What reelfast and bigbruce are forgetting in their "conservation" dream world is that any big breeder cut loose by the rec fleet will very soon be dangling dead on a long line hook. Now that is effective conservation, right?
The target harvest rate of the exploitable biomass for IPHC is 20 percent (except Area 4 whose target harvest rate is only 15%). Areas 2A, 2B, and 2C are the most heavily fished, and the ones with more restrictions.
That means that 80% of the biomass is unfished (i.e. unlanded), and mostly uncaught as there are daily hail-ins. If the TAC biomass for that area is caught - the fleet is shut down - whenever that happens. Since that is a individual quota - it is on a boat-by-boat basis.
I doubt if the commercial long-line halibut fleet throws back many halibut - as once they meet their quota - they stop fishing. They will throw back other bycatch if they exceed their by-catch quota. If there are any "overages" - they come off next years TAC.
The lingcod and other jig fishing boats (e.g. Zn - rockcod) will (however) throw back quite a few halis as they typically have quite a small halibut by-catch quota, unless they are able to buy or lease extra hali quota which has been in short supply...
Another thing to consider is that the IPHC is NOT DFO - it is international - and the Pacific halibut stocks are generally in good shape (prob. because of that), although certain areas do have large annual fluctuations in the total biomass estimates and subsequently the subareas TAC fluctuates.
Another thing to consider is that slot limits have been used (particularly in lake fishing) world-wide - as the idea is to allow the smaller new entries time to grow - while protecting the large, experienced breeders.
Slot limits make particularly good sense for halibut as the largest are mostly females, the older one gets (as a fish or a human) one also gets more experienced wrt when/where to release eggs and spawn; and the number of eggs produced are related to the size of the female (a 50-pound (23 kg) fish will produce around 500,000 eggs while a 250 lb (113 kg) female may produce over 4 million). Females are not sexually mature until they are 11+ yrs old (~+100-120 cm length).
AND one last thing nobody really talks about - but is apparent in the so-called rebounding cod fishery on the East Coast:
older spawners not only give more vitagellin (or nutrients) to their eggs, but novice spawners take their lead on when and where to release eggs from the older, larger fish. I bet that's why the cod fishery is taking so long to "rebound" - too many teenagers left to spawn.
Letting the older female spawners go free (esp. again for halibut who lack a swim bladder and won't suffer the bends that rockcod do) - is a very good conservation tool.