There are tons of opinios about the Salton Sea both good and bad, I've fished and hunted the Salton Sea for over 20 years and I'll give you some of my opinions along with a few facts.
Yes there are fish and bird die offs, the fish dies from lack of oxygen from plankton plumes. The bird die offs are from various reasons all of which I don't know of.
As far as the fishing, I fished in the great days of the late 80's and early 90's when catching unfortunately illegal commercial poachers gill netted tons of the orangemouth corvina and sold it throughout so. California. There were also a handful of guides who advertised and bragged about killing 9 fish limits of Corvina that went over 200 lbs per person. Fishing dipped in the early 90's but has come back in the last 4 or so years.
Typically the fishing starts getting good in the spring time and can stay good into October to Nov. Fish to 20 lbs used to be very common, these bigger fish are coming back, we caught fish up to 18 lbs this summer.
I don't fish there during the Fall and Winter, I'm to busy shooting ducks and geese down around the south side of the Salton Sea.
I fish 20 lb line when throwing Fishtraps and other plastics and use 30 lb line when fishing live croaker for bait or while trolling. Yes the fish are good eating but I release all corvina I catch, as far as polution, the water in the Salton Sea is cleaner than the water in Santa Monica Bay.