Sea Urchin Removal and Effectiveness Evaluation from the Long-Term Monitoring Data in Dokdo, Korea

Dokdo is located 215 km away from the Korean Peninsula and is not affected by anthropogenic impacts. In this area, barren grounds developed between the late 1990s and early 2000s. The Korean government has measured the average urchin density from 1 to 10 sampling stations around Dokdo during 2006–2020 and has been conducting urchin removal projects annually since 2015 all around the bottom of Dokdo within depths of 5–20 m. Here, we implemented an additive decomposition method for time series analysis of the collected data. The timeline was divided into two periods: sample data period for predicting the urchin density (2006–2016) and intervention period (2016–2020). A trend model was estimated from the de-seasonalized data of the prediction period. Subsequently, forecasts were compared with the corresponding observations of the intervention period. While the forecasts reflected the spread of barren grounds and sea urchins, the actual urchin density during the intervention period decreased. On comparing the macroalgal coverage between the springs of 2017 and 2020, the samples with coverage less than 20% decreased from 50% in 2017 to 0% in 2020, while those with 21–40% coverage increased from 30% in 2017 to 90% in 2020. These changes confirmed the effect of urchin removal, and, based on this and other studies, an effective threshold of urchin density/biomass of two individuals and 140–150 g/m 2 was suggested as the target criterion. The study proved the urchin removal as an effective solution to recover macroalgal beds in Dokdo.