WHERE HAVE ALL THE MUSSELS GONE?

RHODE ISLAND AQUACULTURE INITIATIVE
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WHERE HAVE ALL THE MUSSELS GONE?
Sea star predation, pea
crabs, and mysterious
die-offs have all been
blamed for the dwindling
harvest in Narragansett Bay
By Kifle W. Hagos
Photos for this article courtesy Kifle Hagos.
RHODE ISLAND AQUACULTURE INITIATIVE
The blue mussel harvest in Narragansett Bay has been declining for the
past couple decades. The main causes
of this decrease in mussel production
are predation by sea stars and the
infestation of the mussels with pea
crabs. Sea stars move en masse, settling
on blue mussel beds. In large numbers,
sea stars can decimate the mussel bed,
leaving only empty shells behind.
Adult pea crabs live inside the
shells of blue mussels, sharing food
and space. They cause the quality of
the blue mussel to decline.Yet while
the mussel may still survive to be
harvested, the presence of pea crabs
in mussels sold for consumption can
greatly reduce market value. They
provide an appetite-killing presence
inside the mussel, looking, as one University of Rhode Island (URI) scientist
observed, “like bloated dog ticks—not
what you want to see accompanying
your pasta marinara or paella at a local
restaurant.” Two common species of
pea crabs occur from Massachusetts
to Texas, namely, oyster pea crabs and
mussel pea crabs.
Captain Michael Marchetti is a
fisherman from Point Judith, R.I. He
catches lobsters and dredges for
scallops with his 40-foot fishing boat,
the F/V Captain Robert. In the past, he
supplemented his income by harvesting mussels. In the summer of 2005
Marchetti harvested, on average, 6,250
pounds of mussels per day. On productive days he harvested up to 14,000
pounds and on not-so-productive days
he caught only 2,200 pounds, which is
below the break-even point. In addition to the declining mussel harvest,
Marchetti is also concerned about the
inefficiency of the mussel harvesting
process. It takes him longer to unload
the mussels or to get a truck to haul
them than to actually harvest the mussels. “The harvest has been economically viable, but hit or miss due to pea
crabs and lack of equipment,” he said.
Domestic mussel production
does not meet current U.S. consumer
demand, which is supplemented by
imports from Canada, New Zealand,
Chile, and China. In 2005, wild and
farmed New Zealand products made
up 55 percent of total U.S. mussel
imports. According to Seafood Business
Magazine, in 2005, the market saw
healthy growth, and imports increased
14 percent over 2004.
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This growth indicates that there is
a future for the production of mussels
in Rhode Island. The environmental
conditions provided by the shallow
coves and inlets of Narragansett Bay
are ripe for the cultivation of mussels. In the past two years, however,
mussels harvested in Rhode Island have
been infested with pea crabs. Currently, American Mussel Harvesters
Inc. (AMH), the only mussel processor
in the state, and one of the largest in
North America, gets its supplies from
the Gulf of Maine and Chatham in Massachusetts. None comes from Rhode
Island. William Silkes, AMH president,
says that his company used to import
mussels from Prince Edward Island
(PEI) in Canada, as well. But PEI mussel
prices have gone up due the weakness
of the U.S. dollar against Canadian
currency.
Marchetti is optimistic about
the future of mussels in Narragansett
Bay. He notes, “There is usually good
mussel production, as good as ‘old timers’ report. But the nature of the Bay,
closed areas, and available beds make
it suitable only for a few vessels.” He
is also hopeful that mussel culture will
supplement the wild harvest, such as
harvesting mussel seed from beds free
from pea crabs and sea stars and transplanting them to growing beds. This is
the method effectively employed in the
Netherlands, where over 80,000 metric
tons of mussels are produced annually
in an area much smaller than Narragan-
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sett Bay. But this production method
requires large capital investment. Scientists suggest that blue mussel culture
is feasible because mussels are efficient
filter feeders and exhibit more rapid
growth than wild mussels. Culture can
increase the growth rate of blue mussels as much as three- to 25-fold over
that of natural growth.
Blue mussels, which yield a higher
meat-to-total-weight ratio, are nutritionally superior to most traditional
shellfish species. The nutritional value
of blue mussels compares extremely
well with that of more traditional food
items such as beef, according to the
Dietary Guide for Americans, published in
2005. In addition to their economic and
nutritional values, blue mussels perform
an ecological function in filtering the
water in which they live. Blue mussels
and all other bivalve shellfish derive
most of their nutritional needs by
filtering particles from the water. This
action increases the clarity of coastal
waters and facilitates the removal of
nitrogen and other nutrients.
