Shellfish Aquaculture

Varnish Clams

Nuttallia obscurata, the varnish clam, the purple varnish clam or the purple mahogany-clam
Photo by Claudia Mills

Notice to Shellfish Farming Industry: Federal/Provincial Letter of Understanding (LOU) on Varnish Clams

Early in 2002, a letter of understanding on varnish clams was signed under which commercial harvest of varnish clams can be permitted on clam tenures. An amendment to include varnish clams as a licensed species must be made to the Aquaculture Licence. (See below for detailed procedure for amendment application). Harvest of varnish clams for purposes other than sale - for example to eradicate them from a tenure is not permitted under the LOU. In addition, active culture of varnish clams - for example by seeding or moving them from tenures where they currently occur to new areas is not permitted.

Varnish clams offer a good opportunity for some growers to increase revenue from their tenures. Varnish clams require special harvesting and handling protocols to retain their value. The quality of varnish clams also varies seasonally. Growers should also be aware that the market for varnish clams is still under development and that markets could be affected by large quantities of product hitting the market all at once - especially if it is substandard in quality.

Maintaining the Quality and Value of the Varnish Clam (Nuttallia obscurata)

Introduction

Originally from Japan, the varnish or dark mahogany-clam (Nuttallia obscurata), was unintentionally introduced (likely by ballast water) to the Strait of Georgia, BC during the late 1980's or early 1990's. The name, "Savoury clam", has also been approved by Canadian Food Inspection Agency for the purpose of marketing. This attractive-looking clam reaches sizes to 2.75" (69mm) and is found commonly in sand-gravel substrate to 8-10" (20cm) depth in the high to mid-intertidal zone. In winter, this clam is often buried somewhat deeper (at 12-15" or 30-38cm). It is also often found where other clams are sparsely represented, such as in freshwater seeps and in less preferred substrates (loose sand, sand-silt, pebble, cobble). Recent sampling indicates that this exotic species is widely distributed on beaches in the Strait of Georgia, and northern Puget Sound, but it has also been found in Barkley, Clayoquot and Nootka Sounds, on the west coast of Vancouver Island (DFO, 2001). Commercial concentrations have been found on clam tenures in Baynes Sound.

Commercial Potential

A market study was conducted on the BC "Savoury clam" to assist in market development for this new seafood product. Initial response from BC food service industry users has been positive and enthusiastic, but the market demand in response to increased supply is unknown. However, the value of Varnish or Savoury clams in the marketplace will be better for the producer if care is taken to maintain product quality during harvesting and handling.

Keep in mind that this is a "soft-shell" clam that breaks easily with inappropriate harvest or handling. In addition, larger Varnish clams often have commensal pea crabs inside that are very resistant to removal. Each grower should examine the incidence of pea crabs in Varnish clams from his or her site prior to commercial harvesting and marketing.

As for other shellfish, key recommendations for maintaining product quality include:

  1. Ensuring that the product comes from clean, approved growing water and doesn't come in contact with sources of contamination during or after harvest; and
  2. Determining that the clams are unstressed and in good meat condition (e.g. not spawny) at time of harvest.
  3. Observing that the same conditions and regulations concerning water quality and biotoxin (PSP, domoic acid) monitoring and closures that apply to Varnish clams as for oysters and Manila clams.

Harvesting

The suggested harvesting procedure is to start by digging a hole in the beach and gently raking substrate and clams into the hole or trench as you move along. The marketable clams (check with potential buyers beforehand) are removed and placed in a bucket or net bag.

Handling Considerations

  • Washing of clams to remove sediment soon after harvest is desirable and should be done with potable (if fresh water) or approved growing water (if salt water);
  • Avoiding prolonged exposure of product to bright sun (summer) or freezing temperatures (winter);
  • Grading to remove other species, dead or damaged clams or gravel;
  • Labeling or tagging product containers appropriately as to origin (growing or harvest site);
  • Purging and wet storing, if used, must be in approved growing waters, preferably at the tenure where harvested, at or below the tide level where the clams were dug, or in adjacent deep water if suspended. Tank-based wet storage may be an excellent option at shore-based operations;
  • At harvest, keeping clams cool (i.e. prevent temperature abuse) and allowing them to purge themselves of grit and pea crabs (if present);

Purging

The Varnish or Savoury clam requires closer attention to purging than Manila clams for market acceptability. Recommended steps for effective purging are as follows:

  • Place the Savoury clams loosely in a mesh bag (between 12-13 kg. or 25-30 lb.)
  • Spread the clams in the bag so that they are not all bunched up.
  • Place the bags on the beach at about the 4-5 foot tide level.
  • Make sure that the bags can't be tossed around by the waves.
  • Turn the bags one half turn every 24 hours, spreading them evenly so that the clams can purge themselves.
  • Repeat this process for about 4-5 days.
  • More detailed purging procedures and experimental results, especially applicable to processors, are reported by Gillespie et al. (2001).

Transporting

  • Shipping to a processing facility should be done using clean, synthetic materials (e.g. sacks, containers, pallets), and without possible contact with materials capable of contamination;
  • Transporting product in boats and trucks that are constructed and maintained so as to protect the clams from contamination;
  • Refrigerating (or icing) of product during transport is needed for continuous temperature control (45F or 7.2C) when ambient air temperature and travel time are such that unacceptable bacterial deterioration may occur.

Proper attention to quality control and safety procedures in harvesting, handling and shipping Varnish or Savoury clams will maintain product quality and greatly assist development of a reliable reputation and strong prices in the marketplace.

Licence Amendment Procedure

To add Varnish clams to the Aquaculture Licence for a clam tenure, an application for amendment to the Shellfish Management Plan (SMP) requesting the addition should be sent (with cheque for $50) to BC Ministry of Agriculture and Lands, to the attention of Jim Russell, at 2500 Cliffe Avenue, Courtenay, BC V9N 5M6. In completing the amendment application, the SMP Seeding Schedule for Varnish clams would simply state: "Natural recruitment", while the related Production Schedule would state the estimated annual harvests for the projected period. An amendment may be made any time during the term of the licence.

Related Publications

Further information on varnish clam biology, distribution and handling procedures can be obtained from the following sources:

  • Gillespie, G.E., M. Parker and W. Merilees. 1999. Distribution, abundance, biology and fisheries potential of the exotic varnish clam (Nuttallia obscurata) in British Columbia. CSAS Research.  Document 99/193. 38 p.

http://www.dfo-mpo.gc.ca/csas/Csas/publications/
ResDocs-DocRech/1999/1999_193_e.htm

  • Gillespie, G.E., B. Rusch, S.J. Gormican, R. Marshall and D. Monroe. 2001. Further investigations into the fishery potential of the exotic varnish clam (Nuttallia obscurata) in British Columbia. PSARC Working Paper I2001-09. 51 p.

http://www.dfo-mpo.gc.ca/csas/Csas/publications/
ResDocs-DocRech/2001/2001_143_e.htm