2.1 Sea urchin population monitoring
Diadema africanum densities have been intermittently monitored at Abades, Boca Cangrejo and Teno sites, on Tenerife Island, since 2001; and at Pta. Fraile, Playa del Pozo, Pta. Fuencaliente and La Bombilla, on La Palma, since 2008 (Figure 1). Sea urchins were sampled using 10 × 2-meter transect lines laid randomly over the rocky substrate and parallel to the coastline. A total of 8–10 replicates per site were used to estimate average densities. The sites were characterized by shallow rocky bottoms (0–20 m) dominated by sea urchin barrens (Hernández et al. 2008). During the mortality episodes, the densities of sea urchins dropped abruptly at all of the sites. In order to quantify the decline in sea urchin densities, we focused on the years 2008–2010 and 2016–2018 (Figure 2a). We selected these years because we had urchin density data for all of the seven sites cited above, which allowed us to perform a more robust before-after comparison. In parallel, all the divers and colleague’s die-off observations were registered, considering the site and the orientation of the island where it was observed (Figure 1). It is important to notice that the densities estimations, during the Winter Storms years, were done after the first die-off observations were detected.