Gut contents are quite well known because it doesn't require lethal sampling; the fish can be caught and have their stomachs pumped before being released alive. The administrator of Spearboard specifically configured his boat to assist Mote and FSU researchers with this sort of work; the fish here also has fin spines removed for aging and an incision made for tissue biopsies and an acoustic tag.
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Vulnerability | Coastal and Marine Laboratory
https://www.int-res.com/articles/esr2015/27/n027p155.pdf
Now, if someone shoots a fish in front of a goliath or a group of goliaths and just assumes being a ~200 lb beach ape that's figured out how to take air underwater and make a device that flings pointy barbs makes them an "apex predator" as compared to a 600-pound fish with superior speed and reflexes, the fish is going to prove that assumption wrong. The problem in my view - and from talking to fisheries researchers - is that put bluntly, anglers and spearfishers got used to not having competition over the past 30-40 years. They do daft things like leave full stringers out in the open and wonder why a shark or goliath comes over and takes it, fish places with names like "Bull Shark Barge," or light themselves up like Old Sparky when a small shark gets within 30 ft.
Boiled down, my problem with the proposal is that the justifications are flimsy at best and deceptive at worst. After saying they're going to use an alternative set of benchmarks for species recovery since they can't meet federal guidelines under the Magnuson-Stevens Act, they produce no new information from these relaxed benchmarks to overrule their decision three years ago and justify a limited harvest. It won't provide additional scientific data, it won't have an appreciable economic benefit, and it won't produce marketable meat. The cynical view is that it just proves FWC can be pressured to ignore science and approve a cull; while I don't think 100 fish per year will crash the population I also don't think it will satisfy the complainers. A measurable reduction is what fishing interests want to see, and given that this debate has been going on for at least 15 years now it's a fair guess the intent is to push the population back to where it was in the 1990s.