King County's Beach Assessment

Burton Acres, Vashon Island

Site Description

This beach is located in a Vashon Island park located on the northeast side of the Burton Peninsula in Quartermaster Harbor. The upper beach is somewhat steep above +3 foot level, but it becomes quite flat at about MLLW. The upper beach is comprised of gravel and sand on the north side but is more cobbled on the east side. The beach sediment becomes much finer as the slope diminishes. Below MLLW, the beach is quite muddy. On the north shore, erosion does not appear to be a major influence but the east beach is being eroded. (That erosion exposed a Pre-columbian shell midden near the point that was excavated in 1996 by the University of Washington's Department of Archeology. We were able to share some of our measurements with them.) Since Burton Acres is located at the north end of the harbor and is on the north side of a peninsula, its north shore is protected from heavy surf most of the time. The east beach is vulnerable if the winds are come from the south.

The flushing in the harbor is greatly restricted by an isthmus at Portage, which connects Vashon and Maury islands. There are no streams discharging directly onto the beach.

This beach is comprised of finer material than most others surveyed. Low wave action allows finer materials to settle in the protected, shallow water. There are some cobbled areas on the eastern beach above the +5 foot level, sandy gravel below that to about +1 foot level and much finer material below that.

The northeastern point of the beach is comprised of loose sandy gravel, a prime habitat for Manila and littleneck clams. The lower beach on the north side below the +4 foot level is comprised of muddy sand that is excellent habitat for the resident geoducks.

Invertebrates

There were 34 species of invertebrates found on this beach which ranked it seventh highest of the 13 beaches surveyed for invertebrates. The number of invertebrate species on this beach is somewhat limited by the scarcity of hard substrate. On the other hand, because the beach is relatively free of harvesting, there are a few animals found here that are not found elsewhere. The short-spinned sea star (the region's largest sea star) was found here. The specimens observed had arms about 12 inches long. Another animal found only at this beach was Protothaca tenerrina (thin-shelled littleneck clam), a relative of the native littleneck, which is easily mistaken for a butter clam.

There were many geoducks present on this beach, but since they usually are two to three feet deep in the sand and mud, they were not sampled.

Sand dollars were common and a few Solen sicarius (jackknife clams) were identified.

Limpets
Plate Limpet	Acmaea scutum
Finger Limpet	Acmaea digitalis
Snails
Moon Snail	Polinices lewisi
Chitons
Mossy Chiton	Mopalia mucosa
Hairy Chiton	Mopalia ciliata
Bivalves
Native Littleneck	Protothaca staminea
Manila Clam	Tapes japonica
Butter Clam	Saxidomus gigantea
Cockle	Clinocardium nutalli
Softshell	Mya arenaria
Horse Clam	Tresus sp
Inconspicuous Macoma	Macoma inconspicua
Bent Nose Macoma	Macoma nasuta
Jackknife Clam	Solen sicarius
Bay Mussel	Mytilus edulis
Horse Mussel	Modiolus rectus
Geoduck	Panope generosa
Pink Scallop	Pecten hericius
Sea Stars
Mottled Star	Evasterias troschelli
Short-spined Pisaster	Pisaster brevispinus
Purple Star	Pisaster ochraceus
Urchin
Sand Dollar	Dendraster excentricus
Barnacles
Acorn Barnacle	Balanus glandula
Small Acorn Barnacle	Chthanalus dalli
Crabs
Green Shore Crab	Hemigrapsus oregonensis
Black Clawed Crab	Lophopanopeus bellus
Graceful decorator crab	Oregonia gracilis
Dungeness crab	Cancer magister
Red rock crab	Cancer productus
Hermit crabs
Hairy hermit	Pagurus hirsutiusculus
Porcelain crabs
Flat Porcelain Crab	Petrolisthes cinctipes
Anemones
Burrowing anemone	Anthopheura artemisia
Plumrose Anemone	Metridium senile
Hydroids
Tube worms
Plume worm	Eudistylia polymorpha
Isopods
Beach Cockroach

Clams

Generally, this beach had the highest yield of clams of all of those surveyed. The combination of excellent substrate (muddy sand) and limited harvesting is probably the main reason for this.

The clam band covered about 1.2 acres and was one the smaller bands surveyed. Of the 30-plus holes dug, 26 contained clams. One hole held 118 manila clams, 36 littleneck clams, and 7 soft-shell clams for a total of 161 clams. Altogether, there were 867 clams found which weighed 16,017 grams total.

The clam population on this beach was comprised mostly of littleneck clams (64% by weight and 58% by count). The butter clams comprised 10% and the manila clams 14% of the weight. The population density on this beach was 33 clams per square foot, the highest of all the beaches. The average weight of those clams was more than a pound (589 grams) per square foot and was the highest of all the beaches surveyed for clams. The average weight per clam was 23.4 grams, the third highest of all beaches.

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

Some of the clams on this beach were larger than those found on most beaches. The butter clams weighed about four times as much as the average for all beaches; the horse clams were three times as heavy and the littleneck clams weighed about twice the average. The cockles, manila clams and macoma clams were about average compared to other beaches. The soft-shell clams appeared to be two-thirds the average of other beaches which seems incompatible with the high productivity of other types of clams on this beach. This may be due to the relative scarcity of this species on the region's beaches in general. Usually, because only a few are found and one or more of them may be very large, figures for the average size at those individual beaches are skewed.

Of the 250 manila clams collected, only 42 (17%) were 38 millimeters or longer. That percentage was the third lowest of the eight beaches that had manila clams. The low percentage is in part due to the 118 tiny manila clams collected in one sample, which skewed the results. (Without this sample, the percentage would have been nearly 30%.) 235 of the 408 littleneck clams collected, or 58%, were legal-size, which was the third highest of the beaches. Twenty of the 22 (91%) butter clams were legal size, the highest for all of the beaches.

Seaweed

No algae survey was conducted at this site.

Use

This beach is not heavily harvested, although a few craters were observed. The beaches have been closed in Quartermaster Harbor to shellfish and seaweed harvesting for several years because of fecal contamination. The sources of that contamination, probably failing septic systems, have not been corrected.

Because of the inconvenience and cost associated with using the ferries, the Vashon beaches have not received the harvesting pressure that the urban beaches have.

This beach is relatively warm for Puget Sound, and is used for swimming. Current fecal coliform data is unavailable for Quartermaster Harbor, so it is unknown whether swimming is safe.

Essentially, the clam population shows the effects of closing a beach to shellfish harvesting: high diversity, high numbers, and large clams.

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Updated: 11/01/98