spawned, and, after water hardening, the eggs 

 were transported to a laboratory in Corvallis to be 

 hatched. Shortly after arrival, 20 eggs from each 

 of 13 matings were measured as above. Fork 

 lengths were later determined from the frozen 

 dams and sires. 



To obtain samples for determination of possible 

 correlation between egg size and ON size of the 

 hatched fry, we randomly selected 10 fingerlings 

 from each of eight available matings of rainbow 

 trout individually hatched and reared in Corvallis 

 (above). Fork lengths were measured, and otoliths 

 were removed by dissection. 



Storage and Treatment of Otoliths 



The enveloping membrane (sacculus) was 

 removed from each otolith prior to storage. Ini- 

 tially, otoliths were stored dry in coin envelopes 

 before transfer to a clearing solution. Because this 

 method led to breakage of otoliths, later samples 

 were placed in a clearing solution immediately 

 after removal from the fish. Otoliths were cleared 

 from 1 to 21 mo before examination; there was no 

 apparent relationship between clearing time and 

 readableness of the otolith. 



Samples were initially cleared in methyl 

 salicylate. Because some otoliths did not clear 

 sufficiently, a 50:50 mixture of glycerin and water 

 (McKern et al. 1974) was used for the remainder of 

 the samples, but this solution tended to increase 

 the opacity of the entire otolith. Neither burning 

 these otoliths on an asbestos pad over a bunsen 

 burner nor clearing the otoliths in oil of cloves 

 increased contrast between the opaque and 

 hyaline parts. Consequently, it was difficult to 

 discern the nucleus. 



Improved readings were obtained by applying 

 drops of HCl to the medial surface of otoliths 

 preserved in glycerin and water; this resulted in a 

 dissolution of the medial lobes, a consequent thin- 

 ning of the otolith, and clearer definition of den- 

 sity patterns. This method is quick (a few 

 milliliters of HCl applied for 2-4 min for a large 

 otolith) and is easily controlled by periodic inspec- 

 tion of the otolith during treatment. Because the 

 edges of the otolith are dissolved, this method 

 should not be used when age determinations are 

 required. 



Terminology and Examination of Otoliths 



When viewed under reflected light on a black 

 656 



background, the ON of S. gairdneri is hyaline with 

 a narrow opaque ring around the border (Figure 

 2). The metamorphic check is a narrow hyaline 

 ring delineating the nucleus (Kim and Koo 1963). 

 For examination, otoliths were placed lateral sur- 

 face up on black Plexiglas^ depression plates, 

 illuminated with a beam of light at 45° and pho- 

 tographed on 35-mm film through a microscope at 

 50 X . Panatomic-X film (ASA 32) was used, and the 

 negatives were enlarged to 4x5 or 5x7 inches 

 onto grade 3 or 4 (high contrast) paper. A stage 

 micrometer was also photographed and enlarged 

 at the same magnifications so that otolith 

 measurements could be determined from the pho- 

 tographs. The length and width of the nucleus 

 (Figure 2) was measured from the photographs by 

 using a compass and the corresponding pho- 

 tograph of the micrometer. 



RESULTS AND DISCUSSION 



Size of Otolith Nucleus 



The linear correlation between ON length and 

 width was strong in both rainbow trout (r = 0.838) 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



\ METAMORPHIC 

 / CHECK 



i 



I 

 I- 

 o 



t 



[^LENGTH 



B 



Figure 2.-Illustration of (A) otolith and (B) otolith nucleus of 

 Salmo gairdneri, with notation of measurements and ter- 

 minology used. 



