Restoration Project

Chinese University of Hong Kong - Tung Ping Chau Marine Park

Restoration Objective:

In order to prove the cause–effect relationship between sea urchin density and formation of barren ground, a caging experiment was carried out to manipulate sea urchin density in order to evaluate the grazing pressure of this sea urchin on S. siliquastrum, the main subtidal canopy species. Furthermore, the effectiveness of transplantation strategies using adult Sargassum thalli or Sargassum recruits on ceramic tiles to facilitate the recovery of Sargassum beds was also investigated.

Site Selection Criteria:

This site experiences strong wave action, especially during the summer monsoon.

Cause Of Decline:

From 2006, the Sargassum bed in Lung Lok Shui (LLS) in Hong Kong started to diminish. This coincided with a bloom in density of short-spined sea urchin Anthocidaris crassispina, one of the major grazers of Sargassum spp. in Hong Kong. An urchin barren ground was ultimately formed in LLS in 2008, and various Sargassum spp. including S. siliquastrum were no longer found since then, although patches of S. hemiphyllum individuals could still be found in the lower intertidal and shallow subtidal.

Key Reasons For Decline:

Overgrazing

Scientific Paper

Assessing the potential for recovery of a Sargassum siliquastrum community in Hong Kong

Y.H. Leung, C.W. Yeung, P.O. Ang, , Journal of Applied Phycology, Vol. 26.https://doi.org/10.1007/s10811-013-0097-1

Organisation:

Chinese University of Hong Kong

Site Observations:

Observation Date

15th Feb 2012 – 22nd Mar 2012

Action Summary:

Reproductive S. siliquastrum thalli were collected from in mid-February 2012 and cultured in an outdoor flowthrough culture tank in the laboratory. 35 ceramic tiles were placed below the plants to allow S. siliquastrum germlings to settle. After 10 weeks, the number of established recruits on each tile was counted. Twenty-five tiles with recruits in their best condition were chosen, transferred to the field, and mounted in a cage in Lung Lok Shui. The tiles were collected back and the number of surviving recruits on each tile was counted. This experiment was carried twice, once in 2012 and repeated in 2013.

Lessons Learned:

Allowing sea urchin harvest in the marine park and the surrounding areas may be the only option left to restore the Sargassum community in LLS. Additional restoration strategies using transplantation of adult plants may need to be put in place to serve as the seed stock.

Project Outcomes:

The results of this study showed that the transplanted Sargassum thalli could not survive outside exclusion cages, whereas induced recruits were almost wiped out in all tiles even inside exclusion cages. This provides evidence that the sea urchin A. crassispina could have overgrazed S. siliquastrum thalli and decimated the Sargassum canopy in Lung Lok Shui. It also suggested that other than A. crassispina, other grazers also contributed to the removal of S. siliquastrum recruits. Therefore, natural recovery of Sargassum bed seems unlikely if the grazing pressure remains high in this site. Proactive management strategies may therefore need to be adapted to restore this Sargassum bed.

Key Reasons For Decline:

Overgrazing

Area of Restoration (In Square Metres)

0.7

Indicator Data:

Indicator:

Ending Value:

Starting Value:

Percent Survival

5.782
%
100
%
Costings:
Cost Currency:USD

Observation Date

15th Feb 2012 – 1st May 2012

Action Summary:

170 individuals of adult S. siliquastrum were collected in January 2012 (for the first experiment) and in February 2012 (for the second experiment). They were first maintained in an outdoor flow-through culture tank. In the first experiment, all collected thalli were trimmed to 100mm in length. Six thalli were placed in each replicate of each cage setup, with a total 150 thalli transplanted into the field. For the second experiment, all thalli of Sargassum were similarly trimmed to 100 mm in length, but at the same time, the biomass of each thallus was also trimmed down to 6 g (wet weight). The trimmed thalli in both experiments were tied on nylon ropes and then fixed inside each cage.

Lessons Learned:

Allowing sea urchin harvest in the marine park and the surrounding areas may be the only option left to restore the Sargassum community in LLS. Additional restoration strategies using transplantation of adult plants may need to be put in place to serve as the seed stock.

