The question of by-catch impacts on the halibut biomass doesn't have an easy answer, but one must consider the long-term implications of removal of sub-legal fish into the targeted biomass.

The average by-catch from 2010 to 2018 was 7.97 million pounds per year. Most of those fish are taken by the groundfish trawl fleet targeting other species, but of a similar size to halibut. Using yellowfin sole (a major target species in the BOA and BSAI trawl fisheries as an example) one can get a broad inkling of the impact of the by-catch issue. In general terms, the average weight/length of a yellowfin sole is about 3 lbs and 18" in length. A 20" halibut is about 3 lbs and both species occupy similar habitat. Trawl gear at that size is not selective and halibut are lost.

It is one thing to talk about pounds discarded, but on the other side one can look at the number of fish lost each year. The 10 year average by-catch discard of 7.97 million lbs of halibut using a 3 lb average is 2,656,666 fish per year.

How many of those fish would survive to catchable (over 32" for the commercial fleet) size might not be able to be estimated, but one has to wonder what the long-term implications are for fishery managers when such numbers are removed from the population. The halibut fisher (sport, commercial, and subsistence) and the communities that depend on the halibut fishery are all impacted.

Here's a link to an IPHC report that provides some interesting information:

https://www.iphc.int/uploads/pdf/im/im095/iphc-2019-im095-05.pdf