Sentence:  Japanese fishermen have sometimes observed that sardines hauled up in their nets during a tsunami have enormously swollen stomachs ; the <WSD>fish</WSD> have swallowed vast numbers of bottom living diatoms , raised to the surface by the disturbance . 
Meaning of the word marked with <WSD> tags:  any of various mostly cold-blooded aquatic vertebrates usually having scales and breathing through gills