KEPSHIRE and McNEIL: GROWTH OF PREMIGRATORY CHINOOK 



To clarify the concept of daily increment of 

 body weight, fish that can maintain an increase 

 in body weight of 2.0 and 3.0 "^r per day, for 

 example, will double their weight in approxi- 

 mately 35 and 23 days, respectively. 



Fish held in water of 0, 17, and IS'/cc salinity 

 grew at a faster rate and were heavier at the 

 end of the experiments than fish of the same age 

 transferred to water of 24, 25, and SS'/cc salinity. 

 The observed mean weight of fish in individual 

 subgroups is plotted against age of fish in Fig- 

 ure 1. 



Equation (2) can be rewritten in linear form 

 to calculate statistics which are useful for 

 making comparisons of rate of growth among 

 test groups of fish. The linear model is: 



(log Wt — log Wo) = log (l + h) 



t 

 (3) 



Slope of the regression line is given as log 

 (1 + h). This model requires the regression 

 line to pass through the origin since (log Wt 

 — log T^o) = ait = 0. 



Group I fish were weighed on six occasions 

 over a period of 70 days. We have estimated 

 values of log (1 + h) and h for each of the 

 five subgroups of Group I by calculating the five 

 regressions of (log Wt — log Wo) on t. Because 

 the weight of Group II fish was measured on 

 only three occasions, we have not applied a sim- 

 ilar analysis to the second experiment. 



Application of regression methods to obser- 

 vations on Group I fish indicates that fish in 

 fresh water and water of 17^f salinity (Sub- 

 groups la and lb) grew at a significantly faster 

 rate than fish exposed to water of 33%r salinity 

 (subgroups Ic, Id, and le). Equations for the 

 five subgroups are given in Table 3 along with 

 the 95 '^r confidence interval estimates of log 

 {1 + h) and the approximate confidence interval 

 estimates of h. Figure 2 shows growth curves 

 for the fastest (Subgroup la) and slowest (Sub- 

 group le) growing fish. The periodic measure- 

 ments of weight are plotted in Figure 2 to show 

 their correspondence with the growth curves 

 calculated by use of equation (1). 



30 



2.5 - 



2.0 



1.5 



5 1,0 



0.5 



Salinity 

 O 0% 



O 17-18%. 

 A 33 7c 



_1_ 



_1_ 



_1_ 



50 64 78 92 106 



Age (days after hotching) 



120 



Figure 1. — Growth in weight of experimental subgroups 

 of juvenile chinook salmon. 



DISCUSSION 



Chinook salmon used in these experiments 

 were exposed to salt water much earlier in life 

 than they normally would experience in nature. 

 Group I fish were acclimated to high salinity 

 (24%r) 66 days after hatching and 36 days after 

 commencement of feeding. Group II fish were 

 acclimated to high salinity (25'/, o) 54 days after 

 hatching and 24 days after commencement of 

 feeding. There were only 66 deaths (3.7%) 

 among the 1,800 fish which had been exposed 

 to salinities of 24, 25, and 33/{o for periods of 

 25 and 54 days. 



The average rate of growth in water of high 

 salinity (24'/cc and above) varied between 2.1 

 and 2.3% increment in body weight per day. 

 These fish doubled their weight in 30 to 33 days. 

 The average rate of growth in water of low sa- 

 linity (17%f. and 0%o) was 2.6 and 2.7% per day. 

 These fish doubled their weight in 26 to 27 days. 



Although these experiments demonstrate that 

 juvenile chinook salmon can be acclimated to full- 

 strength sea water in an early age, it is apparent 

 that water of high salinity causes a reduced rate 



121 



