Optimizing the biomass balance of macroalgae and sea urchins in kelp beds by removing the urchins

ABSTRACT

In this study, we conducted sea urchin removal (SUR) experiments at two sites along Hokkaido’s southwestern and northwestern coasts in the Sea of Japan and one site along the northeast coast in the Sea of Okhotsk, to elucidate the biomass balance between macroalgae and sea urchins at different urchin barren sites. The experiments resulted in the restoration of macroalgal biomass at all sites and an increase in macroalgal diversity, indicating that it was the impact of high grazing pressure reducing the macroalgae, regardless of the particular experimental site. Notably, the recovery of the macroalgal biomass was characterized by increased kelp biomass, including Saccharina japonica var. religiosa and S. japonica var. ochotensis. In addition, negative correlations were observed between macroalgal biomass and sea urchin density. The lowest sea urchin density inducing an increase of macroalgal biomass was approximately the same in the reproductive growth phase (0.1 kg m⁻²) and in the vegetative growth phase (0.15 kg m⁻²) of Saccharina spp., the dominant kelps, regardless of the experimental site. These values can be helpful indicators for managing sea urchin density in order to restore macroalgae. In contrast, the maximum sea urchin density inducing the lowest macroalgal biomass varied markedly depending on the study site, indicating that different levels of kelp productivity are influenced by oceanographic conditions. Moreover, a strong positive correlation was observed between macroalgal biomass and sea urchins’ gonadosomatic index (GSI). Based on this correlation, macroalgal biomass supporting sea urchins with commercial value (GSI ≥ 15%) was estimated to be 1.6 kg m⁻². This sea urchin density value is also a helpful indicator for managing macroalgal biomass and commercially valuable sea urchins. Moreover, this value will depend on the influence of the oceanographic environment of the site, particularly the nutrient conditions, on macroalgal productivity. In addition, we have also discussed how macroalgae and sea urchins can best coexist at the experimental sites.

Highlights

• Sea urchin removal (SUR) restored macroalgal biomass and diversity.

• Recovery of macroalgal biomass by SUR was characterized by kelps.

• Kelp biomass required to support the commercial sea urchin production was estimated.

KEYWORDS:

• Ecosystem balance
• gonadosomatic index
• kelp bed
• oceanographic condition
• sea urchin barren
• sea urchin removal

Acknowledgements

We are very grateful to the Hiyama, Oshidomari, Kutsugata, and Esashi Fishermen’s Cooperative Association staff, the Kaminokuni and Rishirifuji town office staff, the Hiyama-nanbu, Rishiri, and Esashi Fisheries Technical Guidance Office, and the Hokkaido Prefectural Government. We are also grateful to the following colleagues of the Hokkaido Research Organization: K. Kikuchi, H. Monma, Y. Nozawa, S. Kiyokawa, Y. Kuwahara, T. Kuribayashi, H. Asami, H. Okumura and H. Yoshida. A part of the compilation work was supported by the ‘Regional University and Local Industrial Vitalization Grant Project’(Hakodate City) adopted by the Cabinet Office, Government of Japan in March 2022.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary information

The following supplementary material is accessible via the Supplementary Content tab on the article’s online page at https://doi.org/10.1080/09670262.2024.2306397

Supplementary table S1. Sampling date, sites and number of quadrats (N) in each experimental plot. Macroalgae and sea urchins were collected from 0.25 m2 or 1 m2 quadrats, respectively. The sites are in the southwestern (SW, Kaminokuni), northwestern (NW, Rishirifuji), and northeastern (NE, Esashi) parts of Hokkaido.

Author contributions

S. Akaike: original concept, fieldwork, performed measurements and analysis and manuscript editing; H. Mizuta: original concept, drafting and manuscript editing.

Additional information

Funding

Part of the compilation work was supported by the ‘Regional University and Local Industrial Vitalization Grant Project’ (Hakodate City) adopted by the cabinet office (Japan) in March 2022.