Reduced biomass of the kelp Saccharina japonica cumulatively affects gonad production of sea urchins over ensuing years off northeastern Japan

Abstract

The annual kelp Saccharina japonica grows along the northeastern coast of Japan, making the coast highly productive for the fishery for herbivores such as sea urchins and abalones. The biomass of the kelp along this coast drastically varies each year due to variable environmental factors, i.e., seawater temperature and nutrient concentrations, during the period of kelp germination. We carried out a field investigation to elucidate how the gonad production fluctuates in an environment where the kelp production varies each year. Seaweed biomass and gonad weight of the sea urchin Mesocentrotus nudus were examined monthly from March 2015 through December 2018. The kelp was abundant and the gonad index (= gonad ∕ body weight × 100) of the sea urchins reached more than 20% in 2015. However, in the following 3 years (2016 to 2018), the kelp was nearly absent from the area and the maximum gonad index was significantly less than that in the kelp-abundant year (2015). During the kelp-scarce 3 years, the gonad index showed a significant decrease from one kelp-scarce year to another, indicating that the effect of a poor algal growth on the gonad production of sea urchins was cumulative over the following years.