Yesterday's Island Today's Nantucket
Volume 39 Issue 13 • July 30 - Aug 5, 2009
now in our 39th season

Real vrs. Perceived Dangers:
Swimming with Sharks

by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station

This past week, scenes from the movie “Jaws” have been flashing in my mind for a variety of reasons, which will hopefully become evident throughout this article.  Even on the most idyllic summer day on Nantucket, natural hazards from ticks to mosquitoes to greenheads to rip currents can plague the summer visitor.  Our awareness of these hazards and the efforts we go to avoid or mitigate them can ruin a perfectly good day.  My goal today is to discuss two dangers associated with Nantucket and illustrate how one is simply a perceived danger and the other, is truly a threat. In this instance, the scary looking creature of nightmares and late night television is the least of our worries.

The recent rebound in grey seal (Halichoerus grypus, meaning "hooked-nosed sea pig") numbers, from almost complete localized extirpation (extermination) to levels which seem high to present day human populations has caused many people to worry about a relatively rare predator, the great white shark, or Carcharodon carcharias.  Carcharodon comes from the Greek words “karcharos,” which means “sharp or jagged,” and “odous,” which means “tooth,” certainly an appropriate name.  Great white sharks have 300 teeth and can grow to be 20 feet long and weigh up to 2 tons. These sharks have pointed snouts, spindle-shaped bodies, and large gill openings.  The first dorsal fin is large, high, erect and angular or somewhat rounded.  The second dorsal and anal fins are minute.  The caudal peduncle (area where tail fin is attached) has one or two distinct keels. Great whites exhibit countershading, which means they are dark and a bit mottled on top in order to blend in with the dark water and sand below and white on the underbelly to minimize their silhouette from below (slimming camouflage!).  Great whites are an apex predator which tend to favor warmer waters year-round but can be seen occasionally in New England waters during late spring, summer, and fall and can even hang out into early winter.  In Massachusetts we see a variety of shark species including blue, mako, angel, basking (huge sharks often mistaken for great whites), dusky, porbeagle, sandbar, sand tiger, smooth and spiny dogfish, thresher, sharpnose, tiger and hammerheads (relatively rare tropical species), and the iconic great white shark.

This time of year, the white shark sighting start to increase, some of them hoaxes and jokes, some of them real.  Last week, I received a tip that a very small (approximately three-feet-long) juvenile shark that might be a great white washed up near Smith’s Point.  Last summer, in mid July, a 6.5 foot long young female great white shark washed up onto the south side of Nantucket near Sheep Pond Road.  The Division of Marine Fisheries verified that this was indeed a great white shark.  This was the first time in 21 years that a C. carcharias had washed ashore in Massachusetts.  Dr. Greg Skomal, a Massachusetts Division of Marine Fisheries (DMF) biologist and Martha's Vineyard resident, is the Commonwealth’s resident shark expert and he spends a fair amount of time providing factual information aimed at calming ocean users’ fears.  Some of his colleagues and graduate students have used the UMass Field Station as a base to attend billfish and shark tournaments to obtain stomach content samples in order to learn more about shark and apex fish prey species.  Dr. Skomal has also done an extensive amount of research to look at shark migration in May and June along the Atlantic Seaboard.  Massachusetts is the northernmost limit for several migratory shark species from porbeagles to basking sharks. The porbeagle shark is the only species found year-round in Massachusetts waters because of its preference for colder temperate waters.  Just like many fish species that migrate from their southern wintering locations to the productive waters off New England, so too, do sharks as they chase prey species in from deeper waters.

To learn more about the Division of Mass Fisheries current shark research projects go to  A thorough and accurate summary of great white sighting and non-sightings and the facts behind New England shark encounters and Greg Skomal’s research, can be read in Jeff Harder’s excellent April 2 2009, Cape Cod Life Magazine article on the web at

I especially like this link to a case study for students processing and attempting to understand the many sides of this issue at (accessed July 26, 2009).  The issues are: how many seals are “too many”?  What role do sharks play in seal population control?  And how do recent levels of sharks, seals, and associated species compare to the population of seals and sharks in the 1600s versus later in time when it was acceptable to hunt and kill seals for food or to remove perceived competitors for fish.

In Massachusetts, there have been four reported unprovoked shark attacks from 1670-2007, three of which were fatal, with the last fatal attack occurring in 1936.  Except for sport fishing, there are no major commercial fisheries for sharks except trawl and gillnet fisheries for spiny dogfish (according to the Division of Marie Fisheries website).  Of the 5.7 million pounds of shark landed in the Commonwealth in 2000, 99% were spiny dogfish with a commercial value of $1.3 million.  The remaining 1% was primarily makos, threshers, and porbeagles taken incidental to offshore trawl, longline, and gillnet fisheries based in this state.

The term for an abnormal fear of sharks (which I did not even know existed before I wrote this) is “galeophobia" which is derived from the Greek words "galeos" meaning “dogfish, small shark” with a derivative or alternate meaning from galē for “shark with markings resembling those on a weasel” and the often used "phobos" (fear).  Galeophobia is also sometimes used as alternate term for ailurophobia, or the fear of cats, from the Greek word "galee," meaning "polecat" and/or "weasel." According to, “sufferers from this phobia experience anxiety even though they may be safe on a boat or in an aquarium or on a beach.”  Or even in a crowded movie theatre.

