Biotic and abiotic interactions controlling starfish outbreaks in the Bay of Douarnenez, Brittany, France

Massive outbreaks of starfish (Asteridae) occur periodically in European coastal waters. The origins of these irregular population increases are poorly understood. In the subtidal zone of the Bay of Douamenez, Brittany, France, the occurrence of several species of Asteridae and the polymodal size distribution differentiate these aggregations from the single year-class and monospecific intertidal infestations. An echinoderm population survey from 1982 to 1988 showed an increase in carnivorous starfish (principally Asterias rubens and Marthasterias glacialis) from 1981 to 1984, when the population reached carrying capacity. At this time, a trophic imbalance between starfish and bivalves was observed. From 1985 onward, the total echinoderm biomass decreased coincidentally with an increase in the two super predators the starfish, Luidia sarsi and Luidia ciliaris. From 1986 to 1988, the primary carnivorous echinoderm biomass appeared insufficient to meet the food requirements of the Luidia species. The role of the different factors controlling these starfish population changes is discussed. Whereas the hypothesis of regulation by Luidia predation fits the observations, it is also possible that direct or indirect temperature effects are also involved in population changes.