Individual vessel quotas (IVQs) established on an important target species are a common tool to regulate fisheries. This article investigates fishermen's behavior in a situation in which a primary output quota is decreasing due to policy restrictions. The findings suggest that vessels respond to such situations by increasing pressure on unregulated species, using their spare capacity. This article models fishermen's behavior using a restricted profit function, treating the harvest of regulated species as fixed, while unregulated species are variable outputs. An empirical application is provided for the Polish trawler fleet with permits to harvest IVQ-regulated Atlantic Cod in the Baltic Sea. The results indicate a strong substitute relationship between the cod quota and pelagic species. Derived elasticities of intensity, providing a measure of the impact of a fixedfactor on variable factors, offer strong evidence of increasing pressure on pelagic species caused by decreasing quotas.

JEL Code: Q22.