Grazing pressure of sea urchin Mesocentrotus nudus estimated from seawater temperature and wave height, and its effects on the early growth of annual kelp Saccharina japonica in northeastern Japan

Grazing by the sea urchin Mesocentrotus nudus plays a vital role in the development of marine forests of annual kelp, Saccharina japonica, along the Pacific coast of northeastern Japan. The grazing rate of sea urchins, which are rigorously temperature dependent, affects the chance of germination/survival of the kelp during winter/spring when seawater temperatures in this region are highly variable among years. It has previously been found that if the mean seawater temperature between February and March remained below 5.5 °C, kelp forests in this region would establish later in the year. However, the mechanism leading to this success or failure of kelp forests to develop has yet to be investigated in detail. Laboratory experiments confirmed that the grazing rate and movement of the sea urchin decreased as seawater temperature declined from 12 °C to 3 °C. Field observations, using interval cameras from autumn to spring, revealed that the density of sea urchins on artificial reefs decreased at lower seawater temperatures and/or higher significant wave heights (SWH). Using these relationships, the grazing pressure by sea urchins (= grazing rate × density) in the site was estimated based on seawater temperature and SWH. Comparing the grazing pressures among past years with different states of kelp forests, conditions needed to establish kelp forests were revealed. Extremely low grazing pressure (< 0.5 g m−2 day−1) lasted for a total of 31 days in 2015 when kelp grew densely, while this condition occurred on 11 and 14 days respectively in 2019 and 2022 when kelp was sparse. Kelp did not grow in 2016 when the grazing pressure continuously remained above 0.5 g m−2 day−1. At this study site, such combinations of seawater temperature and SWH as 4 °C and 1.0 m, 6 °C and 2.3 m, and 8 °C and 3.7 m would equate to a grazing pressure of 0.5 g m−2 day−1. To develop a dense kelp forest, grazing pressure was required to be lower than 0.5 g m−2 day−1 for about a month as in 2015. Though these figures should be adjusted, a model and scheme of this study will be applied to the other areas where kelp forest restoration/management plans are required. •Grazing of sea urchins was quantified by lab experiments and field observations•Grazing pressure decreased at lower temperatures and/or higher wave heights•One month of lower grazing on juvenile kelp was needed to establish dense kelp forest