A systemic infection wlth a hexamitid flagellate resembling Hexamita salmonis caused high mortality in chinook salmon Oncorl~ynchus tshawytscha reared at a seawater netpen farm in British Columbia, Canada. Affected fish were anemic and had swollen abdomens containing serosanguinous asc~tes and large blood clots. They also had an enlarged, mottled and congested liver, and an enlarged kidney and spleen. Numerous parasites were observed in the blood. The most remarkable histological changes were found in the liver and kidney. Livers of affected fish showed edema, congestion and inflammation. The renal interstitium was moderately hyperplastic due to proliferation of hemoblasts. The systemic infection was transmitted in the laboratory to chinook by intraperitoneal injection, by gavage of infected ascites and by waterborne exposure (in both fresh and sea water) with a mixture of infected ascites and tissue. The infection was also transmitted in fresh and sea water by cohabitation with infected chinook. Atlantic salmon were refractory to the infection. Based on the ease of transmission of the parasite in both fresh and sea water, and the high mortality associated with the infection, this disease poses a potentially serious threat to aquaculture of chinook salmon.

The effects of natural blooms of Heterosigma akashiwo on freshwater-and saltwater-acclimated juvenile chinook salmon were assessed. Rates of fish mortality in the blooms were independent oi acclimation of fish to seawater and the ambient oxygen levels, but were de endent on concentration oi algae and ambient water temperatures. No pathological abnormality to gilE or other internal organs in the fish were evident. Aeration or oxygenation of fish cages did not enhance or inhibit fish survival in a H . akashiwo bloom. Cause of death was considered to be due to a labile ichthyotoxic agent.

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Changes in the structure of the gonad, skin, interrenal, liver, kidney, stomach, gill and pituitary gland. as well as blood cortisol and haematocrit values were investigated in adult pink salmon during their migration through the Fraser and Thompson Rivers to the spawning grounds. At the commencement of their freshwater migration the gonads of both males and females were in an advanced state of development, the pituitary contained a large complement of well-granulated gonadotrops, and hypertrophy was evident in the interrenal tissue and in the epidermis of the skin. At this time, no change from the normal sexually immature salmon was evident in the structure of the gill, liver or stomach. Sclerosis of the glon~eruli was noted in the kidney. The plasma cortisol level was consistent with concentrations in unstressed salmon.Migration of the fish through a turbulent section of the Fraser River evoked a marked increase in both blood cortisol concentration and in interrenal nuclear diameters.On arrival at the spawning grounds, 1&15 days after entry into fresh water, a general but not marked deterioration of the tissues was evident. The results are discussed in relation to the spawning migration of other species of Pacific salmon.

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