Investigating the causes of reproductive impairment of burbot (Lota lota) in Kemi, Northern Finland

Abstract

Severe impairment of gonadal maturation in burbot (Lota lota), a cold-water gadiform, has been observed for over three decades in the coastal waters of Kemi, northern Finland—an area influenced by pulp and paper mills and other industrial activities. Similar reproductive disruptions have been reported globally in fish exposed to effluents from pulp and paper mills, often linked to endocrine-disrupting chemicals (EDCs). However, previous research in Kemi has not identified a specific causative compound. This study aimed to characterize the histological nature of reproductive impairments in burbot and explore potential underlying causes. Gonadal development was assessed through histological analysis of stained paraffin sections, while liver energy content was measured spectrophotometrically to evaluate the effects of chemical exposure on energy allocation. Additionally, exposure to resin acids and chlorophenols, common endocrine-disrupting chemicals (EDCs) in pulp and paper mill effluents, was assessed in bile and liver samples. Burbots at the study site exhibited delayed or arrested gonadal maturation during the spawning season. Histopathological findings included increased connective tissue, arrested spermatogenesis, and an intersex condition. The gonadosomatic index (GSI) was reduced, while the hepatosomatic index (HSI) was elevated. Liver protein content was significantly lower at the study site and negatively correlated with liver and body size, suggesting possible liver pathology. No measurable exposure to resin acids or chlorophenols was detected. These findings support the presence of EDC-related reproductive disruption in burbot near Kemi. However, the specific source of the responsible compounds remains unidentified, underscoring the need for continued investigation.