In aquatic ecosystem foodweb can be drawn as follows: Producer (phytoplankton) $\rightarrow$ Primary consumer (zooplankton) $\rightarrow$ Secondary consumer (small fish and water bug) (first carnivore) $\rightarrow$ Tertiary consumers (Secondary carnivores big fishes : Birds living in water like Crane and Duck.

Therefore big fishes and birds seen in water like Crane and Duck which a acquires position as tertiary consumers or secondary carnivores.