Any attempt to manage the catch of trawl fisheries species by species, by quotas, by control of mesh size, by closed areas, closed seasons, or by any other device, will inevitably underfish some species and overfish others, leading in the long run to declining catches.
Two complicating factors were variable recruitment, which meant that adjustments to the quota must be made from time to time, perhaps annually; and the fact that a number of species figured prominently in the catch, so that inevitably some would be overfished, and some would be underfished, if quotas were placed on the most desirable species.