Reduced biomass of the kelp Saccharina japonica cumulatively affects gonad production of sea urchins over ensuing years
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23RD INTERNATIONAL SEAWEED SYMPOSIUM, JEJU
Reduced biomass of the kelp Saccharina japonica cumulatively affects gonad production of sea urchins over ensuing years off northeastern Japan Kousuke Yatsuya 1 & Yukio Matsumoto 1 & Kei Sasaki 1 & Norio Shirafuji 1 & Daisuke Muraoka 1 Received: 29 June 2019 / Revised and accepted: 29 December 2019 # Springer Nature B.V. 2020
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. Keywords Annual variation . Kelp . Mesocentrotus nudus . Saccharina japonica . Sea urchin
Introduction Saccharina japonica (Areschoug) Lane et al. growing along the northeastern coast of Honshu, Japan, shows an annual life span and high productivity (Musashi et al. 1993; Yatsuya et al. 2017). The kelp germinates in early spring when water temperature is lowest in a year and withers during autumn and winter. The maximum biomass of S. japonica was reported at 3773 g dry weight m−2 (Yatsuya et al. 2017). Seaweed beds of S. japonica contribute to the abundant fishery landings of herbivores such as sea urchins and abalones (Ministry of Agriculture Forestry and Fisheries of Japan 2019). However, the biomass of the kelp in this region is highly variable each year. Fujiwara (2008) showed that the biomass ranged from 0 kg to more than 14 kg fresh weight m−2. The variability is related to the difference in environmental factors
* Kousuke Yatsuya [email protected] 1
Tohoku National Fisheries Research Institute, Fisheries Research and Education Agency, Miyako, Iwate 027-0097, Japan
such as water temperature and nutrient concentrations during the period of kelp germination (Kirihara et al. 2003; Yatsuya et al. 2017). Cold water temperature might keep herbivores from grazing on gametophytes and juvenile kelps (Machiguchi 1993). Off northeastern Japan where the Oyashio cold current and the Kuroshio warm current meet, water temperatures especially during la
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