What happens during el nino years when coastal upwelling off the coast of south america is disrupted? plankton populations flourish and fishing is very productive plankton populations stay fixed but fish populations boom, creating imbalance in the ecosystem plankton populations die off and fishers are put out of work?

The correct answer is - plankton populations die off and fishers are put out of work.
El Nino refers to very hot phases of ocean water, during which time plankton, and other oceanic organisms, quickly die off because of quite high temperatures. Without the plankton, there would be no food for the fish, which is why it is important for them to survive. If there are no plankton, there are no fish. 

They create genetic variations