Posted by: Rob Viens | August 1, 2012

The Elusive Franciscana

On August 1st the Beagle was nearing the narrow end of the Río de la Plata:

“We have had a famous breeze & are now at anchor about 12 miles from Buenos Ayres.— At one time to day it was just possible to see both the Northern & Southern shores of the river at the same time.— A river of such great size & dimensions possesses no interest or grandeur.” (Aug 1)

Man, Darwin has been a little cranky this week – it seems, at least from his diary, that nothing about the wide river and flat plains has much interest to him.  Again, I have to wonder if this is largely the result of the relative “lushness” of the Brazilian rainforest that he recently left behind.

Regardless of what Darwin might have thought at the time, there were (and still are) several things of “interest” and “grandeur” in the river.  One, that I was drawn to writing about is the La Plata dolphin – also known as the Franciscana (Pontoporia blainvillei).

La Plata dolphins (from the Convention on Migratory Species page):

La Plata dolphin

River dolphins, including the La Plata, are typically smaller (as dolphins go) and have a longer snout than their marine cousins.  Many of the species also have very poor eyesight. Instead they rely on echolocation in order to navigate their dark, muddy environment that they inhabit.

There are four living species of river dolphin today (though there are several more extinct species and several subspecies).  Along with the La Plata dolphin, there is the Amazon River dolphin (known as boto, Inia geoffrensis), the Chinese or Yangtze River dolphin (also known as the baiji, Lipotes vexillifer), and the South Asian river dolphins (the susu and bhulan, subspecies of Platanista gangetica). The later two dolphins live in the Ganges and Indus rivers respectively. Sadly many of these species are endangered and the Yangtze River dolphin was declared functionally extinct in 2006.  Another species that our children will never have the opportunity to meet – gone forever.

All of the above species live in freshwater rivers – typically large, sediment-laden rivers (the Amazon, Ganges, Indus and (formerly) Yangtze). The La Plata river dolphin is unique in this group, in that it lives in the salty waters of the river estuary.

The La Plata dolphins also extend into the shallow marine waters and estuaries of larger rivers from northern Argentina to just north of Rio de Janeiro. Recent genetic studies have shown that there are several distinct populations of Franciscana within this range that tend to remain genetically isolated from other populations.

Like all river dolphins, Franciscana are threatened by several human activities including being caught in fishing nets (“bycatch”), pollution and habitat loss/degradation. They are currently listed as “vulnerable” –  but let’s hope that they do not suffer the same fate someday as the baiji.

A young La Plata dolphin rescued near Montevideo in 2010 (Washington Post image):

La Plata dolphin

Based on their experience with people, it may come as no surprise that these dolphins are very shy and elusive. So it is probably not odd that Darwin did not see them during his time on the river. However, his “nemesis” (and I use that term loosely) the French naturalist Alcide d’Orbigny not only saw them on the river but earned naming rights by being the first to describe them.  More on him a little later… (RJV)

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Responses

  1. More of Chuck’s literary style: “a famous breeze…Yeah, Chuck seems a bit down these days…I wasn’t aware of river dolphins.


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