In Lake Geneva, the surface water temperature has increased by 1°C over 20 years probably as a result of climate change. The effects of changes in temperature on the reproductive cycle of the roach Rutilus rutilus were assessed in a 19 year survey. Over time, spawning tended to begin earlier. The consequences of temperature changes were assessed on two different stages of the female reproductive cycle: the development of the ovaries from the beginning of autumn to ovulation, and the onset of the spawning period. The development of the ovaries was studied for 7 consecutive years from October to June. From 1 October to the onset of spawning, it was possible to assess the gonado-somatic index (I G ) of females in terms of time expressed as a sum of degree-days. The correlation between I G and the sum of degree-days was þ0Á97. The onset of the roach spawning period in Lake Geneva was triggered by a thermal threshold (median and range 190 AE 10 degree-days for the 15 previous days). From October to April, climate warming accelerated the development of gonads, then in May, a thermal threshold that triggered the onset of roach spawning occurred earlier.

Development of a broodstock of Arctic charr was undertaken in experimcntal tanks supplied with water pumped from Lake Léman (Geneva) at a depth of 36 metres. Spawners werc subjected to different thermal rcgimes to determine thc cffects of temperature on spawning time and gamctc viability. In the tanks, thc ovulation occurred spontancously at the same time as the wild fish in the lake. The timing of ovulation was slowed down at 8°C and abovc. At 1 IoC, ovulation was inhibited. When the water temperature of the rearing tanks was higher than 5°C for several weeks prior to spawning, the quality of eggs produced by reared femalcs was poorer than those of wild fish because the process of overripening was very rapid above 5°C. Provided that spawners were reared at 5°C and examined twicc a week to detect ovulation, 80 to 90 percent survival to the eyed stage could bc expccted for eggs of reared fcmales. The fertilization rate of eggs produced by spawncrs transferred from 8 to 5°C in December was high (78%). When the transfer occurred in January, the viability of ova tended to dccrcase (63%).

The effects of water temperature on the timing of spawning of perch Perca fluviatilis in Lake Geneva were assessed in a 20 year survey (1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003). Spawning started at the end of April or at the beginning of May, when the water temperature rose to >10°C, but most of the spawns were laid when water temperature exceeded 12°C. This led to a positive correlation between the date of the mid-spawning period and the date when the water temperature rose to >12°C. Large perch had a tendency to spawn later than the small ones, which led to a positive correlation between the date of mid-spawning period and the yearly mean width of perch egg-ribbons. The coefficient of multiple determination between the date of mid-spawning period and the mean width of the ribbon of spawn, combined with the date when the water temperature rose to >12°C was r 2 ¼ 0Á77. The perch generally chose deeper spawning places (12 m) at the end of the spawning period than at the beginning (4 m), which was related to water temperature.

Résumé -La connaissance des conditions dans lesquelles se déroule la fraie des poissons lacustres constitue le point de départ pour l'aménagement et la protection des frayères naturelles et pour le choix des espèces susceptibles d'être acclimatées dans les lacs et les réservoirs.Parmi les principaux facteurs influençant le succds de la fraie, les points suivants sont abordés :-date de fraie, -régimes thermiques permettant le déclenchement de la fraie et le bon dérou-lement du développement embryonnaire, ainsi que la gamme de degrés-jours de cette étape;-substrats de ponte: la plasticité ou les exigences des espèces vis-à-vis des matériaux constituant les frayères sont discutés; -profondeur des frayères pour chaque espèce et plasticité de certaines espèces vis-à-vis de ce paramètre.Ces informations concernent les espèces suivantes:-salmonidés (corégone et omble chevalier),-percidés ( perche fluviatilis et sandre),-brochet, -cyprinidés (plus particulièrement le gardon).L'existence de races géographiques et de sous-populations présentant des habitudes de fraie très différentes vis-à-vis de l'environnement (date de ponte, type de substrat ou profondeur), ainsi que l'existence de <> pour les aires de fraie, apparaissent comme des phénomènes assez répandus chez les poissons lacustres.Summary -Knowledge of the conditions in which the spawning of lake fishes occurs is the departure point in the management and protection of natural spawning grounds and in the choice of species which can be acclimated to lakes and reservoirs.Some of the main environmental factors affecting successful spawning have been discussed : -spawning date, 118C. Gillet -temperature regimes permiîting the onset of spawning and good embryonic development as well as the degrees-day range for this stage, -spawning substrates: species flexibility of demands in the rnatter of spawning grounds, -depth of the spawning grounds for each species and the flexibility of sorne species in this regard.These data concern the following species :-salmonids (coregone and charr), -percides (river perch, pike perch), -pike -cyprinids particularly roach. The existence of geographic breeds and sub-populations having very different spawning habits in relation with the environrnent (spawning date, type of substrate or depth), as well as a sense of .choming-in regard to the spawning grounds, seem to be rather common phenomena among lake fishes.

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