THE EFFECTS OF TEMPERATURE ON EXTERNAL EGG MEMBRANES IN COHO SALMON (ONCORHYNCHUS-KISUTCH) AND THE OCCURRENCE OF SOFT-SHELL DISEASE

The occurrence of soft-shell disease in salmon hatcheries in British C olumbia was investigated. Soft-shell disease occurred most frequently in chinook (Oncorhynchus tshawytscha) and coho (Oncorhynchus kisutch) salmon, and in years of high water temperature. To test the effects of temperature, coho salmon eggs were incubated in two different tempera ture regimes (8 and 13 degrees C) and then subjected to scanning elect ron microscopy. As incubation progressed, the higher incubation temper ature significantly raised the hydrostatic pressure of the egg (p < 0. 001), thereby increasing the circumferential tension of the zona radia ta. As the surface of the egg increased, the central plugs in the pore canals of the externus were raised upward. At lower temperatures, the surface tension of the egg membrane was reduced and the central plugs retreated into the pore canals. The position of the central plugs in the pore canals of the zona radiata can therefore fluctuate with diffe rent temperature regimes. The central plugs appear to be loosely set i n the zona radiata, suggesting that this region could provide opportun istic pathogens such as bacteria or fungi with an entry point into the inner egg membrane.