Animal Molluscs
Family: Psammobiidae
Classification: Regulated
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Varnish
clam found in Willapa Bay. |
The mahogany clam
has been observed in northern Puget Sound for about ten years now. The clam
apparently was introduced via ballast water from Asia. It tends to inhabit the
upper one third of the intertidal zone, decreasing in the middle and lower intertidal
zone. However, on beaches where no other clam species were present it ranged
into the lower tidal levels. The varnish clam is more freshwater tolerant than
other species, and the most abundant populations are found in areas where there
are significant influxes of fresh water runoff. The clam has rapidly established
densities as high as 1,780 clams per square meter in the southern portion of
the Georgia Straits, and densities exceeding 800 per square meter on some beaches
in the San Juan Islands. The clam is now found throughout Puget Sound, and
may be harvested as part of the 40 clam limit.
Subject: Nuttalia obscurata
Taxonomy: Nuttalia obscurata, Soletellina obscurata, Psammobia olivacea,
Nuttallia solida, and was once thought to be a subgenus of Sanguinolaria (Gillespie
et al., 1999)
Common names: Purple varnish clam,varnish clam, or dark mahogany clam
The purple varnish
clam Nuttallia obscurata was first observed in British Columbia waters
in the early 1990’s. The most likely means of introduction are larval
or juvenile release in ballast water, or as fouling organisms transported on
the hulls of vessels. By the late 1990’s the clam was well established
throughout the Georgia Strait, had spread into north Puget Sound and the Strait
of Juan de Fuca, and along the west coast of Vancouver Island. In 1999 the clam
was found at Nehalem Bay State Park in Oregon. This is thought to be a separate
introduction. The clam now occurs in most Oregon estuaries. In February of 2002
a single live specimen was found on the southern Washington coastline, in Willapa
Bay.
Both Oregon and
British Columbia have established recreational fisheries for the clam, and in
Washington they have been classified as shellfish as of 5/1/04 and may be harvested
as all or part of the 40-clam per day limit. A few were commercially marketed
in British Columbia in 1998. This was halted pending further study regarding
human health risks and the status of the resource under the government’s
Pacific Fisheries Adjustment and Restructuring program. The program utilizes
a rigorous three-phase scientific review process for developing new fisheries.
The first phase
requires collection and review of available information on the species, and
identification of any information gaps that may limit assessment and development
of a potential management framework. Fisheries and Oceans Canada (DFO) completed
this phase in 1999. In 2000-2001 funding was provided for the second phase,
consisting of detailed biological assessments and distribution surveys. DFO
researchers worked with Aquametrix Research, the Community Futures Development
Corp. of Powell River, and clam diggers from the Area C Clam Harvesters Association
to conduct population dynamics studies. Their research was designed to determine
the effect of varnish clams on beaches in the Gulf of Georgia, potential competition
issues between Varnish clams and other species (in particular commercially harvested
Manila clams), and harvest characteristics. Marketability studies have also
been conducted. Twenty thousand pounds of clams were harvested under a scientific
license and marketed as the "Savoury Clam." These studies are complete,
and the results provided to the Pacific Scientific Advice Review Committee for
review and recommendation in January of this year.British Columbia is ready
to proceed with phase three, the implementation of a monitored small-scale commercial
fishery.
The opinion of
the Aquatic Nuisance Species Committee has been solicited regarding the development
of this fishery for the Varnish Clam. This opinion paper considers the Canadian
stock assessments, the work of University of Washington researchers at Friday
Harbor, and the opinions of the Washington Department of Health, shellfish growers,
and others.
The purple varnish
clam, or varnish clam, derives its name from the shiny brown coating (periostracum)
on the outer surface of the shell. The interior surface of the shell is usually
a uniform shade of purple. Occasionally small specimens will have purple coloring
showing on the outer surface. The clam is also sometimes referred to as the
purple or dark mahogany clam. The scientific name is Nuttallia obscurata,
although it has in the past been identified as Soletellina obscurata, Psammobia olivacea, Nuttallia solida, and was once thought to
be a subgenus of Sanguinolaria (Gillespie et al., 1999). The clam is
one of three Nuttallia species native to Japan, Korea and possibly China.
There has been little research done on the clam in Asia, and it is not commercially
marketed there (Mills, 2000).
The varnish clam
is nearly oval shaped, with the posterior end slightly longer and more angular.
