Hi Don / Steve / Mark,
My feeling is that the mussullah is the original species of the kaveri and is now becoming something of a rareity, and the kudhree (blue fin) is the one that has been more recently introduced.
The records from the Tata power Lonavala will confirm the same if closely investigated. I have sent these links to you Mark, and hope you will have a better understanding of how the changes on the Kaveri took place.
To me, the mussullah is the larger of the 2 species, and the kudhree grows to 60% the size of the mussullah.
I don't recall seeing a single blue fin or having seen a picture of a bluefin in the Kaveri prior to 2000.
Although i know that the species cannot be distinguished by the colour of the fish alone, but there is a distinct difference in the look of both species, plus the largest bluefin that i have seen on picture is the one that Darren caught, and this fish although almost black in colour, to me this fish also looks like a kudhree, and it certainly isin't the biggest fish to come out of the Kaveri. I have pictures of fish in the 90+ category that were taken in the 80's and 90's, and none of them had the colouration and look of the kudhree.
Just as the kudhree comes in shades of silver, black, gold, copper, bronze, with blueish fins, so does the mussullah come in shades of gold, silver, black, green, and even copper with orange or white coloured fins depending on the colour of the body of the fish. The gold and copper having orange coloured fins, while the silver had white fins, and the black grey fins.
There is also a difference in the two fish if 2 of the same size are compared, with the kudhree having less of a hump than the mussullah, and in some cases i have seen a drooping tail in the kudhree, which i have never seen in the mussullah.
Both seem to have around 25 scales along the lateral line, but i have not got into measuring the head of the fish with relation to its girth, nor have i made a study of the dorsal or anal fins, but here too there is a difference in both species, with the mussullah having more rounded anal fins.
The mussullah are still breeding in the kaveri, but much slower than the kudhree.
Mind you, I could be wrong