To cite this article: Ruber Rodríguez-Barreras, Alain Durán, Julián Lopéz-Morell & Alberto M. Sabat (2015): Effect of fish removal on the abundance and size structure of the sea urchin Diadema antillarum: A field experiment, Marine Biology Research, Abstract Three decades after its population collapse, the sea urchin Diadema antillarum has yet to return to pre-mortality densities in most Caribbean localities. Circumstantial evidence suggests high post-settlement mortality due to predation as a potential mechanism to explain the lack of, or the slow recovery of this sea urchin. In this study we experimentally reduced the densities of two reef fish, Thalassoma bifasciatum and Halichoeres bivittatus, at one site for one year and measured the effect of their removal in the abundances and size structures of D. antillarum in relation to that of a comparable control site. We hypothesized a significant increase in urchin abundance and a reduction in mean test diameter at the experimental site with respect to the control site after the reduction in fish abundance. Overall, the density of D. antillarum did not vary significantly between control and experimental sites for the different census periods. Fish removal had a significant effect on the size structure of D. antillarum and in the mean density of the medium size classes. Results suggest that removal of these two labrids can significantly increase the relative abundance of medium-sized sea urchins and modify its population structure. Efforts to recover D. antillarum populations would be strengthened if densities of T. bifasciatum and H. bivittatus remain low.
A reliable and harmless mark–recapture method provides valuable information for the management of commercial sea cucumber species. Nevertheless, marking and tracking sea cucumbers is notoriously difficult and represents a serious challenge. In this study, we tested one external and one internal tag in the sea cucumber Holothuria grisea. A passive integrated transponder (PIT) tag was inserted into the coelomic cavity in one treatment, whereas a T-bar (external tag) was attached in the upper surface of the body wall in the other treatment; sea cucumbers were then followed for 17 weeks. The tagging procedure caused no evisceration in the experimental groups, nor was a significant difference in growth rate found between treatments. The retention of the PIT tag was low, with 100% lost by Week 9. Retention of T-bars was higher, with 90% still attached by the end of the Week 8, but retention decreased thereafter as T-bar absorption increased. No relationship was found between initial weight and the number of weeks PIT tags (r=–0.173, P=0.781) or T-bars (r=–0.220, P=0.652) were retained. Neither the T-bar nor the PIT tags fulfilled the requirements of high retention required for long-term studies. However, we do recommend the use of T-bars for short-term studies for H. grisea under laboratory conditions or in a habitat with low substrate complexity.
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