Clam gauge | WWF Hong Kong

Clam gauge



Unregulated clam-harvesting can lead to overexploitation of marine resources. Clams that are big enough to spawn are frequently being taken away in large numbers, leaving the remaining clams with limited potential to reproduce and supply new clams. Likewise when small clams that have yet to reach maturity are harvested, there will be no clams left to grow and reproduce. Targeted clams will drop in numbers drastically, eventually leading to changes in community structure and a clash in clam diversity1

Based on data from citizen science surveys and research findings, WWF has formulated clam gauges to serve as reference for the public. As an interim measure before clam-digging activity is officially managed, WWF recommends that clam diggers not collect clams  smaller than the clam gauge. Give clams a chance to reproduce and let the small ones grow!

Principles behind clam gauge design
  1. Based on literature reviews to understand the relationship between sizes and maturity for different clam species; the suggested clam gauge size is the size of over half of the individuals of that species deemed mature
  2. Takes reference from the statutory minimum size of catches of the same species in other regions
  3. Based on a comparison of data obtained from WWF citizen science ecological surveys, to ensure the designated sizes on the clam gauges are applicable to local shores
  4. In the long term, clam gauges will be reviewed regularly to accommodate for any changes in the status of the local clam community.
Take the most commonly harvested clam species at Shui Hau decades ago, the Asiatic hard clam (Meretrix spp.) as an example. The largest clam collected by villagers 20 years ago would reach 130 mm in shell length, and weighed as much as a papaya. Clams nowadays, however are becoming fewer and smaller in size, very likely due to unregulated clam-digging activity.
 
WWF has designated a minimum size for capture to be 4 cm. The size chosen covers a wide range of Asiatic hard clam species found in Asia, with reference to WWF’s citizen science surveys.


 
For the four other clam species below, WWF has designated the minimum size for capture to be 30 mm (or 3 cm). This can be applied to other clam-digging hotspots other than Shui Hau.
To manage clam resources, overseas regulatory authorities have implemented fishery plans to control clamming activity. Measures include annual population surveys, exploitation plan, monitoring and control2. Exploitation plans include a permit system limiting the harvest season, zoning, number of people, gear and catch, such as the clam quota and size3.
1Gaspar, M.B.; Barracha, I.; Carvalho, S.; Vasconcelos, P. Clam Fisheries Worldwide: Main Species, Harvesting Methods and Fishing Impacts. In Clam Fisheries and Aquaculture; Da Costa González, F., Ed.; Nova Science Publishers: Hauppauge, NY, USA, 2013; pp. 291–327.

2Gaspar, M.B.; Barracha, I.; Carvalho, S.; Vasconcelos, P. Clam Fisheries Worldwide: Main Species, Harvesting Methods and Fishing Impacts. In Clam Fisheries and Aquaculture; Da Costa González, F., Ed.; Nova Science Publishers: Hauppauge, NY, USA, 2013; pp. 291–327.

3Fisheries and Oceans Canada, the Government of Canada. Notice to recreational fishermen.
http://www.glf.dfo-mpo.gc.ca/Gulf/FAM/Recreational-Fisheries/New-Conservation-Measures-Clams accessed: June 2018