adult fish
Refers to a fish that has reached maturity, as opposed to a juvenile or fry. Used in biological, fisheries, and everyday contexts.
There are many adult fish in this pond.
It takes about a year for a fry to become an adult fish.
稚魚 is a newly hatched, immature fish, whereas 成魚 is a fully grown adult fish.
Compound of 成 (sei, 'become, mature') and 魚 (gyo, 'fish'). The reading is on'yomi.