Abstract
This article discusses the issue of poaching in Biebrza National Park. This is one of the important but often overlooked factors influencing the species composition and abundance of fish in the Biebrza River and its tributaries. This article analyzes the documentation of the Biebrza National Park Guard in the Osowiec-Twierdza from 2007 to 2016. The obtained results show that the scale of the described phenomenon is serious and its methods of combating are not effective. It is also a serious shortcoming in the absence of mandatory analysis of fish found in poaching tools, which prevents proper estimation of losses both in economic and environmental terms.