The shell is relatively flat, with a large external hinge ligament. The clam
has a large foot and long unfused siphons, indicative of a burrowing species
able to live deep in the substrate. There is some uncertainty as to how large
it may grow. The nearest related species on the west coast, Nuttallia nuttalli found in southern and Baja California, reportedly grows to a maximum length
of 150mm. The largest live N. obscurata reported locally to date was
68mm (a little over 2.5 inches) total length and weighed 45.1 grams (Gillespie
et al., 1999). However, Claudia Mills reports finding a shell at least 75mm
in total length at Orcas Island in 1999.
It is difficult
to develop an accurate size to age relationship in varnish clams by counting
the growth rings. These may be affected by density and tidal elevation as well
as by substrate consistency, which can create additional rings due to disturbances
in growth. Therefore, local studies have relied on interpretation of length
and frequency distributions. Research suggests the clam attains a length of
38 mm in about four years, a rate similar to that of the Manila clam (Gillespie
et al., 1999). There are some indications that the clam may grow more rapidly
in Puget Sound (Fidalgio Bay) than in the upper regions of the Georgia Straits.
Like the Manila
clam (Venerupis philippinarum), varnish clams are broadcast spawners,
with a planktonic larvae period. The rapid expansion of populations in the Georgia
Strait and along the Oregon Coast suggests a fairly long planktonic period,
which may result in broader dispersal. Recruitment surveys in British Columbia
indicate reproduction occurs between early spring and summer (Gillespie et al.,
1999). The British Columbia surveys, and density surveys done at Argyle Lagoon
by Dr. David Duggins (University of Washington Friday Harbor Labs) indicate
high settlement rates and/or a very low mortality rate in this region. In the
N.W. Pacific the clam reaches maturity at one year of age, and spawns each year
(Hushando, as cited by Gillespie, et al., 1999). There is every reason to expect
that the varnish clam could become as ubiquitous as the Manila clam in the Pacific
Northwest.
Varnish clams are
capable of both filter (suspension) feeding and deposit feeding. Laboratory
observations indicate that the clam tends to extend its siphons and filter feed
at night. When deposit feeding the clam extends its foot and rotates it, collecting
deposit materials, which are passed into the oral region when the foot is retracted.
Detritus is also picked up and passed into the oral region when the foot is
used for locomotion. This dual feeding capability may enable the clam to outcompete
other species living in the same intertidal zone. The most commonly encountered
clams in the region are the manila clam (also a nonnative species), and native
littleneck (Protothaca staminea), butter clam (Saxidomus gigantea),
Nuttall cockle (Clinocardium nuttallii), horse clam (Tresus spp.),
eastern softshell (Mya arenaria) and macoma species such as the bent-nose
(Macoma nasuta) and the polluted clam (Macoma irus).
The purple varnish
clam has rapidly established densities as high as 1,780 clams per square meter
in the southern portion of the Georgia Straits (Gillespie et al., 1999), and
is expanding into northern beaches. Expansion southward into Washington waters
has been somewhat slower, but it has reached densities exceeding 800 per square
meter on some beaches in the San Juan Islands (Dr. David Duggins and Dr. Paul
Dinnel, personal communication). The clam is found frequently now on beaches
in Admiralty Inlet and in Hood Canal as far south as Potlatch State park. It
is found in substrates ranging from cobble to muddy sand (Mills, 2000) up to
30 cm. subtidal depth. The majority of surveys indicate the greatest abundance
occurs in the upper one third of the intertidal zone, decreasing in the middle
and lower intertidal zone. However, on beaches where no other clam species were
present it ranged into the lower tidal levels (Gillespie, et al., 2002). The
varnish clam is more freshwater tolerant than other species, and the most abundant
populations are found in areas where there are significant influxes of fresh
water.
With the exception
of the manila, which is found in the lower reaches of the high intertidal zone,
most of the local species are found in the mid to low intertidal zone. The manila,
which is a non-native species that was accidentally introduced with Pacific
Oyster seed, has become a primary fishery in both British Columbia and Washington.
The overlap between manila clams and varnish clams is therefore a matter of
concern. To address this concern, British Columbia researchers conducted controlled
field experiments with the two species over a period of five months. Growth
and survival rates in areas occupied by both species were compared in a variety
of situations. The varnish clam had a slight advantage over the manila in the
high intertidal zone, possibly because of its bimodal feeding abilities. Growth
in length was inhibited 47% in the manila clams, and 32% in the varnish clams.
