3.1 Sea urchin population monitoring
In February 2018, we detected another large mass mortality event affecting the Eastern Atlantic archipelagos (Figure 2a). This time, the mortality event was first detected on the eastern part of the archipelago (Canary Islands), and a few weeks later it spread to Lanzarote, Porto Santo and Madeira (Claudia Ribeiro and João Canning Clode, personal communication) (see Figure 1 for dates), and extending during spring and summer months (Gizzi et al. 2020). On Tenerife, there was an overall average population reduction of 93.2% compared with pre-mortality densities (before 2016 vs after 2018; F= 183.81, p<0.01; Figure 2a). On La Palma island, there was an overall population reduction of 93.1% (before 2016 vs after 2018; F=55.30, p<0.01). It is important to note that populations on La Palma never recovered after the first sea urchin mortality event; densities after the event always remained lower than before the first mortality event. For the Madeira archipelago, a rapid recovery after the mortality have been described (Gizzi et al. 2020).