Adventures and misadventures monitoring sea urchin barrens - managing sea urchin fisheries to conserve and restore macoralgal forestsHabitat Restoration, Rehabilitation & Enhancement

Jordi Boada¹, Fernando García-González²

¹Centre d’Estudis Avançats de Blanes, Spain - ²Instituto Español de Oceanografía, Spain

The loss of functionality of predation outside protected areas has facilitated the spread of urchin barrens across temperate reefs. A set of managerial actions has been implemented in different regions to prevent or control the expansion of barrens and to restore the lost macroalgal forests. Among the proposed opportunities, the establishment of new sea urchin fisheries seems to be a way of giving the reefs a helping hand. In the Mediterranean, overfishing, climate factors and biological invasions are increasing the distribution of bare rock extensions. Despite the ecological knowledge around barren formation, there is still limited capacity to preserve macroalgal forests. We have monitored barrens through underwater surveys, unmanned aerial vehicles (i.e. georeferenced drone orthomosaics of 2 cm resolution), and citizen science since 2017. Specifically, we have identified the critical thresholds of sea urchin abundance to preserve macroalgal forests and monitored the total barren extension of a ca. 8 km of coastline for 5 years.
Additionally, we simulated the effect of fishing in 3 barren reefs of 200 square meters and monitored the evolution of macroalgal forests (i.e. cover). Finally, we analysed the impact of a sea urchin harvester reporting catch data at a high spatial resolution level for 3 years on the recovery of the macroalgal cover in the studied reefs. In the years of study, we found an emblematic Fucales species remnant in a small reef patch in the region to be back after sea urchins were removed. We also tracked the effect of an unprecedented storm on sea urchin populations and macroalgal forest cover. We found clear thresholds of urchin abundance above which the macroalgal communities cannot be sustained. We also found thresholds of sea urchin catch promoting the recovery of macroalgal cover. Interestingly, our results show how the indicators found can be used to implement sea urchin fishing plans to preserve and restore macroalgal forests. So far, the results have served international projects, have been implemented in regional biodiversity monitoring programs, and used to advise the sea urchin harvesting commission in the region and will be of relevance in the global context of temperate reefs ecology and conservation.