無「蟹」可擊
Protecting Somanniathelphusa Zanklon
Introduction
In 2019, the renovation of the Fanling Golf Course came to a halt as an ecological challenge approached. This is what South China Morning Post have to say about the incident,
One particular animal, the Somanniathelphusa zanklon – a crab native to Hong Kong – is in danger of being wiped out completely from the area if development goes ahead, according to the World Wildlife Fund, Hong Kong.
Somanniathelphusa zanklon
Somanniathelphusa zanklon is an endangered crab species mainly found along rivers along the Northern border, the Northeastern part of Hong Kong and Lantau Island.
Preferring small, low-gradient, slow-flowing tributaries and freshwater marshes as their habitat, the population of S. zanklon has been declining due to the vast ongoing channelisation and development projects in Hong Kong.
The question is, can we protect this crab species by different means? Perhaps modifying a chanalized river course could be an answer.
The Northern District housed the majority of Hong Kong’s S. zanklon population, with Sheung Yue River, Ng Tung River and Ping Yuen River having the most abundant numbers of the crustacean.
It is not at all surprising as New Territories’ rivers are less modified, which contributes to making the Northern District one of the safe havens of the crab.
Yet, signs of human development have been popping out all over the former rural areas, rivers are getting straightened, river beds are becoming smoother…
How can we further modify these channels to keep its ecological value and protect our endangered invertebrate? A channelized section of Sheung Yue River would be analyzed in the comming section, in hopes that S. zanklon would have a greater habitat to live in.
Study Area
After years of development, Sheung Yue River has been heavily channelized.
- Left: Before channelization (1928)
- RIght: After channelization (2022)
- (river highlighted in blue is Sheung Yue River)
The map shows the study area of Sheung Yue River
Source: JCWISE
Objectives
Our study aims to provide insights in protecting S. zanklon
1. Improve current habitats of S. zanklon
- Identify the ecological status of Sheung Yue River and provide suggestion
2. explore and protect potential habitat of S. zanklon
- Identify potential sites of S. zanklon occurrence in the North District
Materials & Methodology
A. Identify ecological status by hydro-morphological assessment
To evaluate whether Sheung Yue River provides an ideal habitat for S. zanklon, we have carried out a hydro-morphological assessment to analyze its ecological status.
4 parameters have been selected for assessment in different river zone:
For channel zone
- Hydrological regime influence
- Longitudinal premeability
For bank zone
- bank management
- bank vegetation
After analysis, each parameter will be given a class out of I to V:
- Class I : High Status (natural)
- Class II : Good Status
- Class III : Moderate Status
- Class IV : Poor Status
- Class V : Bad Status (heavily modified)
The level of class represent the score given in different parameters, e.g. Class I = 1 mark, Class II = 2 marks
Eventually, a weighted hydro-morphological score will be calculated for differerent river sections to find out their ecological status.
Reference of the methodology: (J Novakova et al 2017 IOP Conf. Ser.: Earth Environ. Sci. 92 01204)
Channel Zone
Hydrological Regime Influence
It is the distribution over time of water in a watershed, among precipitation, evaporation, soil moisture, groundwater storage, surface storage, and runoff.
Since drainage channels affect the surface's and sub-surface water along rivers, the intensity of the influence on hydrological regime can be determined by the density of drainage channel in Sheung Yue River.
Source: Drainage Service Department
The density of drainage channel has been summarized within the Sheung Yue River (left) into 5 classes by natural breaks. The classes are then further reclassified into Class I to V based on the hydrological regime influence. (right)
Higher the density of drainage channel, larger the influence on hydrological regime influence, and higher the level of classes.
It has been discovered that the lower and middle course of Sheung Yue River experienced a greater impact on their hydrological regime, while most in the upper course remain natural.
Longitudinal Permeability
It is the ability of allowing biological continuity like the migration of S. zanklon. Yet, weirs have been built along rivers and become migration barriers, which worsen longitudinal permeability.
The map shows the presence of weirs along Sheung Yue River. Most weirs are located at the middle course.
What do these weirs look like?
Weir at Fanling Highway
Source: Google Map
Inflatable weir at Ying Pun
Source: Google Map
We have summarized the number of weirs found in rivers.
The more the weirs are built, the higher the class the river section obtained. Once longitudinal permeability of the river is disrupted, the river can not be assigned with Class I,II,III. (WalterBinder,AlbertGöttle,DuanShuhuai,2015/06/01/)
Regions without weirs are given Class I.
