The population dynamics of Ophidiaster ophidianus (Echinodermata: Asteroidea) in the Azores, at the north-western periphery of its distribution

The warm water Ophidiaster ophidianus has a protected status in the Mediterranean Sea and also occurs in the shallow subtidal of the Azorean Archipelago. From April 2007 to August 2009, the population structure and seasonal abundance of O. ophidianus was studied at different depths within a marine conservation area offshore from São Miguel (SAC: PTMIG0020, Natura 2000). Ophidiaster ophidianus showed a marked seasonal trend in abundance, with higher numbers being recorded from 5 m depth during spring and autumn, and higher numbers at 10 m depth in summer and winter. Densities ranged from 9 to 16 individuals per 100 m2 over the sampling period. The size–frequency distribution of O. ophidianus was uni-modal for each sampling month. Small individuals dominated at 5 m depth, intermediate-sized individuals at 10 m and larger individuals were more frequent at 15 m. The present study establishes population parameters for O. ophidianus and provides the necessary information for management purposes at the north-western periphery of its range in the Azores.