V. A. 1. Harshell Clams |
The three species of clams generally referred to as hardshell clams in the Puget Sound region include the introduced Manila clam (Venerupis [Ruditapes] philippinarum), the native littleneck clam (Protothaca staminea), and the butter clam (Saxidomus giganteus) (Scholz 1990). The Manila and native littleneck clams are considered the most important to the commercial fishery because of their better keeping quality. The butter clam is more important to the fishery, where it accounts for the greatest quantity of total weight harvested because of its larger size. Butter clam harvest is frequently closed as a result of high levels of the paralytic shellfish poisoning (PSP) toxin. All clams commercially harvested are regulated for both fecal coliform and PSP. Many areas in Puget Sound are seriously affected by both.
On Bainbridge Island, two areas are noted for hardshell clams. One is located on the western shore of the Island just north of Point White (Appendix A– Shellfish Occurrence Map), and the second is located on the south side of the Island near Rich Passage.
Littleneck clam (Protothaca staminea)
The native or littleneck clam (Figure V-1) is an important suspension/filter feeder found along most Pacific coast estuaries where appropriate substrates and salinities exist. (Wolotira Jr. et al. 1989; Emmett et al. 1991). It ranges from Baja California to the Alaskan Aleutian Islands (Fitch 1953; Schink et al. 1983; Cheney and Mumford 1986). The littleneck clam prefers firm gravel or clay-gravel sediments, and reaches a length of about 6 cm (Quayle and Bourne 1972; Goodwin and Shaul 1978; Bulthuis and Conrad 1995). It is often associated with butter clams (Paul and Feder 1976) and is normally found intertidally from –1.0 to 1.3 m MLLW (Chew and Ma 1987). Optimum temperatures for growth appear to be 12 to 18°C with a salinity of 24 to 31 parts per thousand (ppt); however, they may tolerate salinities as low as 20 ppt for extended periods (Quayle and Bourne 1972). Predators of the littleneck clam include oyster drills, moon snails, sea stars, octopus, rock crabs and fishes (Chew and Ma 1987). Littleneck clams are also eaten by sea otters, ducks, and other birds (Schink et al. 1983; Cheney and Mumford 1986). Recruitment is highly variable and may depend on temperature, food supply, predation, currents and appropriate substrate (Peterson 1982). Siltation caused by upland development can cause problems and dredging has been shown to affect subtidal populations (Schink et al. 1983). Similarly, severe weather can affect intertidal populations by producing high freshwater runoff that covers clams with sediment, or alternatively, washing away sediments and exposing them (Cheney and Mumford 1986).
Japanese littleneck or Manila clam (Venerupis[Ruditapes] philippinarum)
The Manila clam or Japanese littleneck is about the same size as the littleneck clam (Figure V-1), and was originally imported with seed oysters from Japan. It has become well established in the Puget Sound region and is found together with the littleneck clam, although it tends to reside at slightly higher elevations (Bulthuis and Conrad 1995). The Japanese littleneck clam is usually found between 0.9 and 2.4 m MLLW (Quayle and Bourne 1972). Optimum growth conditions are temperatures between 13°C to 21°C and salinities between 24 ppt and 31 ppt. Ideal substrate consists of gravel, sand, some mud, and shell (Anderson et al. 1982), although this species can inhabit a wide range of substrate types. Important predators include the moonsnails, rock and shore crabs, rock and English sole, starry flounder, pile and shiner perch, starfish, ducks, and scoters (Quayle and Bourne 1972; Anderson et al. 1982; Chew 1989). Spat settlement areas are dependent on currents, substrate (Chew 1989), and wave damage; extreme temperatures and siltation can adversely affect population sizes (Bardach et al. 1972; Chew 1989). Commercially, manila clams are considered better “keepers” than littleneck clams, which accounts for recent increases in commercial production and aquaculture ventures (Scholz 1990).
Butter clams (Saxidomus giganteus)
Butter clams (Figure V-1) are found predominantly in sandy and gravelly mud substrate. They range from Alaska to San Fransisco Bay and are commonly found in Puget Sound. They reach a length of about 10 cm, somewhat larger than the littleneck and Manila clams, however are less desirable to harvest commercially because of shorter shelf life. Although this species may be buried as deep as 30 cm, it is usually found much closer to the surface. It is generally found in the same tidal elevations as the littleneck clam. Predators of this species include moon snails, Dungeness crab, and sea stars (Wolotira Jr. et al. 1989).
Figure V-1. A variety of hardshell clams
Geoduck (Panopea abrupta)
The geoduck clam (Figure V-2) is one of the largest burrowing clams in the world. It ranges from Alaska to Baja California and is commonly found in Puget Sound. Prior to 1970 the clam supported a small intertidal recreational fishery in the Sound; however, legislative changes in 1969 allowed a commercial fishery to be co-managed by WDFW and WDNR (Goodwin 1990). Geoducks now support the largest clam fishery along the west coast of North America. The average commercial geoduck weighs about 2 pounds but can grow to over 10 pounds. They prefer sand and mud substrates from the lower intertidal zone to water depths of at least 360 feet (Goodwin 1990; Sizemore and Ulrich 2000). Geoduck abundance in Puget Sound is inversely proportional to the latitude, hence they are most abundant in the southern parts of the Sound (Goodwin 1990; Sizemore and Ulrich 2000). The average age of geoducks in most commercially harvested beds is between 30 and 60 years. They cannot completely withdraw their soft body parts within their shell and cannot rapidly crawl or dig to avoid predation. Instead, they develop long siphons and bury themselves deeply in the substrate, many over three feet deep. Predators of juvenile geoducks include moon snails, coonstripe shrimp, rock crabs, English, rock, and sand sole, pile perch, spiny dogfish, starry flounder and other flatfish. Sea stars and sunstars feed on juveniles and adults (Sloan and Robinson 1983; Wolotira Jr. et al. 1989). The tips of geoduck siphons are eaten by the Pacific staghorn sculpin (Andersen Jr. 1971), and adults are also excavated and eaten by sea otters. Numerous commercial geoduck tracts are located along the northern, east, and western shoreline of Bainbridge Island (Appendix A –Shellfish Occurrence Map). No tracts are located along the southern shoreline in the Rich Passage area.
Figure V-2. Geoduck clam (Panopea abrupta) (Source: WA Dept. of Ecology 2002a)
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