Contribution to the study of the food, of the growth and of the fecondity of the perch (Perca fluviatilis L.) in a temperate oligotropic lake
The large-scale hydro-electric reservoir of Lake Sainte-Croix fed by the Verdon river, was dammed in 1974 and holds 765 hm3, spreading over 2 200 ha at 477 meters above sea level. Its average and maximum depths-are 50 and 80 meters and the water level can fall 16 meters in Autumn and Winter, reducing the water cover by a maximum of 23 % during January and February. It is a warm monomictic lake where the hypolimnion is well oxygenated in all seasons. The water has an alkaline pH and is relatively poor in nutritional salts. Controlled by water temperature during the mixing period, the primary production, always tow, is limited by loss of nitrogen in the epilimnion and lack of light in the hypolimnion during the thermal stratified period. The epilimnion temperature varies from 7 °C in February to 22 °C in July, with an average of more than 900 degrees per day per year above 14 °C. In addition to the various indigenous species, perch, roach and bleak are stocked, the latter two species being predominant now. The stomach contents of perch fished with nets and by electrical means during the Winter (December-March) and Summer (August) periods were examined and the animals' ages were determined using the opecularbone method. The scarcity of plankton compels the young as well as the adult perch to feed on macroinvertebrates of benthos. This trophic niche overlap between the two size groups creates intraspecific competition. The availability of benthic prey is conditioned to a great extent by the receeded water line in Winter (Winter tide) which leaves the essential feeding ground dry for several months and limits the development of aquatic vegetation. The existence of an interspecific roach-bleak competition, especially noticeable between the young planktivorous fry, accentuates the effects of the poor nutritional conditions on the rate of perch reproduction which is among the lowest on record. Sexual maturity is reached at the end of the first year for the males, and during the second year for only 50 % of the females. The absolute fertility measured on 15 females between 14 and 40 cm, varied from 2,500 to 50,000 eggs.