FISHERY BULLETIN: VOL. 83, NO. 1 



The fertilized eggs of each female were incu- 

 bated in separate compartments in Heath Trays at 

 Abbotsford and Loon Lake trout hatcheries. In 

 1981, fertilized eggs from two female steelhead and 

 one female rainbow trout were subdivided into 

 three lots and held at 6.5° 9.5° and 15.0°C until 

 yolk-sac absorption. In 1982, all fish were held at 

 11°C. An approximate 12:12 LD photoperiod was 

 maintained through incubation and rearing. 

 Samples of steelhead and rainbow trout eggs or 

 alevins were taken at biweekly intervals in 1981. 

 Alevins only were sampled in 1982. 



Oncorhynchus tshawytscha eggs were taken 

 from the 1981 Capilano River stock and were incu- 

 bated at 6°C under an approximate 12:12 LD 

 photoperiod. Hatchery practice did not allow sepa- 

 rate rearing of groups of eggs from individual 

 females. 



Otolith development in S. gairdneri embryos 

 was studied by dissecting the embryo from the egg, 

 clearing it with carbol xylol, and then squashing 

 the embryo between two microscope slides. This 

 treatment, which made noncalcified tissue trans- 



parent and amorphous compared with otoliths and 

 other hard parts, permitted otolith examination 

 with a transmitted light microscope at 400 x. 

 While we also examined embryos with X-ray and 

 xeroradiographic techniques, satisfactory results 

 were obtained more simply with the carbol xylol 

 treatment. 



Examination of the nuclei of otoliths from ale- 

 vins required that otoliths be ground and polished 

 following the method of Neilson and Geen (1981). 

 The extent of the otolith nucleus in both embryos 

 and alevins was delimited by the first growth in- 

 crement encircling all central otolith precursors or 

 primordia (Fig. 1). The first growth increment en- 

 circling the central primordia generally appeared 

 dark when viewed with a transmitted light micro- 

 scope. The only primordium outside the nucleus 

 was in the anterior-ventral quadrant and was as- 

 sociated with the formation of the rostrum, the 

 pointed anterior extremity of the otolith showti in 

 Figure 1. 



To avoid bias, otolith nucleus length was mea- 

 sured from coded preparations with an ocular 



Nucleus length 



Primordium 



Rostral primordium 



*' I 



lOOiJm 



Figure l.— SagitUl otolith from a Capilano River chinook salmon, Oncorhynchus tshawytscha, alevin showing the otolith 



nucleus, primordia, and rostral primordium. 



82 



