You can change parameters that determine how often the sharks and fish breed and how soon the sharks will starve if they can't eat a fish. If you get the settings right, the populations will live for a long time. Get them wrong, however, and the sharks might eat up all the fish, then starve to death. They might also all die off before finding any fish to feed on.
It is fun to cheer on the sharks and fish as they battle it out, but this game is also a simulation of some ideas from population ecology, although it is a very simplistic model.
The idea for this game comes from A.K. Dewdney's "Computer Recreations" column in the December 1984 issue of Scientific American.