My photos weren’t good enough to be able to ID the fish, but the experts at iNaturalist narrowed it down to a Weed Whiting. The gull did have trouble swallowing it. The nearby gulls were trying to take advantage of this and steal it whenever he regurgitated it.
I shouldn’t have let myself get too distracted, considering how much was going on in the rockpools. I also tried to get photos of birds pooping on this trip but alas, no luck.