This paper relies on a natural experiment to study the short-term consequences of a reduction of the trawler fleet on the economic situation of fisheries. In the context of the Covid pandemic, a scenario of cooperation leading to a weekly rotation of trawlers was set up for four weeks in May 2020 in the port of Le Grau-du-Roi located on the Mediterranean Sea. However, this scenario was not utilized in the nearby port of Sète. Using detailed transaction data, we rely on a difference-indifferences strategy to assess the impact of the large decrease (around 45%) in the number of trawlers selling fish on a daily basis during the cooperation period. We show that the daily sales revenue per active trawler increased more in Le Grau-du-Roi than in Sète (around 20%) due to higher fish catches, presumably due to a decrease in congestion. However, we find that the evolution of the total revenue per trawler was much lower (around 40%) in Le Grau-du-Roi than in Sète because of the cooperation, meaning that the decrease in fishing time had not been offset at all by the increase in daily sales revenue.