I think it's a mouse. At one bait store I used to frequent, the guy at the counter insisted that the best live bait was a live mouse.

He said you tie the line around the mouse like a belt with the hook on the top.

He claimed only the biggest bass would hit the thing.

He sensed my slight disbelief and was about to go out the shed behind the store and try to find me a mouse. I had to talk him down.

I think this bass would use whatever else it was going to eat to pack that mouse further down the gullet.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."