King County's 1996/1997 Beach Assessment

Point Robinson Park, Vashon Island

Site Description

This beach is a Vashon Island Parks facility located north of the U.S. Coast Guard lighthouse on Point Robinson on the east side of Maury Island. Although much of the point is a sand spit, this beach is at the foot of a clay bluff. The beach is somewhat steep because it is exposed to the surf much of the time. The upper beach is comprised of loose cobbles down to the +6 foot level. At that point, gravel becomes a larger component of the substrate. At the +3 foot level, the slope lessens and finer sediments are mixed with the cobbles and gravel. In some areas, clay layers are exposed or lie just a short distance beneath the surface. The bluffs are evidence of the erosive forces at work here.

Invertebrates

No formal invertebrate surveys were conducted but some field observations were made. A small colony of clams were tentatively identified as Spisula falcata (hooked surf clams). Cockles were common in the eelgrass beds as were the striped nudibranchs. Staff found a few Eupentaca quinquesemita (white sea cucumbers) and found both yellow and purple shore crabs to be very common. The clay outcroppings did not appear to be hosting any piddocks.

Clams

The clam band is narrow because of the slope, but there are large compact colonies of butter clams where the substrate is gravel and cobbles. Horse clams are common if cobbles are buried by three to six inches of sand, and large cockles are common in the sandy areas where small eelgrass beds are found. The clam band is about approximately 800 feet long, averages 60 feet wide and covers about 1.5 acres. It is somewhat wider at the west end than at the east. The clay layers create a few interruptions in the band.

Seventeen (17) of the 19 holes dug contained one or more clams. Both from numeric and biomass viewpoints, most of the clams are at the western end of the survey area. Numerically, the composition of the population was dominated by littlenecks (29%), macomas (27%), and butter clams (24%). The composition by weight is very different. Butter clams account for 50% of the population, horse clams, 31%, but littleneck clams accounted for only 9% of the weight. The 154 clams collected weighed 4450 grams for an average of 28.9 grams, the second highest average of the 14 beaches surveyed. The population density was the fourth lowest with only nine clams per square foot, whereas yield, at 251 grams per square foot, was fourth highest. Of the eleven Manila clams found here, none were of legal size and only five of 43 littleneck clams (12%) were legal. Fourteen (14) of the 34 butter clams were legal size, however.

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 relative average sizes of the clam species found here reflect the environmental conditions and the low level of harvesting pressure. The shallower burrowing littleneck and manila clams are about two-thirds as large as the averages but numerically they are not scarce. The deeper, burrowing butter and horse clams are nearly three times the averages for all beaches combined. It is assumed that the shallower living animals are washed out during the winter storms while the others are not and can mature.

Seaweed

Informal observations indicated that sargassum grows in some areas just below the 0 foot level. Since there are few boulders and because the surf is so violent here, the diversity of life here seems to be somewhat limited.

Other

Rough-winged swallows nest in the clay bluffs. A sea lion was observed feeding a short distance offshore of the sand spit on the point. Within a few minutes, he had consumed a 2' fish (probably a salmon as indicated by the orange flesh) and had grabbed another.

Use

The beach provides aesthetics and solitude. Although it is in a park, the apparent level of harvesting seems to be low; distance from urban population pressures may be a factor.

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Updated: 11/02/98