NEILSON and GEEN: CHINOOK SALMON OTOLITH INCREMENT FORMATION 



Table L — List of abbreviations denoting 

 experimental regimes to which On- 

 corhynchus tshawytscha fry were exposed 

 in 1982. Percent ration (% of body 

 weight offered every 24 h ) is given and the 

 water temperature at time of feeding dur- 

 ing the diel cycle, if applicable, is indi- 

 cated in parentheses. Refer to Figure 1 for 

 details of feeding, temperature, and 

 photoperiod regimes. 



' Fish in these treatments were held at constant 

 temperature. 



^Fish in these treatments were fed 2 times per 

 24 h. 



^Fish in this treatment were held at a constant 

 temperature and exposed to a 10-min bout of 

 forced activity at 1900 h every day. 



adjusting total food offered as fish grew or were 

 sampled. Every third day, excess food was removed 

 from the aquaria within 30 min of offering, 

 weighed, and consumption estimated. 



On day 26, we exposed fry for 30 min to a hyper- 

 tonic solution of 1 g/1 sodium chloride and 40 mg/1 

 oxytetracycline hydrochloride. The tetracycline 

 was incorporated into the otolith and provided a 

 time marker which exhibited fluorescence when 

 viewed with ultraviolet illumination. All fry were 

 successfully marked by this method. 



Originally, we had intended to sample 15 fish at 

 days 10, 20, and 40. However, an accidental inter- 

 ruption of the dechlorinated water supply on day 

 19 resulted in the mortality of some fish in treat- 

 ments 4% (cool), 4% (constant), 2 x 4%, and 4% + 

 activity. Complete mortality of starved fish oc- 

 curred at that time. To ensure an adequate (N ^ 

 10) sample on experiment completion, no samples 

 were taken at day 20 for the above four treat- 

 ments. Even so, only five fish remained by day 40 

 in the 4% (cool) treatment. 



Fork lengths were determined immediately 

 after the fish were sacrificed. Fish were then dried 

 to a constant weight (60°C for 48 h) in individual 

 labeled containers, and weighed. Sagittal otoliths 

 were then removed, weighed with an electrobal- 

 ance, and prepared for examination with the SEM 

 or a light microscope. 



Increment counts were conducted as described 

 by Neilson and Geen (1982). No attempt was made 



to distinguish between the daily and subdaily in- 

 crements as did Brothers (1978) and Campana 

 (1983). Such distinctions are often based on subjec- 

 tive appraisals of increment continuity and ap- 

 pearance when viewed with a light microscope. We 

 did not observe any such differences in growth 

 increments of O. tshawytscha. Moreover, as the 

 purpose of this study was to determine the 

 periodicity of increment formation as a basis for 

 detailed study of fish growth, the classification 

 of increments as daily or subdaily was not neces- 

 sary. 



RESULTS 



Eggs and Alevins 



The formation of growth increments com- 

 menced before hatching under all experimental 

 regimes. One increment/24 h was formed on aver- 

 age under all temperature regimes (Table 2). No 

 significant departure from unity was noted (anal- 

 ysis of variance, P > 0.05). However, the appear- 

 ance of the daily growth increments differed be- 

 tween treatments. Otoliths offish subject to a cycle 

 of temperature were characterized by more regu- 

 lar and easily observed growth increments than 

 those held under constant temperatures (Fig. 2). 



Examination with a SEM at 1,000 x revealed 

 that the bipartite nature of otolith growth incre- 

 ments differed between the temperature regimes. 

 After etching with a weak acid (Neilson and Geen 

 1982), the relatively deeply etched portion of the 

 bipartite growth increment (corresponding to the 

 opaque portion of the bipartite structures when 

 viewed with a transmitted light microscope) com- 

 prised a larger average fraction of the growth in- 

 crements (P < 0.01) in otoliths offish subjected to a 

 diel cycle in temperature than those offish held at 

 constant water temperatures. The lightly etched 

 portion of daily growth increments did not differ 

 significantly between fish held in diel temperature 

 regimes with 2°C and 4°C amplitude (analysis of 



Table 2. — Summary of Oncorhynchus tshawytscha otolith in- 

 crement counts for alevins held under various temperature re- 

 gimes. 



'± 1 standard deviation indicated, n = 1 0. 



93 



