TEMPORAL FLUCTUATIONS OF DOLIOLID ABUNDANCE IN THE BAY OF VILLEFRANCHE-SUR-MER (NORTHWESTERN MEDITERRANEAN SEA) FROM 1967 TO 1990

This study describes the temporal fluctuations of doliolid populations sampled in the Bay of Villefranche-sur-Mer (Northwestern Mediterranea n Sea) from 1967 to 1990. The abundance level of doliolids (strongly d ominated by Doliolum nationalis and Dolioletta gegenbauri phorozooids) was estimated weekly according to Frontier's (1969) ordinal scale. Po pulation abundance shows important interannual variations. The series displays a clear upward trend from the late 1960s to the early 1980s; thereafter, the abundances remain at a high and steady level. Five ann ual patterns of abundance level changes are identified, using cluster analysis. The influence of hydrological and meteorological conditions on doliolid abundance is tested using a Markov regression model for or dinal ecological time series (Menard et al., 1993). A reduction of sal inity in the warm surface water due to the establishment of a haloclin e during early summer or autumn creates the main hydroclimatic conditi ons favourable to the development of the populations in question. Thes e conditions mainly occurr during the oligotrophic period of the year, when the regenerated production system is the governing process.