King County's 1996/1997 Beach Assessment

Tramp Harbor, Vashon Island

Site Description

This beach is located on the north side of the isthmus that connects Maury and Vashon islands. It is not located in a park as are the other beaches surveyed, but it is heavily used. The area is now known as Portage.

The beach is relatively flat and is cobbled down to about the +4' foot tide level. It is composed of sand and gravel below that. The eastern end of the survey area lies at the foot of a clay bluff. There, the substrate is quite sandy and the presence of buried wood bearing ax marks is evidence of relatively recent slides. The beach does not show the effects of heavy surf. Perhaps that absence explains the large numbers of sand dollars.

Invertebrates

There were 23 species of invertebrates identified on this beach which placed it second lowest in diversity. Most of the animals noted were mollusks (18 of the 23 species). Eleven (11) of the mollusks were bivalves, four were limpets and three were chitons. It must be noted that the survey here was less complete than at most beaches because of low volunteer turnout on the day of the survey. Thousands of sand dollars live on this beach. A Lucina annulatus (ringed lucine) was found. Even though it is not rare, we observed few on the beaches surveyed.

Limpets
Plate Limpet	Acmaea scutum
Variegated Limpet	Acmaea persona
Shield Limpet	Acmaea pelta
Finger Limpet	Acmaea digitalis
Chitons
Mossy Chiton	Mopalia mucosa
Hairy Chiton	Mopalia ciliata
Woody Chiton	Mopalia lignosa
Bivalves
Native Littleneck	Protothaca staminea
Manila Clam	Tapes japonica
Butter Clam	Saxidomus gigantea
Cockle	Clinocardium nutalli
Horse Clam	Tresus sp
Sand Clam	Macoma secta
Inconspicuous Macoma	Macoma inconspicua
Bent Nose Macoma	Macoma nasuta
Modest Tellin	Tellina modesta
Bay Mussel	Mytilus edulis
Jingle Shell	Posodesmus cepio
Sea Stars
Purple Star	Pisaster ochraceus
Sand Dollars
Sand Dollar	Dendraster excentricus
Barnacles
Acorn Barnacle	Balanus glandula
Thatched Barnacle	Balanus cariosus
Shrimp
Dock Shrinp	Pandalus daneae

Clams

The clam band in the assessment area covered about 2.4 acres. The greatest concentration of clams was in the cobbles on the upper portion of the beach. The band was about 40' to 80' wide along the cobbled portion of the upper beach (see contour map). Numerically, the macomas and native littleneck clams comprised more than 80% (41% each) of the clams found. The proportions of biomass comprised by the clam groups were much different. The littlenecks accounted for 51% of the biomass; the manila and butter clams combined accounted for another 28%.

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

The 344 clams collected from 26 holes weighed 3,381 grams. From those numbers, it is estimated that the average clam weighed 9.8 grams and there were about 13 clams found in each hole. The yield from this beach was 130 grams per square foot, which is below average for the beaches surveyed.

The littleneck clams and the macomas were about the same size as the averages for all beaches combined while the manila, butter and softshell clams were larger than average. Eleven of the 37, or 30% of the manila clams found were legal size. That is the third highest average for all of the beaches. Nine of 14 (64%) of the butter clams were legal size, the fourth highest of the beaches where clams were measured. Thirty-eight percent (38%) (54 of 142) of the native littleneck clams were legal size.

Seaweed

A seaweed survey was not conducted but a few species were noted in the field notes. Clumps of Ahnfeltia sp. were common here, and both Smithora sp. and Punctaria sp. were found growing on the eelgrass blades in the beds at the east end of the survey area. Cockles and moon snails were common in the sandy substrate of the eelgrass beds as were the sand dollars.

Other

Purple martins and an osprey were observed flying over the beach. Nest boxes for the martins have been placed on some of the old pilings north of there.

Use

Clams are harvested from the beach for both bait and for human consumption. A public fishing pier is located to the north. Clams are taken from this beach but the harvesting pressure is not as intense as it is on most of the beaches on the mainland to the east.

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Updated: 11/02/98