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Background While domoic acid (DA) poisoning was first recognized in an outbreak on Prince Edward Island, Canada, most of the known toxic events since that time have occurred on the U.S. west coast. DA was first implicated in the illness and death of brown pelicans and Brandt’s cormorants in Monterey Bay, California in 1991. About one month following the toxic bloom in California, levels of DA above the regulatory limit of 20 µg/g shellfish tissue were found in the edible parts of razor clams (Siliqua patula) and Dungeness crabs (Cancer magister) on the Washington coast. In 1998, impacts of DA to the health of marine life and to the fisheries economy were documented in several regions along the west coast. In particular, California sea lions (Zalophus californianus) in central California were severely affected by DA poisoning and high levels of toxin in razor clams in Oregon and Washington resulted in beach closures for more than a year and a half . In the study region, beach and harvest closures resulting from the toxigenic Pseudo-nitzschia blooms have a severe economic impact on both coastal economies and on tribal communities. In 1991, the closure of Washington State beaches to recreational and commercial shellfish harvesting resulted in a $15-20 million revenue loss to local fishing communities. The commercial Dungeness crab industry on which Washington’s Quileute tribe depends for employment lost 50% of their income in 1998 due to harvest closures. The entire razor clam harvest of the Quinault tribe, on which they depend for both subsistence and commercial revenue, was also lost in the fall of 1998. With sufficient warning, tribal fishers could seek alternative buyers for eviscerated crab, and shellfish managers might have longer lead times to schedule closures. Moreover, mitigation strategies to reverse or moderate cell toxicity may soon be available.
The survey of DA along the entire U.S. west coast continental shelf in summer 1998 also suggests a strong relationship between DA concentration and mesoscale topographic features. Off northern California where large coastal promontories and hence rapid offshore transport occur, DA levels are low. Offshore of the Strait of Juan de Fuca, over Heceta/Stonewall Bank in Oregon, offshore of Monterey Bay (inshore of the Farallone Islands) and near the Santa Barbara Channel, DA levels are higher. One common characteristic of these regions of high DA is their capacity to have longer retention times than other coastal regions. Our study will focus on toxic Pseudo-nitzschia bloom initiation and transport in one such west coast retentive feature for which sufficient data have been collected to formulate a working hypothesis; namely, the Juan de Fuca eddy, a prominent oceanographic feature off the northern Washington coast. Recent studies suggest that the seasonal Juan de Fuca eddy is an initiation site for toxic blooms of Pseudo-nitzschia that impact shellfish on beaches along the Washington coast. This eddy may serve as a “bioreactor” for the development of high cell densities of phytoplankton, especially diatoms. In ship of opportunity studies, Pseudo-nitzschia comprised 4-99% of the > 20 µm phytoplankton assemblage and cells contained DA ranging from 50 to 2000 pg/cell in the vicinity of the eddy.
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As
seen in the figure on the right, a survey of DA along the entire U.S.
west coast continental shelf in summer 1998, both the species of Pseudo-nitzschia
(including P. multiseries, P. australis and P.
pseudodelicatissima) and the relative levels of toxicity vary in
time and space along the west coast. Moreover, it is not uncommon for
potentially toxic Pseudo-nitzschia cells to be present without
detectable DA.
Measurements
made during cruises and beach sampling of seawater and shellfish in
1997 and 1998 are all consistent with the possibility that during some
years DA from this eddy appears to move southward in prolonged upwelling
events and then onshore during the first major storm of the fall season,
where it results in high levels of DA in razor clams on coastal beaches.
For example, the figure on the left shows Pseudo-nitzschia
species cell counts , particulate DA in seawater, and DA in razor clams
from July through October 1998 at Kalaloch Beach (upper panel). North-south
component of surface currents at a nearby mooring during the same time
period is also shown (lower panel). In this region, southward flow (negative)
is usually an indication of winds toward the south and upwelling; northward
flow (positive) is indicative of winds blowing toward the north and
downwelling (i.e., a storm). During upwelling, winds move surface layers
offshore; during downwelling, winds move surface layers toward the coast.
This figure suggests that an increase in Pseudo-nitzschia and
particulate DA occurred after a period of persistent upwelling (southward
and offshore water movement) followed by a storm (northward and onshore
water movement). DA in razor clams began to increase after particulate
DA in seawater increased. Such events result in immediate closure of
coastal clamming beaches, often for the entire season (or longer, due
to the slow depuration of DA from razor clam tissue).