Histopathological evaluation of Atlantic tomcod ( Microgadus tomcod) collected at estuarine sites receiving pulp and paper mill effluent

Abstract

Histopathological examination of liver, pancreas, spleen and gonads was conducted in Atlantic tomcod ( Microgadus tomcod) collected during September–November 1994 from estuarine sites receiving pulp and paper mill effluents in the Restigouche Estuary, the Miramichi Estuary and Pictou Harbour, and reference sites in the Kouchibouguac and Margaree Estuaries. This study was part of a larger investigation into the environmental health of estuaries and the coastal environment of the southern Gulf of St. Lawrence. Density and surface area of pigmented macrophage aggregates (PMA), and presence or absence of inflammatory, preneoplastic and neoplastic changes were evaluated in spleen and liver. Stages of maturity of the gonads were determined. In the Miramichi Estuary only, 28% of the fish had extensive multifocal granulomatous lesions centered on PMA in the spleen, likely caused by an infectious agent. In these affected fish, the density of PMA was slightly increased, and the surface area of PMA was markedly increased (10×) compared to normal fish from the same site. Both the density and the surface area of PMA were lower in spleens of fish from the Restigouche Estuary than at other sites. Condition factor and stage of maturity of the gonads were higher at this site. In the Restigouche Estuary only, more than 90% of the fish had zones of vacuolation in the pancreas. In the liver, density of PMA was generally low and did not differ significantly among sites. No preneoplastic or neoplastic lesions were observed. Further studies are needed to identify the causes and pathogenesis of the two lesions that were observed only at contaminated sites: the multifocal granulomatous lesions in the spleen and the zones of vacuolation in the pancreas. A larger sample of older fish should be examined to evaluate prevalence of preneoplastic and neoplastic changes. Histopathological markers have successfully identified potential effects of pulp and paper effluents on fish health, but the observed lesions cannot, at this stage, be attributed specifically to toxic exposure.