiration of the algea and biofilm led to oxygen consumption. Values as low as 5.9 mg/l O2 were recorded at night time in the surface water.