An Unusual Mortality of Bottlenose Dolpnins

Bottlenose Dolphins are intelligent, social, long-lived, and alien (earthlyissues.com)

Bottlenose Dolphins are intelligent, social, long-lived, and alien (earthlyissues.com)

A lot of Bottlenose Dolphins have washed up dead or dying on the shores of the US east coast. Starting last summer, and increasing month by month, about a thousand have been found. Many others have certainly died offshore. The deaths were first noticed on the beaches of New Jersey and New York. As the migratory populations have shifted south through the autumn and winter, dead animals have now stranded on beaches as far south as Florida.

Bottlenose dolphin strandings have been very high since July. Blue indicaes the average number stranded in each state from 2007-2012, red the number stranded since July 2013 (noaa)

Bottlenose Dolphin strandings have been very high since July. Blue indicates the average number stranded in each state from 2007-2012, red the number stranded since July 2013 (noaa)

What’s going on here?

NOAA has labelled this an Unusual Mortality Event, though such events really are not that unusual. Sixty marine mammal UMEs have occurred in US waters since 1991, and this is the 12th on the east coast since then. It may, though, be the biggest yet recorded.

Bottlenose Dolphins live in warm temperate waters around the world. Along the east coast of the the US they live in distinct migratory and resident coastal populations, a total of about 40,000 dolphins, that may or may not intermix at times. A larger offshore population, another 80,000 animals, live out along the edge of the Continental Shelf, and they may also sometimes intermix with the coastal populations. We really don’t know much about these interactions.

Distinct populations of Bottlenose Dolphins, two of them migratory and at least three of them resident, live along the US east coast. Larger offshore populations live along the edge of the Continental Shelf (noaa.gov)

Distinct populations of Bottlenose Dolphins, two of them migratory and at least three of them resident, live along the US east coast. Larger offshore populations live along the edge of the Continental Shelf (noaa.gov)

It would be good to know, for the dolphins are dying from infection by cetacean morbillivirus, which is a kind of measles virus. It is highly contagious, and though several stranded Humpback Whales carried the virus, so far it seems to be restricted to Bottlenose Dolphins. It doesn’t jump to humans, but it’s probably best not to grub around inside a dead animal without protection. And best if your dog doesn’t chew on one.

The infections and deaths will continue through the winter and should then diminish – assuming that past infections such as the the last major one 25 years ago are typical.

How many may die? If the infections are mostly occurring in just one of the migratory populations, then the impact on that population could be great. These are long lived, highly social, tightly organized and very intelligent animals, and 10 or 20% mortality would be very disruptive. And it could be more. It could spread to other populations. Or it could just peter out.

The Cetacean Morbillivirus kills Bottlenose Dolphins, but feeds vultures (sbs.com)

The Cetacean Morbillivirus kills Bottlenose Dolphins, but feeds vultures (sbs.com)

The question of course is why has this happened? Does it mean anything?

The last outbreak of the virus was in 1987-88 when 700 bottlenose dolphins died on the east coast. Perhaps they get hit by this virus the way we get hit by the flu virus. Then it is basically a common and trivial event, and we note it and move on.

Possibly instead it has been caused by human agent – perhaps some coastal pollution has reduced immune defenses to infection by the virus. But too little is known about the infection to support or reject this, let alone detect a causal polluting agent. Even if it true, the disease should disappear in the spring, we’ll remain ignorant, and then we’ll forget all about it.

The bleakest scenario is that the deaths are an indication of the decreasing long-term health of coastal waters, that the disease will spread to other populations of Bottlenose Dolphins and other species of cetaceans and seals, and that we have another growing disaster unfolding. Would we then be driven to take action, and clean things up? Not likely.

So why do we care?

Bottlenose Dolphins are the most familiar of dolphins – familiar from old TV shows and marine aquariums around the world where they jump around for the entertainment, but hardly the education, of the audiences. The species is an icon of the wild, where dolphins leap out of the water in what appears to be the sheer exuberance of living.

The reality? We know little about them, we are not their friends, and they aren’t ours. They die on our coasts, in our fishing nets, and in Taiji in Japan they are still captured for aquariums and slaughtered annually for food.

Their mouth is shaped in a way that looks to us like a smile.

It is no smile.

Iconic Bottlenose Dolphins (hdwpapers.com)

Iconic Bottlenose Dolphins (hdwpapers.com)

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