alt Hacker Newstrawlers historically haven't really discriminated by size. development of decent selective trawling equipment, and introduction of a minimum size is fairly recent
as a data point, a recent change in regulations regarding eastern Baltic cod had no statistical effect on reported catch https://link.springer.com/chapter/10.1007/978-3-030-03308-8_...
>Data quality for stock assessments has deteriorated, discarding of cod has not decreased despite a reduced minimum size and there are no indications of increased gear selectivity in the fishery
...haven't /INTENTIONALLY/ discriminated by size...
The article says smaller fish could more easily escape the nets. Though it doesn't cite studies documenting that, it does seem reasonable.