Project Outcomes:

The results of this study showed that the transplanted Sargassum thalli could not survive outside exclusion cages, whereas induced recruits were almost wiped out in all tiles even inside exclusion cages. This provides evidence that the sea urchin A. crassispina could have overgrazed S. siliquastrum thalli and decimated the Sargassum canopy in Lung Lok Shui. It also suggested that other than A. crassispina, other grazers also contributed to the removal of S. siliquastrum recruits. Therefore, natural recovery of Sargassum bed seems unlikely if the grazing pressure remains high in this site. Proactive management strategies may therefore need to be adapted to restore this Sargassum bed.

Key Reasons For Decline:

Overgrazing

Area of Restoration (In Square Metres)

0.5

Indicator Data:

Indicator:

Ending Value:

Starting Value:

Wet Weight Kelp

85.86
g WW/m2
100
g WW/m2
Costings:
Cost Currency:USD

Observation Date

15th Feb 2012 – 22nd Mar 2012

Action Summary:

Reproductive S. siliquastrum thalli were collected from in mid-February 2012 and cultured in an outdoor flowthrough culture tank in the laboratory. 35 ceramic tiles were placed below the plants to allow S. siliquastrum germlings to settle. After 10 weeks, the number of established recruits on each tile was counted. Twenty-five tiles with recruits in their best condition were chosen, transferred to the field, and mounted in a cage in Lung Lok Shui. The tiles were collected back and the number of surviving recruits on each tile was counted. This experiment was carried twice, once in 2012 and repeated in 2013.

Lessons Learned:

Allowing sea urchin harvest in the marine park and the surrounding areas may be the only option left to restore the Sargassum community in LLS. Additional restoration strategies using transplantation of adult plants may need to be put in place to serve as the seed stock.

Project Outcomes:

The results of this study showed that the transplanted Sargassum thalli could not survive outside exclusion cages, whereas induced recruits were almost wiped out in all tiles even inside exclusion cages. This provides evidence that the sea urchin A. crassispina could have overgrazed S. siliquastrum thalli and decimated the Sargassum canopy in Lung Lok Shui. It also suggested that other than A. crassispina, other grazers also contributed to the removal of S. siliquastrum recruits. Therefore, natural recovery of Sargassum bed seems unlikely if the grazing pressure remains high in this site. Proactive management strategies may therefore need to be adapted to restore this Sargassum bed.

Key Reasons For Decline:

Overgrazing

Area of Restoration (In Square Metres)

0.7

Indicator Data:

Indicator:

Ending Value:

Starting Value:

Percent Survival

5.7439
%
100
%
Costings:
Cost Currency:USD

Observation Date

15th Feb 2012 – 1st May 2012

Action Summary:

170 individuals of adult S. siliquastrum were collected in January 2012 (for the first experiment) and in February 2012 (for the second experiment). They were first maintained in an outdoor flow-through culture tank. In the first experiment, all collected thalli were trimmed to 100mm in length. Six thalli were placed in each replicate of each cage setup, with a total 150 thalli transplanted into the field. For the second experiment, all thalli of Sargassum were similarly trimmed to 100 mm in length, but at the same time, the biomass of each thallus was also trimmed down to 6 g (wet weight). The trimmed thalli in both experiments were tied on nylon ropes and then fixed inside each cage.

Lessons Learned:

Allowing sea urchin harvest in the marine park and the surrounding areas may be the only option left to restore the Sargassum community in LLS. Additional restoration strategies using transplantation of adult plants may need to be put in place to serve as the seed stock.

Project Outcomes:

The results of this study showed that the transplanted Sargassum thalli could not survive outside exclusion cages, whereas induced recruits were almost wiped out in all tiles even inside exclusion cages. This provides evidence that the sea urchin A. crassispina could have overgrazed S. siliquastrum thalli and decimated the Sargassum canopy in Lung Lok Shui. It also suggested that other than A. crassispina, other grazers also contributed to the removal of S. siliquastrum recruits. Therefore, natural recovery of Sargassum bed seems unlikely if the grazing pressure remains high in this site. Proactive management strategies may therefore need to be adapted to restore this Sargassum bed.

Key Reasons For Decline:

Overgrazing

Area of Restoration (In Square Metres)

0.5

Indicator Data:

Indicator:

Ending Value:

Starting Value:

Wet Weight Kelp

96.2963
g WW/m2
1
g WW/m2
Costings:
Cost Currency:USD