If you found it in the midwest region, it would be a Silver
Unicuspid teeth narrows it to Silver and Northern Brook
The mouth disk looks to be larger in diameter than the body, which would eliminate Northern Brook. It's way too big for a Northern Brook Lamprey anyway. Also Northern Brooks are non-parasitic, so if assuming the likely case that you found it attached to a fish it would have to be one of the three parasitic species anyway
Of course if it was from some other region, this information might well be irrelevant
Fish Species Hunt
If you found it in the midwest region, it would be a Silver
Unicuspid teeth narrows it to Silver and Northern Brook
The mouth disk looks to be larger in diameter than the body, which would eliminate Northern Brook. It's way too big for a Northern Brook Lamprey anyway. Also Northern Brooks are non-parasitic, so if assuming the likely case that you found it attached to a fish it would have to be one of the three parasitic species anyway
Of course if it was from some other region, this information might well be irrelevant
Thank you. Forgot to mention the location - it was caught in Wisconsin River, Northern Wisconsin.
Fish Species Hunt
I agree with Tony, I think it's a native silver lamprey.