The Baltic cod is a substock of the Atlantic cod Gadus morhua, and it is divided into two subpopulations (the western and the eastern stock) living in the semi-enclosed Baltic Sea. This brackish water area is receiving high salinity water from the North Sea through the Danish straits (the Great Belt, Øresund, and Little Belt) and freshwater from precipitation and the drainage areas in surrounding countries whereby marked differences with regard to salinity conditions occur in various parts of the area. The biological and hydrographical conditions determine the parasite fauna found in the Baltic cod, and therefore several of the Baltic parasites are biological indicators. Recommended indicator parasites comprise trematodes (Cryptocotyle lingua, Diplostomum spathaceum, Lepidapedon elongatum, Hemiurus lühei, Brachyphallus crenatus), nematodes (Hysterothylacium aduncum, Contracaecum osculatum, Anisakis simplex, Pseudoterranova decipiens, Capillaria gracilis), and acanthocephalans (Echinorhynchus gadi, Corynosoma spp., Pomphorhynchus laevis).