Depends. Some fish tournaments, like bass, are typically catch and release after weighing. This makes it harder to examine the fish if the goal is to release. Others like kingfish are catch and kill, though people cheat by other means like catching a fish earlier in the week.
The big money tournaments like bill fish, where millions of dollars can change hands, are usually catch and release on the spot but entrants have a judge on board to measure any fish caught.
I've done competitive king fishing in the past, and cheating or suspicion of cheating has been around for decades.
From the article they describe many ways they can cheat in fishing (e.g. putting ice in that then melts) that aren't so easy to catch.
If there is a way to cheat, someone will inevitably give it a go with enough incentives. Cheating is the meta-sport and I would think no sport could ever be completely free of it.