You are assuming there is competition. The first step would be for someone to actually prove that cormorants decrease the number of commercial fish out there. But this is never done, and likely the two are totally unrelated. Population dynamics of predator-prey in complex systems such as the Great Lakes are not linear in the way that they suggest from this. Having said all that, the numbers of Double-crested Cormorants in some of the Great Lakes, particularly Lake Ontario, are huge. The dynamics within colonies are interesting as well, back in the day when I monitored and banded Double-crested Cormorants in Hamilton, Ontario they nested above Black-crowned Night Herons in colonies. They essentially drove out the herons due to the fecal matter dropping down on them. Smaller and growing cormorant colonies (in trees) were much more diverse, with herons in there, and maybe Ring-billed Gulls on the ground, but as they grew they became less diverse as other birds left the colony due to the overwhelming effect of the cormorants. They likely went elsewhere and did well elsewhere, but the dynamics were interesting. On big islands where the colonies were on the ground, the same dynamics did not occur. In any case, I doubt they have any data on cormorant and commercial fish in a way that they could prove that cormorants are having a negative effect.
From: <mbbirds...> <mbbirds...> On Behalf Of Chris Hartzell
Sent: Friday, November 23, 2018 7:44 PM
To: Mbbirds Bay Birds <mbbirds...>
Subject: [MBBIRDS] Proposal to cull Double-crested Cormorants
Complaints about Double-crested Cormorants eating too many fish (impacting commercial fisheries) and causing damage to private property has Ontario proposing a cull on the population in the form of establishing them as a game bird and allowing hunting. The proposal would allow hunting of them eight months of the year with a bag limit of 50 per day and no possession limit. There is no time period in years established, so it would be in effect until they do another population analysis at an unknown time in the distant future. This is being proposed even though the same agency states the population just reached stabilization and/or is declining.
Essentially, this is a proposed cull because the commercial fishing industry doesn't like the competition and locals don't like all the bird poop.
You can read the details at this link and use the "Submit a comment" button to tell the Environmental Registry of Ontario what you think. Public comments will be taken until January 3, 2019.