Bank Zone
Bank Management
During river modification, different types of bank lining can be used, as shown in the table (DSD PN No. 3/2021)
With reference to the table, we have classified the types of lining into 5 classes.
- Natural/Unlined : Class I
- Geotextile Reinforced Grass : Class II
- Grassed Cellular Concrete (grasscrete) : Class II
- Gabion/Mattress : Class III
- Masonry : Class IV
- Concrete : Class V
- other types of lining are not studied in this project
Source: Google Earth, JCWISE, DSD PN No. 3/2021
Types of bank lining
The map shows the location of masonry, concrete walls, geotextile reinforced grass, grasscrete, gabions and natural banks along Sheung Yue River.
Grasscrete is paved on the bank of the lower course of Sheung Yue River
Geotextile Reinforced Grass embankment at the lower course
Gabions at the upper course
Although there is vegetation grown, a large piece of concrete banks remain bare
Masonry along streams in upper course
Natural banks in Ta Shek Wu Tsuen
Source: Google Map
Natural Lining
Since most areas at the upper course lack google street view or images to check natural banks and it is not completely reliable to identify natural banks simply from satellite data, we have utilized the layers of vegetation and human activity around Sheung Yue River to find out potential natural banks.
From the map, we have discovered that all the identified natural banks are surrounded by a large piece of vegetation, while some are close to human activites and some are not.
Therefore, the more influential factor of bank modification might be the area of vegetation nearby.
We decided to find potential natural banks that are next to vegetation with an area of at least 0.04 square kilometers, by the function of finding existing location.
overall bank management
- Left: different types of bank lining along Sheung Yue River
- Right : the corresponding classes assigned to the river sections
Bank vegetation
We will determine the amount of bank vegetation by analysizing the riparian zone along Sheung Yue River.
The green area are vegetation around the Sheung Yue River basin.
To find out the whether the river sections have an ideal size of riparian zone, we have created a buffer zone overlaying with vegetation along the Sheung Yue River with a buffer distance of 100 feet, as it is the minimum width neccessary for providing wildlife habitat (buffer guide by University of Wisconsin–Stevens PointStevens Point)
- Left : buffer zone created
- Right : riparian zone within the buffer
The riparian zone along Sheung Yue River is reclassified into the 5 classes of bank vegetation by summarizing the mean of the riparian zone area into the river sections.
After adding up all the weighted scores, the final hydromorphological score(HMS) of different river sections is calculated.
Conclusion
Left: a general identification of upper, middle and lower course for reference
Upper course: the overall ecological status is the best, all studied sections are in good to high status.
Middle course: the overall ecological status is between moderate to poor status.
Lower course: the overall ecological status is good, except that river at Lei Wah San Tsuen is in moderate status.
- Since some of the sections lack data of the bank management, the final score will be unspecified.
- OHR : Hydrological regime influence
- PPK : Longitudinal permeability
- UBR : bank management
- BVG : bank vegetation
Feel free to explore the scores of different river sections in each paramter and their total hydromorphological score (HMS)
HMS with 9 is unspecified watervourse
The pie chart show the proportion of hydromorphological classes attained by Sheung Yue River sections.
- While it is delighted to discover that Good to High Status share a great proportion, the proportion of Moderate Status is as large as them, with a small yet noteworthy proportion of Poor Status.
- The proportion of unspecified river sections could be reduced if sufficient bank management and Google street view data or pictures are obtained, since the types of bank lining is hard to be deduced when natural banks are unlikely.
Suggestions
River sections that fail to reach the good to high status should be improved (EU-WFD,Directive 2000/60/EC)
How can Sheung Yue River be improved?
1. Improving longitudinal continuity by replacing weirs with ramps
Longitudinal permeability is particularly important for S. zanklon as its life cycle requires both slow moving streams and marshes for adults and adjacent terrestrial habitats for brooding females. (Stanton, D.J. & M.R. Leven (2016)
Hence, the presence of weirs will hinder their migration in search of suitable terrestial habitats and also breeding process, harming their population.
Weirs built with ramps could assist the movement of S. zanklon along Sheung Yue RIver.
2. Enlarging riparian zone
Riparian corridors are of pararmount importance for the breeding of S. zanklon. They can also reduce erosion by slowing velocity of water and by binding Soil (Jon Kusler Association of State Wetland Managers,2016). This help protect the habitat of S. zanklon.
However, Sheung Yue River, particularly its lower and middle course, show insufficiency in the area of riparian zone.
- From the pie chart, Class V shared the largest proportion, together with Class IV, they account for 50% of the proportion.
