It’s on almost every diver’s bucket list: diving with dolphins. You may have pictures from that trip to South East Asia or Mexico where a pod of dolphins was playing in the dive boat’s wake. Every face in the background is lit up by the sheer magic of cetaceans; no one is immune. So, I don’t think I need to explain much to you about oceanic dolphins. You all know quite a bit, I’m sure. River dolphins (yes, they exist!), however, are another story altogether.
Ocean cetaceans returned to the water around 50 million years ago as hippo-like even-toed ungulates. They soon became the obligate ocean dwellers we so love today. Eventually, some of these torpedo-shaped, fish eating, swimming machines started to repopulate fresh water rivers in South America and Asia. Dolphins can now be found in the Amazon, Indus, Ganges, Brahmaputra, Mekong, and Irrawaddy rivers. Some, like the Irrawaddy Dolphin remain mostly oceanic with a tendency to inhabit estuaries and river deltas. But 5 (contested) species of dolphin have fully submerged into the freshwater lifestyle to become “true” river dolphins as early as 6 million years ago.
The Amazon river dolphin, also called the pink river dolphin lives solely in – you guessed it – the Amazon River. There are three heavily debated (sub) species of this dolphin, depending on who you talk to they are considered entirely separate species. But the IUCN considers the Amazon river dolphin, the Araguarian river dolphin, and the Bolivian river dolphin to be one species with three distinct populations. The Franciscana or La Plata dolphin also lives in South America, but has readapted to the salt water estuaries of the Atlantic around Brazil, Uruguay, and Argentina. So they went from the ocean, to fresh water, back to brackish water. I know, it’s a complicated family tree, we’re all confused. Anyway, both these species of dolphins resemble oceanic dolphins more than they do the only other river dolphin species in South Asia.
Oceanic dolphins in the Indopacific also independently moved into the major rivers of the area. There are again conflicting reports on weather the Ganges River dolphin and the Indus River dolphin are separate species, or sub species of the South Asian river dolphin. The Chinese river dolphin or Baiji, found in the Yangtze River, was an autonomous species, but was declared functionally extinct in 2006 with the last individual spotted in September, 2004.
River dolphins are smaller than their marine counterparts, ranging from 5 (South Asian) to 8 (Amazon) feet, and the males are usually larger than the females. Because they live in warm waters with few natural predators, they have little to no blubber, which is highly unusual for cetaceans. They have much longer snouts than their oceanic cousins; almost 4 times longer, in fact! Like other cetaceans, they use echolocation, but living in murky waters with routinely horrible vis has made it the predominant sense. The huge, malleable, spongy part of the dolphin’s forehead, called the melon, directs sound waves out into the cloudy river water, and the waves bounce back through the dolphin’s jaw into the inner ear where the information is processed. This allows them to sense how close and in which direction the tasty fish are. River dolphins are even more highly specialized to compensate for their more visually obstructive environment. As such, their eyes are much smaller than their marine counterparts, even when compensated for size.
Amazon river dolphins are also especially flexible, even for marine mammals, due to their free floating vertebrae. In most mammals, including humans and oceanic dolphins, vertebrae are fused together to form a stiff rod-like base that creates a stronger propulsion. But for dolphins living in shallow, turbulent waters, it’s more important to be agile and adaptable escape artists.
River dolphins live to be about 30 years old in the wild, and rarely survive captivity. They are polygynous animals with the male mating with several females in a season. Females only mate with one male every few years and have sole parental responsibilities of the resulting offspring
Fresh water dolphin populations only inhabit one river and are thus highly vulnerable to changes and dangers in that river. They unfortunately live in rivers that are heavily populated with few governmental regulations. Industrial and agricultural chemicals dumped into the rivers poison the waterways. Heavy fishing pressure brings many gill nets for the dolphins to become trapped in. The struggling dolphins causes mass damage to the fishing gear, increasing the animosity of the fishermen who then intentionally kill entangled individuals and use their meat as bait for food fishes. In Brazil, killing of the dolphins is prohibited, but no program is in place to compensate fishermen on their damaged gear, so it is rarely followed.
The increased boat traffic and subsequent noise pollution also wreak havoc on these echolocating mammals, causing deafness and collisions with propellers of the boats. But the most devastating threat to river dolphins is the implantation of dams and irrigation systems. Dams isolate populations of dolphins, reducing genetic diversity and mating potential. They also cause unnatural water patterns including complete desiccation of riverbeds. The dolphins have no way of escaping from the shrinking river and are stranded to perish. Irrigation only exacerbates the decreasing water level, causing an 80% decline of Indus river dolphins since the 1870s and the entire extinction of the Chinese river dolphin. All fresh water dolphins are listed as endangered because of their extreme vulnerability to human action and development.
Their past may be convoluted and fascinating, but their future remains precarious and uncertain. So consider adding meeting these charismatic anomalies to your cetacean bucket list. And consider taking action to preserve their habitats for future generations of both people and aquatic animals alike. Healthy rivers help make healthy oceans and happy dives, after all.
CLICK HERE to see a map of the habitat loss of Indus River dolphins.
‘Freshwater Dolphins’ was written by Roya