Marchetti has been involved in
test dredging for mussels for two years.
The R.I. Rural Development Council
funded his surveys of wild mussel beds
to identify the natural mussel beds free
of pea crabs and sea stars. The author
joined Marchetti twice for test dredging in the Bay at locations adjacent to
URI’s Bay Campus. During the first trip,
the mussel bed was located with the
boat’s eco-sounder, and the dredge was
lowered six times, operating four to
five minutes each time. The catch included mussels, sea stars, spider crabs,
and other benthic (bottom-dwelling)
organisms. On average, over 75 percent of the catch was live mussels, of
which about 75 percent of those were
infested with pea crabs. The second
trip took place at approximately the
same location as the previous one.
Marchetti, following the same procedure, found that all the mussels taken
were dead—six dredges full of empty
mussel shells. The catch included sea
stars, small lobsters, sponges, spider
crabs, and other benthic organisms.
Based on the large number of sea stars
in the dredge, sea star predation was
implicated in the demise of the mussels.
The amount of dead mussels was
massive and justifies regular monitoring in Narragansett Bay. A close study
of mussel beds, in relation to the
movement of pea crab swarms and
sea stars, might determine the cause
of this problem. A team from URI, the
state, and the private sector has come
together to study this setback. The
team includes URI professors John
King and Barry Costa-Pierce; David
Alves, R.I. Coastal Resources Management Council aquaculture coordinator; Silkes; and Marchetti. The seabed
survey of the Bay carried out by King
for this study was funded by the Rhode
Island Aquaculture Initiative. The team
surveyed and mapped bottom habitat
at five sites to assess the suitability of
on-bottom mussel culture in the Bay.
Another group of scientists,
among them URI’s King and Scott
Lindell, director of the Scientific Aquaculture Program at the Woods Hole
Oceanographic Institution, are also
studying ways to avoid pea crab infestation of blue mussels. They are proposing “to develop a better understanding of the timing and environmental
constraints of pea crab infestation of
blue mussels; to support the development of seed collection, grow out and
harvesting strategies that will yield
crab-free mussels; and to establish pilot
scale demonstration farms” in southern New England.
Fish and shellfish die-offs are not
new to Narragansett Bay. The media
has widely covered the mussel die-offs
of summer 2001 and the fish kill of
August 2003 in Greenwich Bay. They
have generated extensive public and
media discussion. The present mussel
decimation is not close in magnitude
to these incidents, which were created
by hypoxic (deficiency of oxygen)
conditions at the bottom of the Bay.
(See 41°N 3(1) on-line at seagrant.gso.
uri.edu/41N/ for related articles.)
The mussel infestation by pea
crabs in Narragansett Bay merits
further study. Changes in temperature,
salinity, currents, and nutrition may
impact on the prevalence of pea crabs.
Warmer winters, stormy summers,
and shortage of precipitation—all
likely linked to climate change—may
be causing long-term changes in the
Bay. Scott Nixon, URI oceanography
professor, who has extensively studied
Narragansett Bay, observes, “changes
in temperature of coastal waters may
seem small compared to day-to-day,
seasonal, and interannual variation we
all experience in air temperature; they
may be having profound effects on the
ecology of the Bay by changing the
physiological rates and the behavior of
various species.”
These studies are encouraging
and may help discover long-term
solutions. Another way of avoiding
the mussel infestation with pea crabs
may be to revert to a suspended
method of offshore mussel culture.
This may require more work and
more capital investment compared to
bottom culture. The University of New
Hampshire has studied the pea-crab
phenomenon and their experiment
has been successful. Mussel farmers in
New Hampshire are now benefitting
from the study. The same study can be
tried in Rhode Island. This may open
employment and trade opportunities
for fishermen like Marchetti and
others, and provide locally developed
blue mussels for the mussel processors.
—Kifle W. Hagos is a URI Coastal Institute
IGERT (Integrated Graduate Education
and Research Traineeship) Project Trainee
and a URI Fisheries, Animal and Veterinary
Science Graduate Student.
Warmer winters, stormy summers, and shortage of precipitation—all likely linked to climate
change—may be causing long-term changes in the Bay.