A case study of stock evaluation on littoral hard substrata: echinoid populations on the north-east coast of Spain  <B>A case study of stock evaluation on littoral hard substrata: echinoid populations on the north-east coast of Spain</B><P><B>Xavier Turon, Cruz Palacín, Manuel Ballesteros & Lluis Dantart</B><I><P> Dept. de Biologia Animal </I>(<I>Invertebrats</I>),<I> Facultat de Biologia,<BR> Avgda. Diagonal, 645, E-08028 Barcelona, Spain</I><B><P> Abstract</B><BR> When ascertaining the stocks of any benthic littoral population on hard substrata, both the mean density of the species and the total surface area available must be estimated with known confidence intervals. Density estimates are hindered by the aggregated distribution featured by most species and the fact that only a limited set of samples is usually available. Surface measurements must take into account a scale effect depending on the heterogeneity of the substratum and the sampling unit adopted.<BR> Both problems are addressed in this work in the context of a study on the populations of the edible sea-urchin <I>Paracentrotus lividus</I> on a rocky shore of the north-east Coast of Spain (western Mediterranean) at a depth of between 0 and 10 m. About five-thousand 1-m2 sampling units were deployed along the coast to assess the abundance of the species. Several known methods for estimating confidence intervals in an overdispersed distribution were compared with bootstrapping procedures. The surface available above a depth of 10 m was estimated at the scale of the sampling unit (1 m2) from the fractal properties of the shores being studied. The final confidence interval was set at 95%, and the resulting estimate for the population stock was between 70 and 478 3106 individuals (larger than 2 cm of test diameter).<I><P> Keywords</I>: echinoids, stock assessment, aggregated distribution, bootstrapping, fractal dimension.<P>