Here is something I believe has a bit of weight. I am not a scientist but spent enough time in the party boat industry to make educated observation. My observations are also based on my time working in the GOM Our local areas can only support so much of a biomass. When I started working on the boats in 06, Cod made up the majority of the bag and haddock weren't a huge portion of the catch with the exception of those April trips were catching a load of Haddock around the spawn was common. Say the average keeper catch was 3:1 cod/haddock. Regulations at the time were 24 inches on cod and 19 or 21 on the haddock, can't remember. Through the people I worked with and old timers, I was told the early 90's late 80's were the years of the nice market cod. Large cod were not uncommon and from what I am told a Haddock was like catching a unicorn. Correct me if I am wrong but I was too young to participate in the fishery at the that time so these stories are what I have to go off of. Fast forward to today and the fishery is primarily smaller haddock and not quite as many cod as we were used to seeing and certainly not the quality we were seeing early 2000's and 90's.
From what I can see, I appears that the fishery changed, lots of cod/ less haddock to lots of haddock/ much lower amount of cod. Is it possible that there is a carrying capacity of what the local ecosystem can hold? I think there is. I think that the change in biomasses of cod haddock was potentially a product or maybe just sped up due to some poor fisheries decisions around 2010. I am sure some people here will remember the slaughter of the breeding congregation of cod by the Isles of Shoals that took place yearly around Memorial Day weekend. I still remember reports of a head boat that doesn't exist anymore limiting out 40 heads at 10 fish each, one stop shop. These aren't stories, it really happened. By the time they shut the area, it was too late. Then follow up the next year with the size limit dropping from 24 inches to 19 inches for a keeper cod at 10 a head. I tell you if that wasn't a one-two punch to the local biomass, I don't know what is. A couple years of cleaning out the egg layers followed by a couple years of killing the recruitment of fish into the spawning population, it's no wonder why the fisheries scientists aren't seeing the population rebound like they are hoping.
Maybe the lack of cod in their niche created this explosion in haddock due to an abundance of food or favorable conditions for haddock spawn where the picked up the slack left by cod. Maybe until the haddock population in knocked down a bit, the cod population can't recover.
Remember this is just my observations. I know there are those who will say that they catch more cod than they know what to do with. I am only able to comment on the hook and line fishery that I participated in but I can tell you, when you are doing the same thing every day, over seven months a year, year after year, you kind of get a decent picture as to what is going on.