Yes, that was the walleye's air bladder that had all those red lesions, and they are caused by a bacterial infection. This is very common each spring - virtually every walleye has these lesions to some degree. The lesions appear to diminish through the summer as the fish regain health.
Infections like this are usually caused by common strains of bacteria that are always present. Walleye (and other species) get physically run down through their spawning season, and with lowered resistance, become prone to more severe infections. The lesions pose no threat to humans, so send those fillets into the fryer.