From my observation. I believe the UMBEE first drowns the Mouse in his mouth then swallows it when it seize to struggle no more. No damage to the fish. He has eaten up to three of these standard size mice at one eating. Also I believe that the impact from his inital contact may some how stunn the mouse. He hits them so hard sometimes that there is no movement from them. I know this because sometimes there is no movement from tails other times the tail is spinning around for a couple of seconds then stops. So either he drowns them or stunns them prior to swallowing them.