Alaska’s Purple-Hinge Rock Scallops Considered for Aquaculture Development11 June 2012
Purple-hinge rock scallops are highly prized by local communities and harvested for subsistence in coastal Alaska. To evaluate the suitability of purplehinge rock scallops for mariculture in Alaska, R. RaLonde, K. Brenner and A. C. M. Oliveira of the University of Alaska, conducted a fouryear growout study. Taken from the Global Aquaculture Advocate, a Global Aquaculture Alliance publication.
Scallop culture is a well-established
industry in some parts of the world. Japan
began investigating the potential for scallop
culture in the 1930s and started commercial
production in the mid-1960s.
Building on Japan’s success and borrowing
from the procedures developed there,
China and Chile began their own scallop
culture industries in the 1980s.
Additional small but developing culture industries exist in eastern Canada, Russia, Great Britain, France, Norway, Ireland, Italy and Spain. In spite of the success of scallop culture elsewhere in the world, efforts in the United States fail to meet domestic demand and remain limited to small-scale operations.
Purple-Hinge Rock Scallops
In the waters of the U.S. state of
Alaska, the Pacific weathervane scallop,
Patinopecten caurinus, is the only commercially
harvested scallop species. The purple-hinge rock scallop, Crassadoma gigantean,
has potential for aquaculture, and is highly
prized by local communities and harvested
for subsistence in coastal Alaska.
Purple-hinge rock scallops are found along the U.S. Pacific coast from Baja, California, northward to Alaska, although their distribution is patchy throughout the range. Unlike their free-swimming pectinid relatives, the scallops have developed an affinity for permanent attachment. in rocky substrates. This presents challenges for commercial harvest, but is a potential advantage for mariculture.
Aquaculture of purple-hinge rock scallops has been investigated in California, and while results were promising, there were barriers to implementing commercial culture operations. Research conducted in 1989 by Neil Bourne in British Columbia showed promise for mariculture in a more northerly location. Renewed interest in Alaska resulted in a study by the authors that further determined the feasibility of culturing this species.
A survey of Alaska’s shellfish-farming
industry in 1997 recognized the importance
of species diversity, and the production
of high-value shellfish became a top
priority. In January 2006, an aquaculture
planning session sponsored by the governor’s
office again supported the development
of new species as a priority.
In response, the Alaska Sea Grant Marine Program, Qutekcak Shellfish Hatchery and Alaskan Shellfish Growers Association invested in research on hatchery production and field growout trials to develop farming practices for the purple-hinge rock scallop. Native to Alaskan waters, purple-hinge rock scallops are biologically suitable for aquaculture and sell for over U.S. $3/scallop on the local market.
Hatchery production research started in 1997 with funding from the National Marine Fisheries Services. Over a twoyear period, multiple reproduction and larval-rearing trials were performed, with the first successes in 1999. In the following year, a sizable population produced enough seed to begin field growout trials. The most significant challenge to the hatchery was low survival from the pedivelliger stage to a seed size of 7.5 mm.
The Alaska Science and Technology
Foundation has funded subsequent field
growout research since 2001, with purple-hinge rock scallop seed 31.1 mm in
shell length delivered to the Pearl of
Alaska farm near Kake in southeastern
Alaska. The scallops were nursery cultured
in lantern nets, where they reached
52 mm in length. Survival during this
initial phase was density dependent, varying
from 17% at a density of 300 scallops/
chamber to 50% when 100 scallops were
stocked per chamber. For growout, the
scallops were moved to nornets and raised
to 4.3 years of age. A nornet is a production
unit composed of a “stack” of rigid
perforated disks separated by about 10
cm and encased in netting. Scallop seeds
are placed on each disk.
Nornets were used because as scallops grow, they incorporate the substrate into their bottom shells. With lantern nets, the netting would be grown into the shells. Removing scallops from the net would require destroying the nets. The nornets’ chamber flooring of thick, perforated plastic prevents scallops from growing into the webbing, simplifying sampling.
Measurements of the scallops’ adductor muscles were taken at ages 2 and 4 years (Table 1). Shell growth for the four-year period is shown in Figure 1.
Purple-hinge Rock Scallop Shell Growth Through Four Years of Growout.
While purple-hinge rock scallops are
locally prized, they would be a new species
for the seafood market – where quality
assessment is necessary to promote
acceptance. Ten adult purple-hinge rock
scallops were harvested from pens at Elfin Cove Oysters in Port Althorp on
the north end of Chichagof Island in
southeast Alaska and shipped live to the
Fishery Industrial Technology Center in
Kodiak. The scallops were held at 4° C,
and processing was carried out within
two days of arrival.
The wet weights of whole scallops, shells, gonadal-visceral (body) tissues and adductor muscles were measured using an electronic digital scale. Shell length, width and depth, and adductor muscle length and width were determined using a cloth tape measure and electronic digital calipers.
The scallops were shucked after whole weights were recorded. The adductor muscles were individually vacuum packed and stored at -30° C until analysis. A muscle condition index (MCI) assessing the physiological condition of the adductor muscle relative to total soft tissue was calculated for the samples according to the formula MCI (%) = weight of adductor muscle/weight of total soft tissues x 100.
The purple-hinge rock scallop is a large species with massive shells and large adductor muscles (Table 2). The range of the 10 specimen whole weights was 561 to 1,202 g. Shell heights ranged from 138 to 189 mm. Yields of edible adductor muscle were less than 10%. However, MCI values were between 36.0 and 50.9.
The results reflected observations made by other researchers on the size and condition of purple-hinge rock scallops. Individuals can reach shell heights of 250 mm, and the shells become massive and heavy. David Leighton reported purplehinge rock scallop adductor muscle weights from 50 to 80 g, and muscles at 30 to 55% of soft body weight, reported in this study as MCI. In comparison, weathervane scallops, which can also reach shell heights of 250 mm, typically have adductor muscles in the 27- to 48-g range and yields of 10 to 12%.
The adductor muscle and digestive
gland are the primary storage sites of
nutrient reserves in scallops. Carbohydrate,
in the form of glycogen, and protein
are stored in the adductor muscle,
while lipid is stored in the digestive
gland. The gross composition of rock
scallops is shown in Table 3.
Purple-hinge scallop adductor muscle is lean due to its low lipid content. Lipids in the muscle are mainly composed of phospholipids. Furthermore, purple-hinge scallop adductor muscle is an excellent source of high-quality marine protein. With regards to flavor, glycogen, together with a variety of amino acids, impart desirable sweetness to the product.
The purple-hinge scallop is a specialty
seafood item that provides an opportunity
for shellfish growers in Alaska to diversify
their farm production. The scallops can
easily fit in with suspended oyster farming.
The low water temperatures in Alaska are not conducive to fast growth of shellfish, but the pristine water quality yields excellent product quality. Because of scallops’ long growout period to market size, research continues to develop growout techniques that require minimal labor for farmers.