^w^ If the large fish were to suddenly disappear, the apex predators would have to feed on the small fishes or starve, and most would probably starve.
^w^ The small fish and crustaceans would thrive initially and then, in their quest for food, deplete the zooplankton. Disease, overcrowding, and eventually a lack of food would cause the small fishes to die off.
^w^ Depletion of the zooplankton would allow the phytoplankton to flourish for a time. However, the phytoplankton population explosion would prevent sunlight from penetrating to the lower depths of the ocean, which would seriously affect the oxygen/carbon dioxide balance upon which all marine life depends.
