A Skeletal Deformity of Northern Pike (Esox lucius) Related to Pulp Mill Effluents

Increased prevalences of a skeletal deformity affecting the skull of northern pike (Esox lucius) were related to three pulp mills in the Gulf of Bothnia, Sweden. At one mill the later disappearance of the disease, from an original prevalence of 35%, coincided with a marked improvement of effluent treatment. A typical deformed pike showed distinct upward bending of the jaws. The disease always affected the anterior part of the parasphenoid and the frontal bones. In the most severe cases the nasal, vomer, and medial rostral were also deformed. The degree of deformation was expressed as the ratio between two measurements made on the skull.