Weight gain was inhibited 34% in the manila and 26 % in the varnish clam. However,
in the mid intertidal zone varnish clams did not gain weight at all when manila
clams were present, while the impact on the manila clam was about the same as
in the high intertidal zone (Gillespie, et al., 2002). These studies are not
conclusive, because the varnish clam does not exhibit the same high correlation
between growth in length and weight as the manila does, and the varnish clam
may put more resources into gonad development at certain times of the year.
Lastly, the densities between treatment effects in the study were not consistent
- combined densities were greater than when either species was alone. The average
density of varnish clams alone was 800 per square meter, while manila clams
averaged 400 per square meter, and the combined densities were at 1,200 per
square meter. The data would seem to suggest that the varnish clam does establish
itself in areas where other species are already present. Further research is
needed to determine whether other species, such as the manila, would have difficulty
re-establishing populations after being harvested if the varnish clam moves
into the area.
About 35% of varnish
clams greater than 45mm in length are host to immature native pea crabs (Pinnixia
faba). The infestation rate is lower in smaller varnish clams and in other
clam species. This has two negative marketing implications. Consumers that see
the crabs may find them unappetizing. A greater concern would be that a consumer
who is allergic to crustaceans might unknowingly consume them and become ill.
British Columbia growers unsuccessfully tried a variety of purging methods to
rid the clams of the crabs.The varnish clam (like the cockle, littleneck, horse
and manila clams) is susceptible to predation by Lewis' moonsnail (Polinices
lewisii). Gulls, crows, and black oystercatches have also come to recognize
them as prey (Gillespie, et al., 1999). Dr. Paul Dinnel (Dinnel Marine Research)
conducted some tank experiments, and found that dungeness crabs easily pry the
shells open and eat varnish clams. Research conducted by James Byers of the
University of Washington also indicates that the species suffers high mortality
from crab predation when it is unable to find loosely packed substrate where
it can readily bury itself. The clam’s ability to live relatively high
up in the intertidal zone may limit predation by most species of crab. On the
other hand, it is possible that predation by crab and moonsnails may play a
role in limiting recruitment in mid and lower intertidal zones. However, in
the lower intertidal areas even being able to bury itself did not offer adequate
protection from crab predation. It appears that native predators may be excluding Nuttallia obscurata from areas inhabited by some native species. However,
there is not enough known about the interaction between the varnish clam and
other species to accurately determine the role this interaction plays in mediating
community level impacts or the success of invasion.
Limited test harvests
were conducted in British Columbia during the summer of 2001 to determine size
distribution and to evaluate harvest efficiency and breakage. The average length
of the clams harvested was 41mm, and the average weight 13.7 grams. Standard
equipment was used to collect the clams and place them in 5 gallon buckets.
The harvested clams were checked for breakage, weighed and measured. The harvested
plot was then hand excavated and screened to remove remaining clams, and determine
total breakage during harvest and harvest efficiency. Efficiency was estimated
to be between 60% and 80%. Total breakage was estimated to be less than 2%,
although this can be expected to vary depending upon substrate type. Breakage
is higher when large gravel or rocks are present than in fine gravel or sand.
Higher breakage and lower efficiency can be expected under normal harvest conditions
when harvest may occur at night, time is limited, and the crew may have less
experience.
The clams can be
purged to acceptable levels of grit accumulated by deposit feeding in twenty-four
hours. However, no method has been found to purge the clams of pea crabs. Careful
selection of harvest sites and harvesting before the clams get over 40mm in
length may be the only way to minimize pea crab infestation. The minimum harvest
size for manilas in Washington is 1.5" or 38mm. Targeting smaller varnish
clams could lead to incidental harvest of undersize manilas. It is also unknown
whether a smaller clam will be as marketable.
Varnish clams in
dry storage retain water better than manila clams, and have a longer shelf life.
According to Dr. Paul Dinnel, the clams can be dry stored in a refrigerator
for up to 30 days with no significant mortality. Clams held in wet storage (sub-tidal
or in a tank) increased their weight by almost 9% over a 9-day period (Gillespie,
2002). These factors, coupled with the overall aesthetic attractiveness of the
clam contribute to a good potential fishery.
The clams were
featured in an “Iron Chef” demonstration at a food show in Vancouver
B.C., with good response from persons attending. There was also a trial market,
through one processor, in restaurants in the Vancouver and Victoria area. The
response was favorable, in spite of fluctuations in product availability. The
clams were sold to the restaurants at a price equal to higher than that of manila
clams.
