Population dynamics of the northern sea urchin Mesocentrotus nudus in Southwestern Hokkaido, Sea of Japan: Dinamika populasi bulu babi Mesocentrotus nudus di Hokkaido Barat Daya, Laut Jepang

Tadashi Kawai; Hideki Akino

doi:10.29244/HAJ.7.2.69

Authors

Tadashi Kawai Hokkaido Reseach Organization, Central Fisheries Research Institute, 238 Hamanaka, Yoichi, Hokkaido 046-8555, Japan
Hideki Akino Hokkaido Research Organization, Hokkaido Research Plaza, 11-chome, Kita 19-jo Nishi, Kita-ku, Sapporo 060-0819, Japan

DOI:

https://doi.org/10.29244/HAJ.7.2.69

Keywords:

age, density, growth, recruit, sea urchin, yearly fluctuation

Abstract

Kelp bed degradation due to environmental changes and overgrazing by herbivores such as sea urchins has raised concerns about the sustainability of coastal ecosystems and fisheries in northern Japan. Over a 35-year period (1985–2020), population dynamics of the northern sea urchin Mesocentrotus nudus were monitored in Shimamaki Village, southwestern Hokkaido, focusing on age-specific density fluctuations. First-year juvenile recruitment varied substantially, while adult density showed smaller fluctuations but a long-term declining trend. No significant correlation was found between juvenile recruitment and adult density, or between adult density and catch volume. Field data were collected through annual scuba surveys using 1 m² quadrats, and age was determined by test diameter and genital plate rings. Statistical relationships were assessed using Spearman’s rank correlation. The weak correlation suggests that post-recruit mortality, food limitation due to kelp loss, and harvesting pressure may decouple recruitment from adult population size. These findings imply that sustainable sea urchin fisheries require more than just enhancing recruitment; ecosystem-based management including macroalgal restoration is essential. Understanding the role of Isoyake in reducing kelp availability provides critical insight into population stability and fishery yields in northern Japan.

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