NEILSON ET AL.: DIMENSIONS OF SALMONID OTOLITH NUCLEI 



Figure 6. — Development of a steelhead trout otolith nucleus 

 resulting from a peripheral primordium (top) and the typical 

 pattern of nucleus development (bottom). Note compression of 

 otolith growth increments in the postrostral quadrant. Otoliths 

 were from progeny of the same female parent. 



We did not find any correlation between mean 

 increment width through the various stages of 

 development and nucleus area in either species 

 «-test, P > 0.05). In addition, examination of re- 

 gressions of increment counts on nucleus area 

 indicated that the frequency of increment forma- 

 tion did not vary as a function of nucleus dimen- 

 sion (P > 0.10 for both S. gairdneri and O. tsha- 

 wytscha). 



DISCUSSION 



Sagittal otoliths in S. gairdneri embryos arise 

 by fusion of primordia, the first calcified struc- 

 tures to appear during development (McKern et al. 

 1974). Radtke and Dean (1982) reported similar 

 results for mummichogs, Fundulus heteroclitus, 

 and also noted that the otolith nucleus was first 

 apparent as an amorphous gel-like mass in the 

 area of the labyrinth in the developing larvae. 

 Calcified primordia appeared later although 

 Radtke and Dean did not describe any variability 

 in their number or position. 



The number and position of the primordia were 

 variable, even within the progeny of a single 

 female. This variation affected the extent of the 

 otolith nucleus. In addition, we observed that 

 water temperature influenced nucleus size. The 

 observed variation in nucleus size limits the util- 

 ity of this feature as a criterion for stock identifica- 

 tion. However, differences in nucleus size did not 

 affect the number of growth increments sub- 

 sequently formed and had no significant influence 

 on their width. 



In our studies eggs were fertilized with the 

 pooled sperm of several males. It is possible that 

 the observed variability in otolith nucleus size was 

 related to the differences between the male par- 

 ents. There was little difference in the size of the 

 males used, either within the group or relative to 

 the females. We cannot rule out genetic differ- 

 ences between males as a factor affecting variabil- 

 ity in nucleus size. However, any genetic effects 

 influencing our results would be no greater than 

 would be expected in natural populations. The 

 numbers of males from which sperm was pooled 

 was usually three, a number frequently involved 

 in fertilization of eggs of a single female in nature 

 (Schroeder 1982; Gross in press). 



In developing a hypothesis to explain the basis 

 for use of otolith nucleus length as a means of 

 distinguishing races, Rybock et al. (1975) 

 suggested that nucleus length was related to egg 

 size, although no data were presented. While we 

 found that greater nucleus lengths were as- 

 sociated with larger eggs on average, and larger 

 eggs originated from larger female parents, the 

 slope of the regression of nucleus length on egg 

 weight was not significant (Fig. 3). Furthermore, 

 the variability of otolith nucleus dimensions in 

 rainbow and steelhead trout from south-central 

 B.C. made their measurement much less useful for 

 stock identification that has been suggested for S. 

 gairdneri from the Deschutes River, Oreg. (Rybock 



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