Cetaceans in brackish habitats.

The boundary between river and ocean is a closely linked to human development. Rivers have long been used for transportation to inland areas and many ports/harbours today are situated in the mouth of a river. These dense forms of development on a limited ecosystem can severely affect the species which reside in this niche.

In a paper from 2002, Smith and Jefferson look at two cetacean species: the Irrawaddy River Dolphin and the Finless Porpoise, respectively classified as Endangered and Critically Endangered. Their intent was to collect information about their populations in order to determine what conservation actions should be taken in light of their findings.

The Irrawaddy River dolphin’s (see below) range spans from the coastlines in the Bay of Bengal, south-east Asia to the Java Sea, and Northern Australia to Papua New Guinea. However, within this range, a large percentage of the population resides in the waters off of Bangladesh. This main population is considered to be vulnerable, while the spread out, smaller sub-populations are critically endangered.

The Finless Porpoise (see below) population spreads even further, however it remains closer to the coast line as it prefers shallower waters; spanning from the coasts of the Persian Gulf, all the way to Japan. This species also has a few isolated populations in rivers, such as the Yangtze River in China (not to be confused with the actual Yangtze River Dolphin which is a different species altogether and considered to be functionally extinct even though it is only listed as critically endangered), which are classified as endangered, while their coastal counterparts are classified as threatened.

Both the Irrawaddy River Dolphin and the Finless Porpoise are threatened by pollution due to the nature of their habitat. Rivers carry pollutants downstream, creating high concentrations at the river mouth. Pollutants such as organochlorides bioaccumulate and the concentrations found in both brackish species have been reported as very high.

Another threat to these species is that because of their ability to survive in fresh water, they are sought out by dolphinariums due to the fact that it is cheaper to have a fresh water tank than a salt water one. Their proximity to the coast and their slow movement also makes them easier targets for capture. And, of course, the fact that they’re very friendly doesn’t hurt. There are reports of the Irrawaddy River Dolphin helping herd fish into fisherman’s nets in the Ayeyarwady River Basin off the coast of Myanmar!

Overall the main threats posed to the Irrawaddy River Dolphin and the Finless Porpoise is habitat degradation and entanglement in coastal fishing nets.

ž AP. Study: Bangladesh hosts 6,000 rare dolphins. 01 April 2009. 13 October 2011 .

ž Smith, B D and T A Jefferson. “Status and conservation of facultative freshwater cetaceans in Asia.” Raffles Bulletin of Zoology (2002): 173-187.

ž http://en.wikipedia.org/wiki/Finless_porpoise

ž http://en.wikipedia.org/wiki/Irrawaddy_dolphin