King County's Beach Assessment

Dockton Beach Park, Vashon Island

Site Description

Located on Vashon Island, this King County park lies at the middle of the west shore of Quartermaster Harbor, which separates Vashon and Maury islands. The beach is fairly flat. Since it lies in a cove in a harbor, the limited wave action permits the deposition of fine sediments; thus the substrate is mostly mud below the +1 foot level. The beach is sand and gravel above that level. There are a few patches of gravel near the sea wall and the boat ramp. The beach is protected from erosion both by its location as well as the sea wall.

Intertidal Inhabitants

This beach, added to the program in 1996, has been mapped and the clam population has been sampled, but the marine algae and invertebrates have not been surveyed yet. There is some marine life here but the muddy substrate does not encourage diversity.

Clams

The clam band was about 700 feet long, 120 feet to 160 feet wide, and covered about 2.2 acres. The assessment area included the entire length of the beach below the 3 foot elevation contour. Twenty-four of the 30 holes dug contained clams, more than on most beaches. A total of three hundred twenty-one clams were found in those samples. The density of 13 clams per square foot is not remarkably high, and the 15.8 gram average weight per clam was the fifth highest for all of the beaches surveyed; consequently, the yield of 211 grams (one-half pound) per square foot was the fourth highest of the beaches surveyed.

Although, there are more macoma clams than other species here (50% are macomas), they only account for 16% of the weight. The littleneck clams contribute 30% of the weight. The horse, soft-shell, and manila clams are evenly distributed and they account for 21, 13 and 15%, respectively.

Check out the following graphs for more information about the clam population:
Numeric composition of clam population
Weight composition of clam population
Numeric distribution of clam population
Biomass distribution of clam population

It is assumed that the lack of harvesting pressure here accounts for the clams being relatively large. The littlenecks are about half again as large as elsewhere and the macomas are five times as large as the average for all beaches. The manila and soft-shell clams were about twice the average; horse clams were three times the average, and the butter clams four and half times the average of all beaches. Forty-one percent (25 of 61) of the manila clams were legal size, the highest for all of the beaches and 57% (43 of 75) for the littleneck clams, fourth highest. Sixty-seven percent of the butter clams were legal size, the third highest, but results are skewed since this was based on only three clams, with two being longer than 38 millimeters.

Seaweeds

No formal survey was conducted.

Use

The park is used mainly as a marina, but people also beachcomb, swim during hot weather, and children seem to wade anytime. Harvesting is limited. The clam population may be protected by the reputation the water has for fecal pollution as well as the nature of the substrate. The substrate is so muddy that one of the volunteers became mired to the point of needing assistance to get free. A third person was required to assist the volunteer and the first rescuer.

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Updated: 11/02/98