TEMPORAL VARIABILITY IN ABUNDANCE OF THE SEA-URCHINS PARACENTROTUS-LIVIDUS AND ARBACIA-LIXULA IN THE NORTHWESTERN MEDITERRANEAN - COMPARISON BETWEEN A MARINE RESERVE AND AN UNPROTECTED AREA

Sea urchin populations were monitored in the Medes Islands Marine Rese rve (NW Mediterranean) and an adjacent unprotected area in order to (1 ) describe temporal variability in abundance and population size-struc ture of sea urchins within each area (from 1991 to 1997), and (2) to c ompare these areas to investigate the role of fish predation level in determining sea urchin population structure over time (from 1995 to 19 97). Abundance of the sea urchin Paracentrotus Lividus (Lamarck) was m onitored at 7 sites (4 inside and 3 outside the reserve), and 2 distin ct habitats (fields of big boulders and vertical walls). P. Lividus ex hibited significant variability in density over time on boulders, both inside and outside the reserve, whereas its density did not vary sign ificantly on vertical walls. P. lividus populations differed significa ntly in size-structure across sites, varying from unimodal to bimodal among sites and years. Density and mean size of P. lividus were not si gnificantly different between the protected and the unprotected area i n either of the 2 habitats. Abundance of the sea urchin Arbacia lixula (L.) was monitored from 1995 to 1997 on vertical walls and boulders ( 4 sites inside and 3 sites outside the reserve). Density of A. lixula differed significantly over time on boulders, but it did not on walls. On vertical walls, density and mean size of A. lixula were not signif icantly different between areas in both habitats. This study shows tha t in the Medes Islands region, sea urchins exhibit striking short-term fluctuations in abundance, which can lead to misinterpretation of lar ger-scale temporal patterns. The comparison between the protected and the unprotected area does not support the hypothesis of fish predation as the most important factor affecting P. lividus populations in the Medes Islands, as patterns of lower sea urchin density relative to the unprotected area nearby were not maintained over time. These facts in dicate that factors other than fish predation are very important in de termining sea urchin population structure in the northwestern Mediterr anean.