Email
PrintWhy Dolphins Strand
The phenomenon of marine mammal strandings has been occurring for hundreds of years. But as the United States Navy continues to catch heat for its use of underwater sonar, the debate surrounding the possible causes has reached an all-time high.
Photograph courtesy of IFAW
On the morning of August 15, 2007 a burst of sound rippled through the clear ocean water off the coast of Andros Island in the Bahamas.
A short time later the noise happened again—only this time it was slightly louder. The sequence continued for six weeks with periods of silence interrupted by emissions of mid-frequency sonar ranging from 80 to 160 decibels.
Boaters might not have heard this deep-water pulse, but then again, they were not its targets. Some beaked whales, swimming more than a mile beneath the waves, were making their way directly into the path of the oncoming sounds.
These whales sported highly sophisticated digital monitoring devices known as D-Tags, which had been suction-cupped to their leathery backs. The tags not only sense any underwater sounds in the vicinity including those heard by or made by the animals, but they also pick up on changes in swimming patterns. Any shift in behavior, such as an animal ceasing to vocalize or changing course suddenly would be logged for later analysis.
So for Brandon Southall and his team of scientists, all they could do was wait.
Southall, the director of the Ocean Acoustics Program for the National Oceanic and Atmospheric Administration (NOAA), led this comprehensive behavioral response study aimed at learning more about how marine mammals react (or don’t react) to underwater sounds. Favorable weather conditions, a relatively high density of beaked whales, and an existing array of hydrophones on the sea floor that make the process of locating whales easier, made the Atlantic Undersea Test and Evaluation Center on Andros Island the perfect spot for the research to take place. In particular the study focused on two species of beaked whales, the Cuvier’s and Blainville’s, of which very little is known and whose deep diving patterns have made live observations challenging.
The research is part of a collective effort by an international team of scientific experts from Woods Hole Oceanographic Institute (WHOI) in Woods Hole, NOAA, and the Sea Mammal Research Unit at the University of St. Andrews in Scotland. They hope the information collected in the study will show whether sound might either be a physical or behavioral trigger for whale and dolphin strandings.
Cape Cod is considered one of the world’s “hot spots” for strandings and when harsh winter storms stir up the seas the chances of whales or dolphins coming ashore are even more likely. Between the months of January and July 2007 alone, the Cape Cod Stranding Network reported nearly 200 marine mammal stranding events, 95 of which were cetaceans, which is the category that includes dolphins and whales. The numbers include not only the stranding of solitary sick animals but also five different unexplained mass strandings events, categorized as a group of more than two unrelated stranded animals.
There is great speculation and a mountain of hypotheses regarding why whales and dolphins strand. While certain animals are clearly weakened by injury (ship strikes are common), malnutrition, parasites, or bio-toxins from harmful algal blooms, the causes of other strandings are shrouded in mystery. Mass strandings are particularly disturbing, as scientists usually have no clear evidence of their cause.
When it comes to explaining local events some people chock it up to the shape of the Cape, believing that cetaceans may follow their food sources right into the shallow creeks and get lost within the scoop of the great sandy arm. Other people hypothesize that there may be a unique geomagnetic disruption in the area inhibiting dolphins and whales’ natural ability to navigate. Due to the social nature of the animals, it is also entirely possible that herds of perfectly healthy animals might be led to danger while traveling and feeding alongside animals with compromised health.
While most scientists agree that there is no single reason why whales and dolphins strand, there is a great desire to reduce the amount of human impact and find ways of preventing any unnecessary mass tragedies. Undoubtedly, the most high-profile debate surrounding the issue has been that of the potential impact of military sonar. Spearheaded by NOAA and heavily funded by the United States Navy, the live study in the Bahamas comes after ever-increasing outcry from animal rights groups.
Many activists hypothesize that the military’s use of mid-frequency sonar (MFA) may directly or indirectly be triggering unexplained mass stranding events. The theory is based on the fact that certain stranding events were reported as being located in the nearby area and at the same time as military maneuvers.
In September of 2002, for example, a group of whales stranded on the beaches of the Canary Islands the same day international naval exercises were conducted. In July 2004 more than 150 melon-headed whales swam into the shallow waters of Hanalei Bay, in Kauai, Hawaii at a time when the navy was conducting exercises in the area.
Unfortunately, the evidence supporting the sonar theory has been circumstantial at best and the absence of concrete empirical data on the actual impact of the sonar on whales and dolphins has led to massive disagreement within the scientific community as to whether the claims might be true.
Back in the Bahamas the rush for the critically needed data continues.
“We want to understand how loud a sound has to be for it to cause hearing damage,” says Southall, who will watch how the whales react to low-level sonar. The study, however, was carefully designed to ensure the safety of the animals and avoid inducing any negative consequences. “It’s sort of like going to a rock concert. The effects are only temporary,” says Peter Tyack, a senior scientist at WHOI and the co-investigator of the behavioral research study.
Another theory being tested is whether the sounds created by navy sonar might mimic the cries of killer whales, which are a natural predator of whales and dolphins. Because the D-tags normally don’t stay attached for longer than 12-18 hours, the data from the acoustic trials will be limited.
The trials are the first step in a project that will span several years. While the 2007 sonar trials are low powered and emitted from a research vessel, in 2008 scientists plan to use actual MFA sources on animals that commonly encounter such sound sources, and in 2009 the trials will repeat in an area where animals would likely have had no previous exposure to sonar. Only after the results have been collected and carefully scrutinized by experts will conclusions be possible.
Up until now, most of the data collected on the physiology of stranded animals was collected primarily through examinations conducted after death. Darlene Ketten, a senior scientist at WHOI is one individual who shares Southall’s interest in studying how marine mammals hear, but her research tends to be more hands-on in nature.
Ketten, who is a marine biologist and neuro-anatomist, works in the computerized scanning and imaging lab (CSI) within WHOI’s brand-new marine research facility. The facility, which opened in the fall of 2005, has taken traditional animal autopsies, known as necropsies, into the digital age. New equipment includes CT and MRI scanners, a large animal lift, and a giant freezer kept at a cool, minus 20 degrees centigrade.
Standing beneath a pair of medical examination lights, Ketten gently tears open a black plastic body bag that holds a common dolphin found beached on a marsh in Wellfleet’s Indian Neck. “I have one gloved hand in the stranding world and another in the modeling and basic research world,” says Ketten, who utilizes state of the art biomedical imaging tools to identify abnormalities of the central nervous system, spine, head and neck, as well as study the neural mechanisms of natural behavior.
Ketten’s research, which is focused on auditory systems and hearing loss in marine mammals as well as other species, has revealed how whale ears deteriorate over time much like human ears do. She also examines stranded animals for signs of illness or trauma. “The body can be a roadmap to what happened to the animal,” she says.
What her research has not shown, however, are signs of acoustic trauma that would suggest sonar is to blame. “If there was damage to the auditory system you would see blown out eardrums or breaks or damage to the middle ear bones and we’re simply not seeing that,” says Ketten. Other injuries, such as bleeding lungs, are inconclusive because the injuries can have many possible causes. Simply lying on the beach is often enough to cause the heavy animals trauma to their internal organs.
According to Ketten, exaggerated media hype has contributed to misrepresentations of scientific data and led to the false assumption that there is empirical proof of sonar injury. The truth, she says, is that marine scientists are just beginning to uncover clues to what is a deep and complicated issue. With no shortage of ideas on the table, the challenge scientists now face is to isolate science fact from fiction. Only then will a life-saving message be possible.
Copyright: This article originally appeared in the Nov 2007 issue of Cape Cod Magazine
