Juvenile rainbow trout were exposed to 36, 171, 1017 and 4887 mg/1 of suspended kaolin clay for 64 days. Gills were analysed quahtatively and quantitatively using histology. Branchial pathology was absent at all sample times in fish exposed to 36,171 and 1017 mg/! kaolin, althoughintracellularkaotinwas visible within filament and lamellar epithelium in gills exposed to 171, 1017 and 4887 mg/1 kaolin at 4, 8, 16, 32 and 64 days. Lesions were found in gills exposed to 4887 mg/I kaolin at 16 and 32 days but the marked lamellar proliferation was associated with moderate to large numbers of protozoa identified as Ichthyobodo necator. Gills regained normal architecture by 64 days despite continued exposure to kaolin. It is suggested that while the gill lesions were probably a direct result of protozoan infection, kaolin was indirectly responsible for creating a favourable environment for protozoan colonization. The recovery of normal architecture and reduction of protozoan numbers between 32 and 64 days was probably a function of increased mucus secretion, immune response and turnover of branchial epithelium. The authors conclude that exposure to prevailing levels of this inert clay particle, even for periods as long as 64 days, probably has little direct effect on gill structure at the light microscope level. The action of noninert and organic particles remains to be investigated.

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The efficacy of iodophor egg disinfection (IED) against egg surface-associated infectious hematopoietic necrosis virus (IHNV) on green and eyed eggs of rainbow trout (Oncorhynchus mykiss) was investigated. Viable IHNV was isolated from the surface of both green and eyed rainbow trout eggs that had been coated with I0 6 plaque-forming units (PFU) of IHNV per milliliter and disinfected in a static solution of iodophor containing 100 mg iodinc/L for 10 and 60 min. Viable IHNV was also isolated from eyed rainbow trout eggs coated with I0 6 PFU IHNV/ mL and disinfected in circulated iodophor containing 100 mg iodine/L for 60 min. Although IHNV was recovered in each experiment, at least 99.98% of the virus was destroyed following IED.