Alki Beach, Seattle
This page describes the beach and its marine life. For information about the park, its recreational amenities, directions and open hours, please visit City of Seattle's Alki Beach Park page (external). Site Description Alki Beach was sampled west of the Alki sewage treatment facility, south of the lighthouse and adjacent to Schmitz Park, a Seattle City Park. This beach was one of the five sampled in 1974 by Thom and Armstrong. The beach is relatively flat, but is comprised of outcroppings of layers of clay and of Vashon till. Since the layers have different hardness, they have eroded at different rates. Consequently, there is a series of ridges relatively parallel to the sea wall that protects the upper beach from erosion. The spaces between the ridges are filled with gravel. The beach is exposed to the surf but it is highly resistant to erosion. Much of the site consists of exposed bedrock, which consists of alternating layers of clay and sandstone. In places, the clay layers have eroded leaving the sandstone exposed as ridges. Intertidal Inhabitants Animal diversity here is extremely high because of the variety of habitats available. Various clams, crabs and shrimp live in the gravel in the tide pools between the ridges. Chitons, limpets, worms, snails and sea stars were commonly found on the hard rock. Piddocks burrow in the soft clay layers. The rocks have heavy growths of seaweed although they seem to be diminishing compared to a photograph taken more than 20 years ago. In 1994, both orange and white sea cucumbers were common, but they were found to be scarce two years later. Invertebrates Forty-nine (49) species of invertebrates were found on this beach, which is the second highest of all 13 beaches surveyed. The diversity of life on this beach is quite high, but it is diminishing because of little protection from harvesters.
| Limpets |
| Plate Limpet |
Acmaea scutum |
| Variegated Limpet |
Acmaea persona |
| Finger Limpet |
Acmaea digitalis |
| Snails |
| Sitka Periwinkle |
Litorina sitchana |
| Checkered Periwinkle |
Litorina scutulata |
| Moon Snail |
Polinices lewisi |
| Hairy Triton |
Fusitriton oregonense |
| Fringed Dogwinkle |
Thais lamellosa |
| Channeled Top |
Calistoma canaliculatum |
| Bivalves |
| Native Littleneck |
Protothaca staminea |
| Manila Clam |
Tapes japonica |
| Butter Clam |
Saxidomus gigantea |
| Cockle |
Clinocardium nutalli |
| Softshell |
Mya arenaria |
| Inconspicuous Macoma |
Macoma inconspicua |
| Pilsbry's Piddock |
Zirfaea pilsbryi |
| Common Piddock |
Penitella penita |
| Bay Mussel |
Mytilus edulis |
| Jingle Shell |
Posodesmus cepio |
| Native oyster |
Ostrea luridea |
| Sea Stars |
| Mottled Star |
Evasterias troschelli |
| Purple Star |
Pisaster ochraceus |
| Sea Cucumbers |
| White |
Eupentacta quinquesemita |
| Orange |
Cucumaria miniata |
| Barnacles |
| Acorn Barnacle |
Balanus glandula |
| Small Acorn Barnacle |
Chthanalus dalli |
| Crabs |
| Green Shore Crab |
Hemigrapsus oregonensis |
| Black Clawed Crab |
Lophopanopeus bellus |
| Graceful kelp crab |
Pugettia gracilis |
| Red rock crab |
Cancer productus |
| Shrimp |
| Barred shrimp |
Heptacarpus pugettensis |
| Anemones |
| Colonial anemone |
Cribrina elegantisima |
| Pebbled Anemone |
Telia coriacea |
| Green Anemone |
Anthopleura xanthogrammica |
| Hydroids |
|
| Segmented worms |
| Corrugated worm |
Glyseria |
| Shellbinder worm |
Thelepus crispus |
| Scale worm |
Halosydna |
| Irridescent worm |
Hemipodus borealis |
| Sand worm |
Nereis virens |
| Tube worms |
| Calcareous tubeworm |
Serpula vermicularius |
| Flatworms |
| Large speckled |
Kaburakia excelsia |
| Small speckled |
Freemania |
| Nemerteans |
| Purple ribbon worm |
Emplectonema sp. |
| Isopods |
| Brown Isopod |
Idotea urotoma |
| Amphipod |
| Skeleton Shrimp |
Caprella kennerlyi | 
Clams Thirty-four (34) holes were dug during each sample date; nearly all contained at least one clam. The largest yield from a single hole was 1.5 lbs. By both count and weight, the manila, littleneck, and butter clams were the most common. Mya arenaria (soft-shell) and Macoma clams and cockles are present. The clam population densities ranged from one to 65 per square foot but the average of all sites was about 14 clams per square foot. The manila clams were larger than the average for all beaches but the rest of the species were a little smaller. About one third of the manila clams and about one fifth of the littleneck and butter clams were legal. That average was the second highest of all of the beaches. Littleneck clams were found wherever a few inches of gravel covered the sandstone. Those gravel patches were scattered, but nearly all of them were inhabited. The clams appeared to be most common above MLLW to about the +3' level; they were scarce below MLLW. The clam band, which occupies about two acres, is about 700 feet long and 80 to 100 feet wide beginning at the jetty at the south end of the beach and continuing a northerly direction (see topographic map). Beyond that point, the gravel patches are very discontinuous. The sampling grid that we used was established by the Seattle Aquarium staff several years ago. The grid is 300 feet long x 80 to 120 feet wide and begins at about the +3 foot level at the south end of the park near the jetty. It extends north from there for 300 feet. The grid is 50 feet by 40 feet on this beach rather than the 100 foot x 40 foot format used on most other beaches. Check out the following graphs for more information about the clam population at Alki: Seaweed Twenty-six (26) species of marine algae were found, making this site the second highest among the beaches surveyed. The brown algae (13 species) were the most varied overall, but there were more red species (10) at the lower tide levels on the beach. Only three species of green algae were noted, but they were the dominant species at the higher tide levels. Rockweed and Gigartina cristata (grapestone) were very common on the rock outcrops at the higher levels. Sargassum appears to be invading the area. Green (Chlorophyceae) - Cladophora sp.
- Enteromorpha intestinalis
- Ulva lactuca
Brown (Phaeophyceae) - Costaria costata
- Desmarestia intermedia
- Desmarestia munda
- Fucus sp.
- Hedophyllum sessile
- Leathesia difforimis
- Petalonia debilis
- Punctaria hesperia
- Pylaiella littoralis
- Ralfsia sp.
- Sargassum muticum
- Scytosiphon bullosus
- Scytosiphon lomentaria
Red (Rhodophyceae) - Ceramium pacificum
- Cryptosiphonia woodii
- Gigartina cristata
- Gigartina exasperata
- Gigartina latissima
- Iridea cordata
- Porphyra perforara
- Prionitis lanceolata
- Rhodymenia palmata
- Smithora naiadum
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