Netpen liver disease (NLD) of salmonid fishes reared in sea water: species susceptibility, recovery, and probable cause

Abstract

A severe toxicopathic liver disease of netpen-reared Atlantic salmon, Salmo salar , referred to as netpen liver disease (NLD), recurred in 3 consecutive summers in Port Townsend Bay, Washington, USA. The first histological changes associated with the disease are necrosis of individual hepatocytes and bile preductule cell proliferation. As NLD progresses, diffuse necrosis, vacuolation and megalocytosis of the liver parenchyma are observed. In 1988, NLD was also observed in chinook salmon, Oncorhynchus tshawytscha , and Donaldson steelhead (rainbow x steelhead) trout, Salmo gairdneri , reared in Port Townsend Bay. NLD is most likely caused by a water-borne toxicant, the source and identity of which remain unknown. Chemical analysis has revealed no unusual concentrations of contaminants, and the sites are in apparently unpolluted waters. This suggests that the toxicant may be a natural toxin, possibly an algal one.