However, when a
trial wild harvest was conducted, processors were reluctant to develop a market
without assurance of a consistent supply. Prices offered were less than 60%
of the price for manila clams (Gillespie, 2002). Clearly, the economic potential
has not been fully evaluated, and a number of management questions remain unresolved.
Theoretically,
size limits may help limit crab infestation, and contribute to an ongoing harvest
over the years. However there is inadequate information available to accurately
determine an appropriate size limit, or how effective setting a limit may be.
Setting a limit similar to that set for manila harvest may be a method that
would allow combined harvests. However, it remains unknown what the impact will
be on manila clams in areas where both species are harvested together. In overlapping
areas it may be possible to harvest the manila without disturbing the deeper
varnish clams. If this occurs, will the varnish clams flourish and create a
negative impact on future manila harvests? The proposed small-scale limited
fishery proposed should provide answers to these and other management questions.
The fact that the
purple varnish clam is a bi-modal feeder raises human health concerns. Biotoxin
events currently tend to be related to seasonal algal bloom activity. However, Alexandrium catenella is known to form an encysted resting stage that
settles to the bottom during cooler winter months. These deepwater areas may
serve as seed beds that occasional upwelling or storm activity may disturb and
carry back up into the photic zone to create offshore plankton blooms. It is
thought that such a seed bed may exist beyond the mouth of the Strait of Juan
de Fuca.
A study of the
toxicity in resting cysts of the red-tide dinoflagellate Gonyaulax escavata from deep water sediments off the coast of Maine (Dale, et al 1978) found the
cysts to be approximately ten times more toxic than the motile stages. There
is some concern that cysts could be ingested by shellfish, particularly by deposit
feeders. This would explain reports of PSP that occur when there are little
or no motile dinoflagellates observed in the water, and times when toxification
levels far in excess of maximum published rates have been observed.
Although Claudia
Mills has collected varnish clams from San Juan and Samish Bay at PSP monitoring
sites in recent years, there is very little data available. In 1998 varnish
clams were collected for testing in an area in B.C. where a PSP bloom had occurred,
and sea mussels (Mytilus trossulus) used for monitoring peaked at 960
micrograms per 100 grams of weight. Although the clams never exceeded the action
level of 80 micrograms of PSP toxin per 100grams, there were indications of
higher uptake. The clams were tested twice, once when sea mussels measured at
870Fg/100g and once when the sea mussels measured 210Fg/100g. Varnish clams
tested at 77Fg/100g and 62Fg/100g respectively. This was a markedly higher uptake
than that of manila clams, which tested at 44Fg/100g the first time and less
than 42Fg/100g the second time (Northwest Fisheries Science Center, 1998). Tim
Determan, from the Department of Health Office of Food Safety and Shellfish
Inspection, felt that the facts taken together suggest that varnish clams have
the potential to contain toxins year around, and that more vigilance by DOH
biotoxin monitoring staff may be necessary.
The fact that the
most abundant populations of the clam are found in areas where there are significant
influxes of fresh water may create other human health issues. At certain times
of the year these sites may include stormwater runoff. In developed areas stormwater
frequently contains fertilizers, pesticides, oil and grease, and heavy metals
such as copper zinc and lead, as well as pet waste. Even relatively undeveloped
areas may have considerable animal waste washed into them by runoff. These waste
products contain bacteria such as fecal coliform and fecal streptococcus, and
may contain viruses as well. Bacterial contamination has resulted in shellfish
contamination and closed beaches in many areas of the state. Further study is
necessary to determine whether varnish clams, due to their propensity to settle
in runoff areas and their bimodal feeding habits, may ingest unsafe levels of
bacteria from the sediments before the contamination is high enough to impact
other shellfish and overall water quality.
Interviews with local shellfish
growers indicate that they are not concerned about the varnish clam becoming
an aquatic nuisance species. Member states of the Western Regional Panel have
taken a stance that the development of a fishery is not an acceptable management
tool for aquatic nuisance species. However, the clam has already become well
established in British Columbia and throughout Puget Sound, and it is unlikely
that any sort of control effort could ever eradicate it.
Dense populations
in Washington waters are presently confined to a relatively small area. However
they are being found more and more frequently on beaches in Admiralty Inlet,
North Puget Sound, and in Hood Canal. There may be populations building along
our coastal beaches as well, a live varnish clam was found in Willapa Bay recently.
More than one researcher has suggested that developing a fishery now while the
distribution is limited might fish it out, or at least minimize further spread.
