The Environment Agency is trying to prevent some brown trout from becoming fish out of water.
The hot weather is causing the River Teme's water levels to fall, raising fears that the fish could become trapped in shallow pools without enough oxygen to keep them alive.
Agency workers were in north Herefordshire on Saturday using mild electric shocks to stun the trout so they can be caught.
The fish are then being placed in holding tanks and moved to a stretch of the river where the water is deeper.
The operation began on Saturday morning and was expected to last all day.
The Environment Agency workers' exact location is being kept secret to protect the fish from poachers.