Contracaecum osculatum (sensu lato) infection of Gadus morhua in the Baltic Sea: inter- and intraspecific interactions

•The frequency distribution of nematode larvae in cod follows a negative binomial distribution.•Larvae in cod reach a length of 27 mm and adult nematodes in seals measure 66 mm.•A crowding effect is indicated as the mean length of larvae decreases in heavily infected cod livers.•Baltic cod contains antibodies against excretory/secretory compounds produced by C. osculatum larvae.