Mangrove forest ecosystem in estuaries plays an important role for the life of living things in waters and land, in which mangrove ecosystem can function as a barrier to ocean waves, a source of fuel and food, recreation, fauna habitat and the richest carbon stock storage, especially in the tropics. Thus, this book aims to identify and inventory the environmental conditions of the waters in the mangrove forest in Kayan-Sembakung Delta, North Kalimantan Province. This book contains a description of the study of fish species found in the waters of Kayan-Sembakung Delta. It is hoped that this will provide an overview of the potential for diversity at the species level and become a database for better fisheries resource management strategies in North Kalimantan. Therefore, this book is present as a source of scientific reference that can be used as reference material for students, lecturers, and researchers who study aquatic biodiversity in coastal area and peat ecosystem.