This indicates the need of extending riparian corridor along Sheung Yue River.
B. Identification of potential sites of S. zanklon occurrence
Watercourses within Hong Kong are often fragmented or channelized, which is likely to inhibit the movement of crabs. Hence it is likely there are now a number of isolated sub-populations. (Stanton, D.J. & M.R. Leven (2016))
To aid conservation actions and prevent further destruction to S. zanklon habitat, prior identification of potential sites of species occurrence is required.
As S. zanklon is most abundantly distributed in North District, we have chosen North District as the study area.
To identify these potential sites, the following habitat characteristics of S. zanklon will be analysed :
- slow-flowing, low-gradient streams
- wetland and ponds
- lowland of altitude below 220m
The map shows the distribution of wetland, ponds and watercourses in the North District.
Since low gradient streams with altitude below 220m is prefered by S. zanklon, we have utilized contour lines to filter out watercourses below 220m.
The blue contour lines are below 220m, while the green ones are above 220m.
- Left: unfiltered watercourses
- Right: filtered watercourses (below 220m)
As gastropods are typically recorded from watercourses and wetlands where S. zanklon occurred (Stanton, D.J. & M.R. Leven (2016)), the distribution of gastropods (the preys of S. zanklon) can also be used to deduce potential sites of S. zanklon occurrence.
We have collected the distribution data of gastropods in the North District from iNaturalist. To ensure data accuracy, we have only selected observations that attained Research Grade, the highest level of data quality. (Some of the records even contain image url for viewing the gastropods.)
- RIght : a density map with 10 concentration classes has been created to show the concentration of gastropods in the North District.
Next, wetland, ponds and watercourses that are within or intersect with the class regions of the density map will be assigned the corresponding classes. (Right: classified wetland, ponds and watercourses)
The potential occurrence of the sites is considered :
- High : with class 6 - 10
- Moderate : with class 3 - 5
- Low : with class 1 - 2
We have discovered that some of the sites with gastropod individuals nearby are classified as Low since they are away from the clusters.
Yet, the potential sites of S. zanklon occurrence prediction might not be accurate as other gastropod clusters might be missed simply from public observations in iNaturalist.
Therefore, we have also find out wetlands, ponds and watercourses that are close to the supply of gastropods (within 100 meters from gastropod individuals) but classified as low. They are then converted to "moderate" to further increase the accuracy of our prediction.
The final assessment of potential sites of S. zanklon occurrence is shown in the map.
Conclusion
- Sites in green is where undiscovered S. zanklon can most likely be found, since there is an ideal physical environment with abundunt supply of gastropods.
- Sites in orange shows a moderate chance of species occurrence.
- Other areas only satisfy the physical characteristics of S. zanklon habitat and is thus less likely been found with S. zanklon.
We hope that more study can be carried out on these potential sites to prevent further losses of this endangered species in future development.
These are some of the examples of the matched potential sites of S. zanklon occurrence, which are proved by the records of S. zanklon in research (Stanton, D.J. & M.R. Leven (2016)) and news (SCMP)
- The tagged location is the general position where S. zanklon has been recorded
Limitation
- Our study is mainly based on the research of Stanton, D.J. & M.R. Leven (2016) since there lacks latest research on S. zanklon. As a result, the distribution of the crabs are subjected to changes and it might affect the accuracy of our study.
- There is also a lack of open spatial data concerning river channels from the government , such as the bank management method used in Sheung Yue River, which might lead to overdependence on satellite data. Data such as the types of channel bed sediments are also hard to be determined from observation, which limits the extent of our hydromorphological assessment.
- Lastly, the distribution data of gastropods can only be obtained from public observations in iNaturalist, where data accuracy might vary as the coordinates of identification sites uploaded by the public might not be reliable.
This is therefore necessary to have updated research to provide more accurate and reliable data on S. zanklon and its habitat in order to boost conservaton.
How can we help Somanniathelphusa zanklon ?
We can help by recognizing S. zanklon in daily lives with iNaturalist. This could assist researchers in analysing the distribution of S. zanklon in Hong Kong or even other regions in Asia, so as to have deeper investigation on their habitat requirements and behaviors. New distribution sites can also be discovered more rapidly by the help of the public.
Having more profound knowledge of S. zanklon, more conservation work can be performed efficiently and the chances of saving the crabs can be enhanced.
We hoped that our study can raise the awareness of the public in conserving S. zanklon and assist their understanding of the species habitat requirements.
Let's explore the potential habitat of S. zanklon and aid in habitat conservation!
