September 25, 2009
Noise pollution and algae are some of the reasons marine mammals strand themselves
By Sharon Young
In March and April of 2004, more than 100 dolphins died in and around the Florida panhandle. Much like an episode of the popular television show CSI, investigators rushed to the scene to search for clues to help uncover the cause of their deaths. The investigators—mostly members of local stranding response networks, government agencies and non-profit organizations—try to save animals who are ill and examine the bodies of those who have died. Samples of tissues and stomach contents are sent to laboratories for detailed examination. Researchers also search for clues in the ocean environment from which the animals came in an effort to gather evidence pointing to the cause of the deaths.
The phenomenon of large numbers of whales and dolphins beaching themselves has mystified people for millennia. Even in ancient Greek literature we can find references to these baffling events. And for centuries, humans have tried to figure out why these strandings occur. Thanks to advances in medical technology, we are increasingly able to provide answers.
When a single animal strands, it is often old, sick or disoriented, and the stranding is a result of its weakened condition. Saving these animals can be a difficult task. Other times, single animals may be found dead as a result of drowning in fishing gear, their bodies floating to shore when they are removed from the net.
Mass strandings are more puzzling. Most of the species that strand in groups are "social" species, such as dolphins and pilot whales. They often live in extended family groups for their entire lives. Some of these social species, such as coastal bottlenose dolphins, have a very limited range and may spend their entire lives in the vicinity of a single bay. Others, such as pilot whales, range more widely and may even follow seasonal migratory routes. These observations have led to a number of theories to explain strandings, each of which seems to be true in at least some cases.
Social Bonds.When dolphins spend their lives within a single group, they may be reluctant to leave behind a sick group member. For example, in 1994, there was a mass stranding of white-sided dolphins in which all the animals were found to be healthy except one exceptionally ill individual.
Geography. Some researchers theorize that distortions in the earth's magnetic field may result in animals stranding if they are using magnetic fields to migrate. Otherwise healthy dolphins have also been found stranded in areas such as Cape Cod, where some believe that the gently sloping beaches and rapidly changing tides lead to navigational miscalculations that result in animals becoming trapped in shallow, enclosed areas.
Acoustic Testing. Whales and dolphins rely heavily on the use of sound to survive. Loud noises from naval sonar and seismic testing have been implicated in recent mass strandings. For example, in the Mediterranean in the 1990s and in the Bahamas in 2000, examinations revealed hemorrhages in the ears and brains of many stranded animals, as one might expect to see following intense acoustic or pressure pulses. Marine mammal scientist Naomi Rose represents the HSUS on a national advisory committee convened by the U.S. Marine Mammal Commission that is looking into this area of growing concern.
Biotoxins. Some marine algae (plant plankton) produce toxins. At times these algae grow rapidly in what are called harmful algal blooms—what we know as "red tide" is one such bloom. Toxins from these blooms build up in the sardines and anchovies who feed on them; the toxins can then accumulate in animals like the sea lions and dolphins who feed on the smaller species. As these biotoxins are passed up the food chain they may lead to sudden death when an animal's nervous system is damaged. In other cases, the toxins may weaken the animal's immune system, leaving the animal vulnerable to other infections that may cause stranding. Large numbers of animals may be affected in an area with a harmful algal bloom. These biotoxins can also affect human health if people eat small fish and shellfish with accumulated toxins.
So which of these culprits was responsible for the large number of dolphins dying in Florida? In 1999, I was appointed to the Working Group on Marine Mammal Unusual Mortality Events, a group of independent experts that is consulted to help guide the investigation when unusually large numbers of marine mammals strand. A look at the evidence eliminated some suspects right off the bat. More than just dolphins were dying—jellyfish, horseshoe crabs, small bait fish and birds also died. Captive dolphins being kept in facilities that drew their water from this area of the Gulf of Mexico also sickened. This evidence seems to exclude geography, social bonds and acoustic testing as culprits. But was there evidence of disease or biotoxins?
Investigators from around the country worked feverishly to examine tissue samples collected from the dead animals. The dolphins showed no sign of trauma or viral infection, but there was some evidence of biotoxins. Many of the dolphins had full stomachs, and brevetoxins—naturally occurring toxins produced in red tide—were found in high concentrations in their stomach contents. This implicated a toxin, but southwest Florida has significant red tide blooms almost annually without unusual numbers of bottlenose dolphins stranding, so the investigation produced perplexing results. Investigators will continue to explore the relationship between toxins in the stomachs and tissues of the dolphins, and will work with an interdisciplinary team of scientists using an ecosystem approach to understand the factors that may have contributed to large numbers of dolphins dying in a short period of time.
Unlike a television show, in which the investigation is neatly wrapped up in a single hour, actual field and laboratory investigations take much longer. Given the caliber of the personnel involved, it is usually just a matter of time until they can identify find the cause of strandings. But finding answers isn't cheap.
In 2003 over $1 million was devoted to the Marine Mammal Health and Stranding Response program. This federally-funded program keeps a tissue bank with samples from past stranding events, and from healthy animals that can be used for comparisons; it also funds many costly tests necessary to determine the cause of death in mass strandings. In the past two decades, there has been an increase in the size and number of harmful algal blooms. And in at least some cases, the blooms are the result of pollution, as fertilizers from farms enters rivers and inadvertently fertilize toxic algae in the ocean environment.
Many of the species that strand are top predators in the ocean, and their health may be a good indication of the health of ocean ecosystems. Studying the causes of their deaths can provide early indications of the potential for illnesses that might negatively impact human health and affect economically important fisheries. The deaths of birds and sea lions on the West Coast in the 1990s resulted in warnings being issued by state health departments, and closed several fisheries in order to prevent humans from eating the same fish and shellfish that killed the sea lions.
Unfortunately, Congress cut off funding to the Marine Mammal Health and Stranding Response Program for 2004, and it was only through the combined efforts of a number of groups—including The HSUS—that it was finally restored. You can help by letting your senators and representatives know that you support programs to help marine mammals, and that you expect them to support for adequate levels of funding for them to continue.
If you find an animal stranded on a beach near you, what should you do? Do not try to put it back in the water. Call local police immediately if you do not know the number of the nearest marine mammal stranding network. In any case, it is important to get trained rescuers and investigators on the job as soon as possible so that they can begin the important work of collecting evidence and saving lives.
Sharon Young is The HSUS's marine issues field director.