"They catch fish."
Translation:Ili kaptas fiŝojn.
A grammar note on "ili": In English, "they" can be used for specific people OR for generic, unspecified people ("they say that eating fish is good for you"). The second case is "oni" in Esperanto. Older English texts also use "one" for the unspecific case ("One can eat fish for good health") but it is much less common in colloquial English.