The Environment Agency said an angler discovered the "brutal" weapon at a club near Canterbury and handed it in.
Spokesman Steve Smith said poachers would have used the 4.9 ft (1.5m) spear to stab fish near river banks.
He said fish spawned at this time of the year and could be especially vulnerable because they needed to swim close to the banks to lay eggs.
Killing spawning fish would harm fish populations by reducing the number of young fish that would hatch later in the year, he added.
"This is an illegal and unsustainable way of fishing, which would have a significant impact on the local wildlife," he said.
He said it was an offence for anyone to have such a spear if they were intending to use it to take or kill salmon, trout, or freshwater fish.