Three men from the Glynneath area were arrested on Friday following a joint operation between Natural Resources Wales and South Wales Police.
Officers carried out raids at three addresses as part of an ongoing investigation into illegal fishing on the River Neath.
The trio, aged 23, 43 and 51, were taken into custody and questioned about the use of prohibited equipment to catch salmon and sewin.
A number of items were seized by officers for further investigation.
The men were later released on bail.
A spokesperson for Natural Resources Wales said:
“This type of activity represents a significant threat to our wild fish stocks which have great social and economic benefit to Wales.
“If anyone else sees any illegal fishing or something suspicious they should call our hotline on 0800 80 70 60. All information is used in our fight against illegal fishing and poaching.”