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| Lungworm in a Guiana dolphin (H&E) |
These sort of strandings are fortunately as rare as they are dramatic - part of the newsworthiness in this case was so many happened in such a short time. Fortunately, the great majority of cetacean "strandings" aren't like this at all. In most cases, the cetaceans are already dead, or nearly so, by the time they wash up. Furthermore, most of them are dolphins, simply because there are more dolphins than whales in the world, and they are also more likely to be swimming in coastal waters in the first place.
After all, every dolphin dies eventually, and the body has to go somewhere. Often, they sink, leading to the spectacular scavenger feeding frenzy of "whale fall" when a vast and blubbery carcass sinks into the depths of the deep ocean. But, especially if they happen to die close to the coast, there's every chance that the body will float for long enough to eventually wash up somewhere. It happens every now and then to humans washed overboard from ships, so, when you think about it, it's unsurprising that it happens to at least some dolphins, too.
Sad though this clearly is for the individual dolphin concerned, it's also a potential conservation tool, in that once we have the bodies we can try and find out what killed them. And one question that's surely important in that regard is "was it us?" Is there something we're doing that's killing dolphins before they would otherwise die of natural causes?
Although the exact number is open to debate, there are over eighty species of cetacean in the world today, about half of which are dolphins or porpoises. Thirteen of these species are considered "threatened" by the International Union for the Conservation of Nature, and it's likely that the number is only that low because, for over forty of the species, we simply have no idea how well they're faring. We have, of course, established a number of reserves worldwide for the protection of cetaceans, but it's precisely in these places that studies of natural "strandings" may help us work out how well we're doing. Patrolling a stretch of African savannah to keep out poachers is hard enough; keeping the open sea free of hazards - some of them perhaps unintentional - isn't exactly easy either. For that matter, since many risks, such as pollution, affect everything, spotting hazards to cetaceans may give us some indication as to what's happening with other animals, such as fish.
One such conservation area is the Paranaguá biosphere reserve in southern Brazil, rated as a World Heritage Site by UNESCO since 1999. It consists of a couple of national parks preserving relatively untouched remnants of the Atlantic Forest as well as the estuary around the port city of Paranaguá and adjacent stretches of coast - although not the sea proper. The port, of course, is a functional industrial and transport area, and inevitably has a degree of waste output, even before we consider accidental oil spills or the like.
Over a five year period, Brazilian researchers checked the estuary and coastal beaches at regular intervals, searching for dolphin carcasses. They examined any that they found to assess the species, age, and general condition, and took a number of samples from bodily organs of the less badly decomposed specimens to examine under the microscope.
"Examine under the microscope" does, perhaps, imply that one simply puts a chunk of something on a slide and take a look at it, but there is, of course, rather more to it. (Trust me on this; it's what I do for a living). It has to be fixed in formalin to stop it decaying, processed through a number of reagents to a firm embedding medium such as paraffin wax, be cut into slices less than 1/200th of a millimetre thick, and then stained with something that makes the tissue components visible. In this case, in addition to the general "haematoxylin and eosin" that shows up the general tissue architecture, the researchers added a number of stains for infectious organisms, among others. All of this, in fact, is more or less exactly what you'd do with a medical biopsy taken from a human; it works just as well with dolphins.
Over that five year period, they found over 200 cetaceans along the beaches - which is, perhaps, rather more than one might expect in an stretch of coast no more than about 70 km (45 miles) in length. Most were Guiana dolphins (Sotalia guianensis), with several bottlenose dolphins (Tursiops truncatus) and La Plata river dolphins (Pontoporia blainvillei), as well as a smattering of other species, including some genuinely large ones, such as humpback whales. Many, especially the largest, presented too many problems - not the least of which were logistical - to evaluate, or else the cause of death simply wasn't obvious, but that still left 46 where it was.
Yes, poking around in the decomposing bodies of over 200 dead animals to get data on 46 of them. Ah, the glamour of zoological research.
So, what were the results of all this? Are we humans off the hook when it comes to the death of dolphins in Brazil? In a word: no.
The researchers estimate that 30 of the animals - two thirds of the total - had died as a result of interaction with humans. Most had linear bruises or cuts suggesting they had been caught in fishing nets, although the authors note that a dolphin caught in the middle of a large net and surrounded by fish wouldn't necessarily be injured by the nets themselves, so that the real number could be higher. Two, while apparently having avoided any nets appeared to have hit boats or their propellers, which is just as likely to be lethal.
Most of the animals caught in fishing nets were Guiana or La Plata dolphins, and its perhaps worth noting that these are the two species that tend to stay closest to the coast, while the bottlenose dolphins stay further out to sea. It is, of course, hard to say whether that jut makes it more likely that they'll wash up on the beach, or whether they're more likely to be accidentally caught by fishing boats in the first place. It may well be a bit of both.
This is not to say, of course, that humans were responsible for all of the deaths. 16 of the dolphins appeared to have died of natural causes; in most cases this was parasitic pneumonia, caused by a species of lungworm (Halocercus brasiliensis) related to similarly named worms infesting a range of terrestrial mammals and, more distantly, to the hookworms found in domestic cats and dogs. Many of the net-caught dolphins also suffered from this form of pneumonia, which might suggest that they were already ill - possibly this makes it more likely that they will blunder into nets if they come across them.
Only a minority of animals had died of any other causes. Three had apparently starved, one had septicaemia, and one had choked to death on an overly large fish. As with the pneumonia, there's not really a lot we can do about this, and sooner or later every animal has to die of something. It's a tough world out in nature, and very few animals get the chance to die simply of "old age", so, unless they're eaten by something larger, disease of some kind is going to prove fatal in the end.
The animals caught in the nets, however, are a different matter. Yes, they would have eventually died of some other cause, and, since many of them had pneumonia, that might well have won out. But how long would that have taken? We don't know that, although dolphins can certainly survive with some degree of lungworm infection for quite some time. If they could have survived for long enough to have, and raise, their own offspring, then an early death that prevents that is a cause for concern.
The number of deaths isn't the problem, it's how it compares to the birth rate. We don't know the latter, but increased death from fishing nets certainly won't be helping matters. And, unlike dolphin pneumonia, it's something we can try to do something about.
[Microphotography from Domiciano et al 2016, reproduced under a Creative Commons Attribution License.]
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