Those observations about fishing quotas do not mean that they do not work. You do not need 100% proof for that, same way as you do not need to check tickets of 100% of passengers in public transportation or vaccinate 100% of people during pandemic. After all, the purpose of the quota is not to reach certain number, it is to set such constraints where population of fish will not fall beyond irreversible level. If the majority of fishermen are one or another way compliant and the rest cannot significantly exceed their quota without being caught, then the goal will be achieved. I do not know if that was the case with the specific quotas, but the quotas could be even stricter than necessary to take the non-compliant behavior of fishermen into account.