Dam kills dolphins???

[Brian Ellis]

This youtube video, taken from a BBC report, clearly blames the Three Gorges Dam to be a major cause of the extinction of the baiji or Yangtze Freshwater Dolphin. Another example of man's thirst for energy creating natural damage.

To be fair, I don't believe the dam to be the main cause: very few specimens caught over the last decade have not had deep scars caused by boat propellers; fishing will also have taken its toll and their health in one of the most industrially polluted rivers cannot have been brilliant. However, since the dam was closed, their main migration route has been blocked.

[Technoid]

Unfortunatley, the species as such where already extinct years ago

What most of these reports fail to make known is that when the total induviduals of a species falls belove a certine point (300 or was it higher?) they are already extinct

[Dr Mordrid]

Right; they had neither the population or the genetic diversity to continue.

[Brian Ellis]

Right; they had neither the population or the genetic diversity to continue.

I strongly disagree on both counts. The population had not significantly changed for centuries, estimated at between 5,000 and 7,500, according to a number of accounts, right up to 50 years ago, when one count gave an estimated 6,000. They had survived for at least 20,000 years. As they hunted in pods of 5-10, it is thought there were at least 200 distinct families. Don't forget that this was possibly the most researched cetacean species in the world and a great deal of DNA knowledge was obtained.

There is no doubt that the extinction was entirely due to human activities of one sort or another. To say that it was because of lack of numbers or genetic diversity is a weaselling excuse to try to say that man was not responsible. My guess is that drowning due to wholesale fishing and the use of motorised boats are the main causes. Reduction of habitat is a secondary cause.

[Dr Mordrid]

Yes, the numbers droped because of human activity...I never said they didn't, you presumed that...so the population was low setting them up for trouble.

In most cases a low population will have a limited genetic diversity, as is the case with cheetahs and other endangered critters. This causes increased susceptibility to some diseases & inbreeding, making matters worse especially in mammals.

[DukeP]

The point about a set number of members of a speciee needed for reproduction is called the Alleen effect - and its observed in _almost_ all social critters (Incidently, this is why the Cod of the Georges and Great banks of the east coast havent recovered from the overfishing).

The fact that a specie with a low number of members also have a low genetic diversity is only right if the specie normally have a high number of members.

Species, usually predators, with a natural low number of members can and often do have a relatively high genetic diversity.

Other much more nummerous species can have a really LOW genetic variation (take the european population after the huge amounts of disease in the middle ages) - its all up to the individual species evolutionary history.

The Dolphins in question have propably been stressed for a good number of years; until the suddenly have reached their species breaking point.

This is often observered with longlived animals, on witch 10 or even 20 years of failing to reproduce wont change the number of members of the specie, only the composition of the agegroups. Then suddenly they all reach a nonreproductive age, and the specie is effectively extinct (Think of Lonely George).

~~DukeP~~