Restoration Objective:

Here, we review three SUR experiments, which were conducted in SW, NW and NE Hokkaido in Japan over the last three decades, to determine the effects of SUR on macroalgal biodiversity and biomass. We also investigated the environmental characteristics enabling the coexistence of macroalgae and sea urchins through experiments involving the manipulation of sea urchin density. Finally, we discuss the indicators that could be valuable for managing coastal macroalgal forests based on observed environmental characteristics.

Site Selection Criteria:

The sites in this study were located in the southwest (SW; Kaminokuni), northwest (NW; Rishirifuji) and northeast (NE; Esashi) of Hokkaido, Japan, which are all in the subarctic zone and cold climate regime. The two sites face the Sea of Japan, and the one additional site faces the Sea of Okhotsk (Fig. 1). Regarding oceanographic conditions, the Tsushima warm current (TSC) flows northwards along the coast of western Hokkaido facing the Sea of Japan. The current divides into two; the Tsugaru warm current (TGC), which flows through the Tsugaru Strait to the Pacific Ocean, and the Soya warm current (SC), which flows through the Soya Strait to the Sea of Okhotsk (Fig. 1). The northeastern side of Hokkaido, facing the Sea of Okhotsk, is affected by the southward East Sakhalin cold current (ESC). The eastern side of Hokkaido, facing the Pacific Ocean, is affected by the Oyashio cold current (Kurile current) (OC) year-round. Sea urchin barrens primarily occur on the coasts influenced by the Tsugaru warm current (TSC) and the Tsugaru warm current (TGC) (Fig. 1). Submerged landscapes in the barren area are usually recognized by the overall disappearance of macroalgal communities, especially canopy species such as Saccharina japonica var. religiosa, Undaria pinnatifida and members of the Sargassaceae family. Crustose coralline algae and sea urchins dominate the subtidal barren area.

Cause Of Decline:

Crustose coralline algae and sea urchins dominate the subtidal barren area. The primary persistent cause of this phenomenon is overgrazing by herbivorous invertebrates (Agatsuma, Citation1995). In addition, the disappearance of canopy-forming macroalgae was thought to be maintained by oligotrophic oceanic conditions and low productivity. However, barrens are not observed in eastern Hokkaido, which are greatly affected by cold currents with high nutrient concentrations and have large communities of S. angustata and S. longissima.

Key Reasons For Decline:

Overgrazing

Scientific Paper

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

Akaike, S., & Mizuta, H. (2024).

https://www.tandfonline.com/d...0.1080/09670262.2024.2306397

Organisation:

Abashiri Fisheries Research Institute, Hokkaido Research Organization

Action Summary:

Sea urchins were repeatedly removed from 5600m2 by scuba divers by hand and were transferred away from the experimental area. 2000–120 000 sea urchins were removed from the experimental area annually.

Lessons Learned:

Macroalgae in nutrient-rich environments were more resilient to sea urchin grazing and persisted at higher urchin biomass, while nutrient-poor conditions shifted to barrens at lower urchin densities, indicating that barrens formation is driven by both bottom-up environmental conditions and top-down grazing pressure. Sea urchin removal also improved gonad development.

Project Outcomes:

In conclusion, the SUR experiments we conducted to elucidate the optimal balance of biomass between macroalgae and sea urchins in barren sites restored macroalgal biomass at all sites and increased macroalgal diversity. Notably, the recovery of macroalgal biomass was characterized by increased kelp biomass. Based on the negative correlation between macroalgal biomass and sea urchin density, the sea urchin density was set as ca. 0.1 kg m−2 in the reproductive growth phase and 0.15 kg m−2 in the vegetative growth phase of kelps, regardless of the experimental site, which provided an appropriate balance between sea urchin density and macroalgal biomass. The ability to support the maximum sea urchin density was greatly influenced by oceanographic conditions. Given the significantly positive relationship between macroalgal biomass and sea urchin GSI, the macroalgal biomass supporting sea urchins with commercial value (GSI ≥ 15%) was estimated to be 1.6 kg m−2. The above values are useful indicators of conditions under which commercially valuable macroalgae and sea urchins can successfully coexist.

Area of Restoration (Ha)

0.56

Indicator Data:

Indicator:	Ending Value:	Starting Value:
Wet Weight Kelp	2428.3400 g WW/m²	45.6000 g WW/m²
Animal biomass	49.0200 g WW/m²	132.2500 g WW/m²
Species richness	44.0000	–

Abashiri Fisheries Research Institute, Hokkaido Research Organization - Harauta

Abashiri Fisheries Research Institute, Hokkaido Research Organization - Harauta

Restoration Project

Abashiri Fisheries Research Institute, Hokkaido Research Organization - Harauta

Restoration Objective:

Site Selection Criteria:

Cause Of Decline:

Overgrazing

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

Organisation:

Abashiri Fisheries Research Institute, Hokkaido Research Organization

Site Observations:

1st Sep 2010 – 1st May 2013

Action Summary:

Lessons Learned:

Project Outcomes:

0.56

Indicator Data:

Indicator:

Ending Value:

Starting Value:

Wet Weight Kelp

Animal biomass

Species richness

7. Gains Cumulative Value When Applied At Large Scales