The Halibut Is...

...that millions of kilograms of halibut are being caught by the big boats while they're trying to catch other species. A problem called “bycatch”.

Pacific halibut from Cook's Inlet, Alaska.
Photo by Jlikes2Fish via Wikipedia

The problem on the west coast of North America is primarily in Alaska, where the rules state that all fish, bycatch or not, must be identified, measured, and sexed by below-deck observers. The process can take up to two hours, and even though part of that time is spent in fish tanks with some water, when they are finally returned to the ocean, most halibut don't make it alive.
Other jurisdictions, such as British Columbia, allow the fish to be sorted on deck, making for a quicker return of bycatch into the water. A practice that significantly lowers the mortality rate.
The head of Seattle's Groundfish Forum, Chris Woodley (representing five companies operating 14 factory fishing vessels in Alaskan waters), has said that trawlers have cut down bycatch, making flatfish only about 0.5 percent of current catch levels of flatfish.
This brings up the question “If 0.5 percent of the flatfish caught in the Bering Sea is millions of kilos, then how much is being caught overall?” It's no wonder that modern fisheries are unsustainable and our oceans are collapsing into nothing but jellyfish, krill, and plastics. Halibut, for example, can grow larger than 2.4 metres (~7 feet long) and weigh over 225 kilos (about 500 pounds). But they cannot grow that big overnight. Fish of that size are survivors, maximum breeders, and targeted by us. Fish that big don't get taken by canoes, dories, or small boats, leaving them plenty of opportunity to re-stock any volumes humans take. But the advent of trawlers and big factory ships meant that these big fish became prime targets.
Overfishing of all species has lead to a quota system, where boats are only allowed a set amount of a specific fish over a limited time. But by allowing the factory ships and their massive nets into productive waters has meant that while the ships might be limited in their catch size of a target fish, bycatch numbers have stayed very high. And this is a worldwide problem.
Current quota-based jurisdictions on the West Coast, other than Alaska, have also instituted a bycatch quota, meaning that vessels must stop fishing when they hit their bycatch limit. I won't say that this is best practice, but it is better than the unlimited bycatch allowed in Alaska.

This piece references Keven Drews' Canadian Press article printed in the Times-Colonist of 20 January, 2015.