INFLUENCE OF EMBRYONIC STAGE ON SURVIVAL OF CUTTHROAT TROUT EXPOSED TO TEMPERATURE REDUCTION

We assessed the influence of developmental stage on the survival of em bryos of cutthroat trout (Oncorhynchus clarki) fertilized at 7 degrees C and subsequently cooled to 3 degrees C. Survival of embryos cooled to 3 degrees C at or prior to 11 d after fertilization was significant ly lower than controls held at 7 degrees C. Survival did not differ fr om controls when embryos were cooled 13 or 15 d after fertilization. E mbryos that had developed through neural tube closure did not show hig her mortality than controls when cooled from 7 to 3 degrees C. Cooling embryos to 3 degrees C after 13 or 15 d at 7 degrees C prolonged the time to hatching by about 40 d when compared with embryos maintained a t 7 degrees C throughout development.