Démographie du hotu, Chondrostoma nasus (LINNE) (Teleostei,: Cyprinidae) dans l'Ourthe (bassin de la Meuse, Belgique

The investigation is based on a large sample (around N=3000) of C. nasus caught by electrofishing from a 20 km reach of the river Ourthe (20-30 m wide, Huet's barbel zone) in 1971-1976. Age was determined by examination of scales and opercular bones revealing a maximum longevity of 23 years for males and 25 years for females. The annual survival rate is 70 % for both sexes. Females were observed to grow faster than males : 426 mm as opposed to 409 mm over 10 years and attain a greater ultimate size. Annual variation of the relative growth rate (taken from annual growth indices) are correlated with the mean air temperature (taken as an index of water temperature ) in summer (June-August). The males and females are sexually mature in the age groups 4-5+ and 5-6+ respectively. The sex ratio of all age groups combined is 1,2 : 1 (males:females); however, males are dominant in the younger age groups and females in the older ones. Absolute fecundity increases with fish length up to 420 mm (log fecundity = -6,282 - 4,180 log length in mm); after this a senility effect leads to a decrease in fecundity. Seasonal length-weight relationships for each sex and size group are given in Table 3 and the seasonal cycle of gonad development and condition is shown in figure 5 and Table 3. C. nasus is a rheophilous, lithophilous cyprinid which spawns from March to early May following a temperature rise to 8-9 °C. Analysis of the age structure shows marked fluctuations in year-class strengths. Such variations of recruitment success are correlated with hydrologic and climatic conditions. At spawning time in spring there is a positive effect of low river flow on reproduction success; in the summer months there is a positive effect due to high temperature and a positive effect of abundant rainfall. A mark-recapture experiment made in 1973 estimated a population density of 71 fish (> 2+) /ha, a biomass of 51,1 kg/ha and a reproductive potential of 100 ova/m2. The angling yield during the 1974 fishing season amounted to approximately 35kg/ha. A cohort analysis indicates the critical ages and sizes at which the maximum biomass and fecundity (5-6 years;340 mm) and somatic production (3-4 years; 260 mm) occur. The European populations of C. nasus exhibit a wide variability in growth rate, age/size of sexual maturity and absolute fecundity with size or age. Other biological characteristics such as sex ratio and reproductive biology are much less variable between populations. From the demographic characteristics observed in the river Ourthe and other Belgian running waters, it is concluded that C. nasus is to a high degree, a r-strategist species.