OBSERVATIONS ON THE LONG-TERM POPULATION DYNAMICS OF THE PERENNIAL ASCIDIAN, ASCIDIA MENTULA O. F. MÜLLER, ON THE SWEDISH WEST COAST

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Abstract
Six populations of A. mentula O. F. Mueller on subtidal vertical rock walls were monitored continually for 12 yr (1971-1982) using stereophotographic techniques. Three stations at 2 depth levels were observed, along a hydrographical gradient extending 100 km from the sheltered inner parts of the Gullmarsfjord to the exposed archipelago off the Swedish west coast. Population densities increased during 1971-1976, and gradually declined from 1976. Recruitment was density-dependent; mortality was density-independent. Temporal covariation in recruitment between stations and depths separated fjord stations into 2 independent, correlated patterns as follows: the exposed archipelago station and the shallow semi-sheltered fjord station and from the shallow-sheltered fjord station from the sheltered and deep semi-sheltered fjord station. Temporal covariation in mortality separated the exposed station from the sheltered fjord stations suggesting different mortality factors. Histological analysis of gonads and analysis of photographically monitored recruitment revealed a seasonal reproductive pattern at 15 m depth; continuous reproduction and recruitment was observed in deeper populations. Temporal patterns of population density appeared to be related to longterm hydrographic changes mediated by variation in recruitment. Mortality caused by predation was not observed but disturbance and dislocation by sea urchins was an important mortality factor, especially at exposed sites.