As of May first 2004 the species is classified as shellfish under WAC
220.12.020 and may be harvested as all or part of the 40-clam per day recreational
limit. It is unlikely, for a number of reasons, that WDFW will create a commercial
fishery for the varnish clam at any time in the near future. The director would
have to recommend to the Fish and Game Commission that limits be set, and a
cooperative management plan for the fishery would have to be developed with
the Tribes. There is also the concern that if a fishery were developed, the
public may be inclined to spread the organism for purposes of enhancing the
fishery. However, the clam usually occurs so high up in the intertidal zone
that the average individual clamming for other recreational species is not likely
to find them without publicity and public education efforts.
Considering the fact that the mussel has spread throughout Puget
Sound far more quickly than anticipated , the question of a commercial fishery
is one that may need to be addressed at some time. If or when it is, Washington’s
system for developing an experimental or emerging commercial fishery differs
somewhat from Canada’s.
The director of
the Department of Fish and Wildlife may declare an emerging commercial fishery
by rule. He then appoints a five-person advisory board representative of the
affected fishery to review available information and make recommendations on
rules relating to the number and qualifications of the participants eligible
for permits under RCW
77.70.160. Participants provide the agency with harvest information. This
method provides scientific information necessary to manage an emerging fishery
while preserving and protecting the resource. After five years of operating
an experimental fishery, the director must report on the fishery and make recommendations
regarding its future operation to the appropriate legislative committees.
Conversely, the
director may make it a trial fishery and not limit the number of participants.
In this case he may classify the species by rule as a food fish or shellfish.
He may then issue trial fishery permits to holders of commercial fishing licenses.
Applications for trial commercial fishery permits specify the area to be fished,
and the methods to be used. A trial commercial fishery may be re-designated
as an emerging or expanding commercial fishery, or it may be closed at any time
for conservation reasons (WAC
220-88-040).
I thank Graham
E. Gillespie, Fisheries and Oceans Canada, for providing the data from the second
phase of fishery development research, which has yet to be published, and for
granting permission to cite both research documents. I also wish to thank Valerie
Anderson (Research Apprentice, University of Washington School of Marine Affairs),
Claudia Mills (Friday Harbor Labs, University of Washington), Tim Determan (Office
of Food Safety and Shellfish Protection, Washington State Department of Health),
Dr. Paul Dinnel (Dinnel Marine Research), and Dr. David Duggins (Friday Harbor
Labs, University of Washington) for sharing information regarding varnish clam
distribution and ecology in Washington.
Byers, James,
2001. Physical Habitat Property Mediates Biotic Resistance to Nonindigenous
Species Invasion. Oral presentation at the International Conference on marine
Bioinvasions, April, 2001.
DFO, 1999, Varnish
clams. DFO Science Stock Status Report C6-13 (1999).
Gillespie, G.E.,
B. Rusch, S.J. Gormican, R.Marshall, and D. Munroe, 2002. Further Investigations
of the Fisheries Potential of the Exotic Varnish Clam (Nuttallia obscurata)
in British Columbia. Pacific Scientific Advice Review Committee Working Paper
I2001-09. (Unpublished).
Gillespie, G.E.,
M. Parker, and W.Merilees, 1999. Distribution, Abundance, Biology and Fisheries
Potential of the Exotic Varnish Clam (Nuttallia obscurata) in British
Columbia. Canadian Stock Assessment Secretariat Research Document 99/193.
Mills, C.E.,
1999-2000. Nuttallia obscurata, the purple varnish clam or the purple
mahogany-clam. Electronic internet document available at: http://faculty.washington.edu/cemills/Nuttallia.html.
Published by the author, web page established March 1988, last updated 23
August, 2000.
NOAA - Northwest
Fisheries Science Center, 1998. Test Results Indicate Varnish Clams are Safe.
West Coast Marine Biotoxins & Harmful Algal Blooms Newsletter. Electronic
internet document available at: http://www.nwfsc.noaa.gov/publications/newsletters/index.cfm
Puget Sound Gillnetters
Association, 2001. New clam appears on Oregon coast.
Sea Grant, 2001.
MIST Aquatic Non-native Species Update.
Prepared for the Aquatic Nuisance Species Committee
by Pamala Meacham, Washington Department of Fish and Wildlife
For more information on fish management issues,
please contact WDFW Fish Management Program.
E-mail: fishpgm@dfw.wa.gov
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