These shows are show contrived I would not be surprised if they netted a river and put a hook in the biggest fish's mouth. Every time the big elusive fish is caught in the last minute before the trip is over. Amazing how that happens.
What kind of show would it be if in the first five minutes they show him catching the biggest fish possible and then for the other 55 minutes they show him pounding the water and not catching much of anything? In previous seasons, it seemed like he spent more time diving or using a remote control vehicle to see what else was down there. I know in some of those episode they saw fish much bigger than the one they finally caught.
How many guys do you know that caught their biggest salmon ever the first time they went out, first couple casts? I'm sure there are a couple but fishing takes time and learning the fish no matter what fish it is.
Also I have seen an episode (unless I am just misremembering) where he left without having caught the monster he was looking for.