Then why did they die?

There are fish that can survive various levels of salt and other conditions. But if the conditions were right for them to survive, reproduce, etc, then why suddenly die? It seems like there could be a cycle since other said they experienced the same die off, but then what is the cause of the cycle and how do any survive to create the carcasses for the next die-off?

The salinity of the Salton Sea is very high. All of the fish that were introduced have died except for tilapia and the desert pupfish. They are the only fish that can tolerate the high salt levels.

https://speciesfishing.com/fishing-the-salton-sea/

There are periodic mass die offs. The most common is because of botulism.

Avian botulism, caused by bacteria, usually occurs in the spring time when the winds are very strong and push all of the old, deoxygenated water from the bottom of the Salton Sea to the top, depleting the oxygen available for the fish, causing them to die. As the fish die, the birds eat them, and get sick from contracting the bacteria. The birds then die and get feasted on by maggots which then infect birds feasting on the maggots. This can result in massive bird mortality. The species most affected by avian botulism are waterfowl and shorebirds.

https://wildlife.ca.gov/Regions/6/Salton-Sea-Birds

Thanks!

The fish and bird population has been in continuous decline. The lake used to get enough freshwater replenishment to maintain a relatively balanced salinity level and saltwater ecosystem. Now, the salinity is continuously increasing over time. The most hearty specimens are able to survive and reproduce, but much of the wildlife has died out. Large amounts of biomass soak in the saline solution, becoming pickled fish and birds, adding to the scent that others have commented on.

Collapse is not instantaneous. Nor is it all-encompassing. Even left unchecked for another 100 years, you would still find some forms of life present, but not the ones you see today.

It's boom and bust cycles. Big booms of fish when the conditions are right and the food is plentiful; big die-offs when the conditions swing the other way. There's always been enough fish that survive the bad times to spark the next boom.