Alrighty now Rance, now pull the other leg...
Eighty pounds is a respectable halibut, but certainly not a BIG halibut. Truely big ones go easily in the multiple-hundreds, I believe the derby winner in Homer this year was about three-fifty or so and the state record is around five. With fish that size, you DO have to worry about what happens when (or IF

) you finally get them out of the water. If your boat is big enough (like a commercial fishing boat), just bleeding the gills or drowning them in air will do the job. If it's too crowded or too small to risk it (or you just like shooting things

) you can use a "bang stick" or just shoot it.
Problem is, not too many folks know EXACTLY where such a big halibuts brain is, and not every halibut feels dead enough to stop moving even after it's been shot a few times.

A halibut in the multiple-hundred pound class can easily injure or damage when it gets irate enough (what with you trying to kill it and all), so it's not a minor concern. I suspect that since it's so dang much work actually getting one that big into the boat, there's plenty of chances to shoot while it's at the surface but still in the water.
And on the soapbox side of things, you need to consider if you actually WANT to kill a fish that big. Yes, it's probably a once-in-a-lifetime fish. Yes, there's a LOT of meat on a fish that big. Yes, you might just win a bunch of money
if you remembered to buy a derby ticket. But...keep in mind that any halibut over about a hundred pounds or so is
always an older female (they get much larger than the males). Older fish in that size class contribute disproportionately to the reproduction of the species, and the bigger they are the better they lay eggs. A three-hundred halibut doesn't lay three time the eggs of a hundred pound halibut, it's more like ten time more. She's a proven survivor just by having gotten so large, and more likely than not she lays the best quality, healthiest eggs in addition to the larger quantity. Thats a LOT of future little halibut you're filleting there with that monster.
Another thing to consider is just how much halibut can you (and your friends, and your relatives, and your neighbors) actually freeze and eat in a reasonable amount of time? Even when you vacuum pack it in high-quality freezer bags, you've got maybe a bit over a year before you're looking at chowder-grade, freezer-burned fish. It's less if you're just zip-locking them. I love halibut, but could I eat my way through a mammoth like that in a year?
Also keep in mind that fish that big can sometimes get wormy, something you almost never find in smaller fish (it's mostly an esthetic issue, cooking kills them). Fish that big have such big fillets that you have to butterfly them three or four times to get them to reasonable cooking thickness, which is a lot of additional work at the cutting table when you're already looking at a HUGE job. The oldest and largest fish are also somewhat mealy-fleshed compared to younger and smaller fish. It's still decent meat, but not quite as good as say...a fifty pounder, which is coincidentally about the largest fish the better restaurants will buy.
If it sounds like I'm trying to talk people into catch-and-release on the "big mamma halibut", well...I am. I'm not usually a proponent of that practice, as I think the more appropriate term would be "playing with your food". I don't fish just for fun (though I do enjoy it) and consider "catch and release only" streams to be essentially fished-out for all practical purposes. I see no point and certainly no need to repeatedly haul the same poor surviving trout back and forth until they finally die just for some "fishing purist"/snobs enjoyment.
What I do consider important is the future survival of the species that we use for food, and it's economic impact on the state. The Alaska/North Pacific fisheries are perhaps the most logically and scientifically managed in the world, but that's not saying they're perfectly run. Halibut is by all reports a very robust and healthy species overall, but there's a LOT of evidence that the easier-to-reach, near-shore halibut fishery is getting more than a bit stressed in a lot of the more popular spots. Charter captains and sport fishers are catching smaller and smaller halibut every year, and going further out to do so. While the halibut by-catch of the commercial trawlers reportedly exceeds the entire sport fishing quota, it isn't going to benefit the hundreds of charter captains and the related businesses that rely on that traffic. It would certainly help if more people considered letting the "big mamas" go with just a photo or two instead of the gaff and the bang stick.