Everything we have talked about so far, although the sight of a shark fin or the vision of large jaws evokes an extremely visceral and gripping image, is an example of a perceived danger that is, in reality, a very rare and unlikely hazard.  This is in contrast to some of the real dangers and hazards we ignore every day like cancer risks from over exposure to the sun, rip currents offshore, and the hundreds of traffic fatalities that occur every day.  The real dangerous “creature” floating in one of our local ponds is the size of a pin head or smaller and appears as a bright, bluish-green sheen on the water.  I am talking about a couple of cyanobacteria species, also known as blue-green algae, which form large algae blooms if given an opportunity due to a “perfect storm” of human use and nature’s rains.  I bet you are shaking in fear already.  Well, the type of blue-green algae species measured in the past two weeks in Hummock Pond can be lethal to dogs and pose a health hazard to humans too, and in this case, are more common than our big-jawed friend, the great white shark.

These outbreaks of harmful algae blooms occur when an overabundance of food in the form of nutrients from fertilizers and other sources is washed into ponds after large rainfall events.  In this case, two culprits were identified from water samples taken in Hummock Pond over the past two weeks. A person trained to look at these samples under a microscope can easily identify the offending parties which were Microcystis aeruginosa (the primary one found) and to a lesser extent, Anabaena circinalis. Both species produce dangerous neurotoxins that are considered toxic to humans, dogs and livestock.

Cyanobacteria are blue-green bacteria or Cyanophyta, which is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria, from the Greek word “kyanós” for “blue”.  They are a significant component of the marine nitrogen cycle and an important primary producer in many areas of the ocean, but are also found in habitats other than the marine environment; in particular cyanobacteria are known to occur in both freshwater and in hypersaline inland lakes and in arid areas where they are a major component of biological  soil crusts.  The ability of cyanobacteria to perform oxygenic photosynthesis is thought to have converted the early reducing atmosphere into an oxidizing one, which dramatically changed the composition of life forms on Earth by provoking an explosion of biodiversity and leading to the near-extinction of oxygen-intolerant organisms.

Microcystis aeruginosa is a phototropic bacterium.  It grows readily in nutrient-rich, slowly moving water.  In most bodies of fresh water and most weather conditions, Microcystis aeruginosa does not pose a hazard to wildlife or human beings. However, under certain conditions (such as when the water is warm with abundant nutrients) Microcystis aeruginosa can grow more rapidly than normal.  The result can be large colonies that form floating masses on the water; they look like huge swaths of green paint and often exude a foul or musty odor.  Under these conditions, Microcystis aeruginosa can produce natural toxins (called microcystins) that are very potent. The microcystin toxins are produced and contained inside the Microcystis cells, and are released to the water when the cells die and disintegrate.  Since the cells are very small, they can be ingested along with the water.  Toxin levels in a waterbody tend to be higher near shorelines and at the surface of the water where animal and human contact is most likely.

Aside from being a nuisance and a danger to fish and wildlife, a bloom of Microcystis aeruginosa may be harmful to humans during recreational water use, either through body contact, inadvertent ingestion, or inhalation of water droplets.  Body contact during water recreation may lead to minor skin irritations or allergic reactions of skin, including eye irritation and blistering of the lips.  It is believed that polysaccharides derived from the cell wall of the bacterium cause these external health effects.  On a more serious level, inadvertent ingestion or inhalation of Microcystis aeruginosa may, within several hours after exposure, lead to abdominal cramps, nausea, vomiting, diarrhea, fever, sore throat or hay fever-like symptoms, with recovery within 48 hours.  It is noteworthy however, that very young children, splashing vigorously in the water and having a small body mass, may be more likely than adults to experience these internal reactions.

 As would be expected, the risk of developing these adverse reactions increases as the intensity of the bloom increases.  It is also characteristic of these blooms to wax and wane from day to day.  Given these facts, it is reasonable for public health authorities to post an advisory cautioning against swimming in water where a bloom is readily visible rather than outright closure of the beach for all activities.  However, it is important to keep in mind that very young children may be at higher risk than older persons.  It may take as long as three weeks for the toxins to drop to a safe concentration. The Nantucket Health Department posted signs at Hummock Pond on July 24 after they were notified of the blooms and these algae will be monitored over the next few weeks until they have dissipated.  Interestingly, there is not a lot of evidence that the bacterium is concentrated into yellow perch tissue, but I would still avoid eating fish from Hummock for a few weeks.

Where the real worry comes in is if strains of these blue green algae form microcystins that causes damage to the liver.  Pets and livestock are especially susceptible to the toxic effects of microcsytin.  For example, in Nebraska in May of 2004, three dogs died shortly after swimming and presumably drinking the water in a private, residential lake with an algal bloom; subsequent analysis of the lake water and tissue specimens indicated that microcystin caused those deaths.  Other scientific papers corroborate this effect, so please keep your dogs out of posted areas. Deaths in livestock have also been reported in the scientific literature, particularly where droughts may induce animals to drink algae-contaminated water when fresh water is not readily available. Human beings are likely to experience serious illness from drinking water containing large concentrations of microcystin, but it is important to note that wells nearby are safe.

I find it beyond ironic, that something that seems so safe and at most just a noxious puddle can be a significant health risk while the odds of Jaws swallowing us whole off Surfside are probably in the 1-in-a-billion range.  Either way, I hope you will stay safe and still enjoy the summer. Swim in the ocean and admire the ponds and enjoy each day.

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