Grandpa answers a question

Grandpa was taking me fishing.  We gathered his fishing tackle and a small pole for me. He sent me out with an empty soup can to find some worms for bait. We had not had much rain lately.  Worms were scarce and the ones I did find were skinny little things.  Grandpa did not do any fly fishing that I knew of.  Grandpa always fished from the boat anchored in a quiet bay.  He was content to sit for hours  listening to the bird song, watching the clouds drifting overhead, his line tossed out a ways from the boat and allowed to drift slowly along beneath the water's surface.  Catching a few fish just added to the fun.
Grandpa's cabin was on a small lake.  Most of the fish he caught were perch or bluegills (sunnies to some).  Once in a while he would hook a
Northern pike--a real fighter, - Not worth much for food as they were so full of tiny bones.  
Using the sad bunch of worms I had gathered, putting two or three on the hook at a time  in order to make anything worth a fish's time to check out,  we used up the short supply quickly and were not having any luck. Finally Grandpa caught a small Bluegill and when he had removed the hook from it's lip, he held the little fish by the tail and whacked it hard against the side of the boat.  The fish stopped flopping.  Grandpa took out his knife and began to cut the fish into chunks.  He dropped the chunks into the bait can reserving one piece which he fastened to the hook on the end of my line.
As I watched his competent performance I felt both sad and a bit ill. 
"Grandpa", I whispered," doesn't that hurt the fish?"
 "We hope not", was all the answer he gave.
Throughout my life with animals, and especially being a Vet Tech and
Wildlife Rehabber, I have witnessed animals in pain.  All types of animals, from baby mice to race horses.  The only question I got wrong on my wildlife exam was in regard to animal pain.  I knew the answer they wanted (multiple choice question), but I answered in the way I had come to believe.
Perhaps animals do not understand pain in the same way we, as humans, do.  That does not mean they do not feel the pain.  I believe they tolerate it - deal with it in a different manner because they were never taught that pain could be devastating.  Little humans are programmed to acknowledge pain and fear it and try to avoid it.  The toddler falls on the sidewalk.  He skins his knee.  Mother runs to him, expressing great concern and sympathy.  She cleans the wound and tells the child the iodine she applies will hurt.  She makes a fuss over what might actually be a small pain.  The animal mother, if she is even around, simply licks the wound.  She does not know what sympathy is so has none to offer.  All bodies are designed with nerves, which fire off signals to let it be known something is amiss or has been damaged.
I just don't believe that animal nerves, especially those in warm-blooded and/or mammals send different messages to the brain of the
creature.  I believe it is the way those signals are interpreted that makes pain seem different to the signal receiver.
So yes, I think animals do feel pain in the same way we, as humans do.
I acknowledge that they do not respond to it in the same way.
This is why Grandpa knocked that fish unconscious before he cut it up.  He hoped the fish no longer felt the pain he was about to inflict.