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Seafood FAQ: Don't let sales ebb when red tide hits
Educate consumers about the causes and effects of this natural event
May 01, 2005
To the average beachgoer, red tide — a naturally occurring phenomenon that
affects the nation’s shores year-round — is nothing more than a nuisance. But
to seafood restaurateurs and retailers, red tide can be bad for business.
When state officials are forced to close shellfish beds to harvesting due to
red tide, buyers can find it difficult and expensive to get hold of certain
items, particularly clams, mussels and oysters. But addressing consumers’
inquiries about red tide without losing business can be even trickier; it’s a
subject scientists still don’t understand completely. Here are answers to a few
Q. What’s red tide?
Ride tide is a generic name for a harmful algae bloom. An HAB is caused by a
population explosion of phytoplankton, or algae, microscopic plants at the base
of the marine food chain. When phytoplankton reaches a certain density, the
water can appear red, hence the term “red tide.” The water can also look green,
yellow or brown, depending on the algal species.
Among the thousands of algal species are more than 100 that produce toxins,
which are ingested by filter-feeding mollusks such as clams, mussels and
oysters. Filter feeders can become toxic before algal concentrations are high
enough to discolor the water, and red tide does not alter the appearance of
Q. What causes red tide?
The right mix of ocean conditions, including low salinity, high nutrient
concentrations, warm surface water and calm seas, causes algae to grow rapidly,
according to the Woods Hole Oceanographic Institution in Woods Hole, Mass. Red
tide often occurs when an extended period of sun follows an extended period of
Q. Is red tide predictable?
No. “I can be fairly sure that southwest Florida will get a red tide each
year, but exactly when and where blooms occur is unpredictable,” says Cynthia
Heil, senior research scientist for the Florida Fish and Wildlife Research
Institute’s HAB program in St. Petersburg, Fla. “We have blooms of [the algal
species] Karenia brevis nearly every year between the Tampa Bay and Naples
regions. Southwest Florida is one of the [nation’s] red-tide hotspots.
Q. Is seafood safe to eat during red tide?
Filter-feeding mollusks such as clams, mussels and oysters harvested from
waters affected by red tide are not safe to eat. Cooking, smoking or freezing
does not destroy the toxins.
But crustaceans such as crabs, lobsters and shrimp and finfish from affected
waters are safe to eat because they do not accumulate the toxins.
Q. What happens if toxic mollusks are consumed?
Eating toxic mollusks can cause humans to develop one of four types of
shellfish poisoning: amnesic (ASP), paralytic (PSP), neurotoxic (NSP) and
ASP is named after one of the ailment’s most severe symptoms, short-term
memory loss, but is characterized by both neurological and gastrointestinal
symptoms, which usually surface within 48 hours and 24 hours, respectively, of
consuming contaminated mollusks. ASP is caused by Pseudo-nitzschia multiseries
and P. australis, which occur off New England and the West Coast. It can be
PSP can also be life threatening, but its symptoms are solely neurological,
including ataxia, drowsiness, fever, rash and tingling, numbness or burning
around the mouth. In severe cases, respiratory arrest can occur within 24
hours. PSP is caused by Alexandrium spp., which also occurs off New England and
the West Coast.
NSP is caused by K. brevis, which occurs in the Gulf of Mexico and,
occasionally, in the Atlantic from North Carolina to Florida. NSP is marked by
both neurological and gastrointestinal symptoms, but people don’t have to eat
toxic mollusks to develop respiratory symptoms. Outbreaks of K. brevis, like
the abnormally large one that stifled southwest Florida late this winter and
early this spring, cause beachgoers to cough and wheeze. NSP is not fatal.
Nor is DSP, which is characterized by gastrointestinal symptoms that can
occur within 30 minutes of eating toxic mollusks. DSP is caused by Prorocentrum
lima, which has been reported off northern New England. Cases of DSP are rare
in North America.
Q. Are cases of shellfish poisoning common in the United States?
No. “Occurrences are extremely rare, because states where they occur have
effective monitoring programs,” says Heil. “In Florida, we close commercial
shellfish beds to harvesting when concentrations of K. brevis reach 5,000 cells
per liter and open them only when the toxins in shellfish meat fall below a
federally regulated level that is acceptable for human consumption.” (The
tolerance level is 80 micrograms per 100 grams of raw edible meat.)
“It’s illegal harvesting that causes most occurrences of ASP, PSP, NSP and
DSP in the United States,” she adds.
Q. What do I tell my customers when they ask about red tide?
That’s up to you. Some restaurateurs and retailers prefer to ignore red tide
to avoid the risk of scaring customers away.
But others, like Bill Deacon, owner of Foster’s Seafood and 33 American
Bistro in Scottsdale, Ariz., opt to address the topic.
Deacon researched and wrote about the causes and effects of red tide in his
biweekly e-mail newsletter on March 31 after one of his 16,000 readers asked,
“Ever heard of red tide?” If asked, he explains to diners why he’s forced
occasionally to remove a menu item or raise its price.
“I like being completely candid with my customers,” says Deacon, who menus
New England softshell clams, Prince Edward Island mussels and Blue Point, N.Y.,
“People are good about it. They understand that [those mollusks can be hard
to buy because] we’re 3,000 miles away from the source.”
Matt Asen, executive fishmonger of the Prawnbroker Restaurant Group, doesn’t
shy away from answering diners’ questions about red tide, either. Five of his
10 Florida restaurants — The Prawnbroker, The Timbers, University Grill,
Sanibel Grill and Matzaluna — are in Ft. Myers and Sanibel, at the southern tip
of Florida’s recent K. brevis outbreak.
“We tell people that it’s a naturally occurring
event,” says Asen. “We assure them that none of the seafood we buy is from