Over 400 dolphins were found dead on the coast of Peru in January, according to the Christian Science Monitor, and scientists are baffled by what is becoming a disturbing trend.
In 2012, at least 877 dolphins and 1,500 birds were found dead on the Peruvian coast, according to the New York Times. The government of Peru was slow to investigate and never came to any definitive conclusions. With the latest round of die-offs, scientists are scrambling to figure out the answer, but they have struggled to pin down a definitive cause.
The theories surrounding the dolphin die-offs center around three factors and their potential connections: a virus, toxins, and sonar.
Some have suggested that a virus, such as morbillivirus, could be the main cause of the dolphin die-offs. Morbillivirus was indicated as the culprit in thousands of dolphin deaths in the Mediterranean Sea in the 1990s. Dolphins found dead off the coast of Florida in November were believed to have died from morbillivirus.
Toxic contaminants are another possibility. Man-made chemicals end up in the ocean and go up the food chain, increasing in concentration at each step. Dolphins, who are near the top of the ocean food chain, can have high levels of toxins from plastics and other chemicals. Some have suggested that industrial and residential waste (a.k.a. raw sewage) dumped into the ocean could be the culprit.
Sonar, used by the navy and oil companies (to search for oil under the ocean floor) is often implicated in the deaths of marine mammals, and some evidence suggests that it could be a factor here. In 2012, veterinarian Carlos Yaipen examined twenty of the dolphins that died in Peru. All had middle-ear hemorrhage and fracture of the ear's periotic bone, lung lesions and bubbles in the blood, according to Scientific American. These injuries suggest that sonar was likely a factor in their deaths.
What could well be happening is that more than one of these factors are combining to cause mass dolphin die-offs. Sonar or toxic contaminants could suppress the dolphins’ immune systems, making them vulnerable to morbillivirus or another disease.
Whatever is going on, it is likely that human activity is playing a part, and we have a moral obligation to determine what is killing these dolphins and to cease our role in their deaths.