FISHERY BULLETIN: VOL. 73. NO 1 



tance may be the protection afforded the embryo 

 by the surrounding egg membranes which sep- 

 arate the internal from the external environ- 

 ment. However, in a second study of zinc toxicity 

 to zebrafish embryos, Skidmore (1966) found no 

 evidence of protection of the embryo by the egg 

 membranes. He found that embryos with ruptured 

 outer membranes actually survived longer in a 

 zinc sulphate solution than embryos of the same 

 age with an intact membrane. If the outer egg 

 membrane impermeability were a major factor in 

 preventing ammonia toxicity, all alevins would 

 be instantly vulnerable at hatching. No sudden 

 susceptibility to toxicants in newly hatched fish 

 was observed in this study or in several others. 



We can see no satisfactory explanation for the 

 observed high resistance to ammonia and other 

 toxicants during early developmental stages of 

 teleosts. The higher resistance of sac fry than 

 eggs to toxicants indicates that the egg mem- 

 branes are not always protective barriers and that 

 the explanation is more complex. 



In our study, the susceptibility to ammonia 

 developed during the transition from alevin to 

 fry, toward the end of yolk absorption. This transi- 

 tion, although gradual, is probably more of a 

 physiological change than the changes that occur 

 at hatching. The newly hatched alevins are more 

 "embryo" than "juvenile." They normally reside 

 in the incubation gravels, have few voluntary 

 responses to changes in their environment, and 

 continue to develop by catabolizing their yolk. 

 As the alevin develops and becomes prepared for 

 emergence, susceptibility to some toxicants in- 

 creases. The alevins are now more juvenile 

 than embryo, even to the point of preemergent 

 feeding as concluded by Dill (1967) for sockeye 

 salmon alevins. 



Our results indicate that rainbow trout embryos 

 and alevins are safer from ammonia toxicity than 

 are older salmonids (Burrows 1964; Larmoyeux 

 and Piper 1973). A dramatic increase in the 

 excretion of ammonia (Rice and Stokes in press) 

 and sensitivity to ammonia appears to begin about 

 the time the fry complete absorption of their 

 yolk. Chronic exposure to ammonia would prob- 

 ably exert its greatest effects beginning at this 

 stage also. 



ACKNOWLEDGMENTS 



We appreciate the aid of the staff of the Auke 

 Bay Fisheries Laboratory in the preparation of 



this paper and the Bureau of Sport Fisheries 

 and Wildlife Bowden National Fish Hatchery for 

 providing the trout eggs. 



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