FISHERY BULLETIN; VOL. 78. NO. 3 



2.10 



2.08 



2.06 - 



2.M - 



z 



Q 



2.02 



Q 



Z 



< 



I- 



uj 2.00 - 



1.98 - 



1.96 



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25 50 75 



SAMPLING TIME (PERCENTAGE EMERGENCE) 



Figure ll. — Average developmental indices (Bams 1972) of 

 pink salmon fry that emerged from gravel incubators seeded 

 with indicated numbers of eggs. Samples of 50 fry were taken 

 from each incubator when 25% , 50% , and 75% of emergence from 

 that incubator had occurred. Bars represent 95% confidence 

 limits. The sample for 25% emergence at the 25,600 density was 

 lost. 



lower than the highest densities in our experimen- 

 tal incubators. Densities of pink salmon alevins in 

 the stream sampled ranged from to 352 

 alevins/0.1 m^ (mean, 21 alevins/0.1 m^), and the 

 highest concentration of NHg was 0.1 ppb. For 

 comparison, the alevin densities in our experi- 

 ment were >1,700 alevins/0.1 m^. 



Oxygen consumption rates per individual in- 

 creased with time. The highest rate observed 

 (0.028 mg/h per alevin) occurred near the end of 

 incubation when 25% of the fry had emerged from 

 the incubators seeded with 1,600 and 6,400 eggs. 

 Further increases in rate of oxygen consumption 

 per alevin might have occurred before emergence 

 was complete, but this potential for stress was 

 eliminated because fry left the incubators. 



The respiration rates of salmon eggs and alevins 

 are oxygen dependent (Fry 1957); consequently, 



low dissolved oxygen concentrations in the water 

 will limit normal metabolic rates. Our data 

 suggest that oxygen concentrations below about 6 

 mg/1 may decrease the normal metabolism of pink 

 salmon alevins, although survival to emergence 

 may not be noticeably affected. Alevins in the 

 incubator seeded with 12,800 eggs were probably 

 stressed and began to emerge early, about 60 d 

 earlier than alevins in lower density incubators 

 (Figure 8), when oxygen concentrations decreased 

 to about 5 mg/1 (Figure 5). Just prior to peak 

 emergence (201 and 208 d after fertilization), ale- 

 vins in the incubators seeded with 6,400 and 

 12,800 eggs probably had their oxygen consump- 

 tion limited because oxygen concentrations had 

 decreased to <6 mg/1 at a time when their demand 

 was greatest. If oxygen concentrations were not 

 limiting, oxygen consumption rates (Figure 7) 

 would have been greater than the rates for alevins 

 in the 1,600-egg incubator as in the earlier mea- 

 surements. 



Although ammonia (NH3) alone did not in- 

 crease to concentrations that decreased size at 

 emergence or reduced survival, it may have acted 

 synergistically with relatively low oxygen concen- 

 trations to create stressful conditions in our study. 

 In earlier studies, survival time offish exposed to 

 ammonia (NHg) was reduced at adequate but low 

 oxygen levels (Wuhrmann 1952; Downing and 

 Merkens 1955). 



Stressful conditions associated with low dis- 

 solved oxygen concentrations alone, or in combi- 

 nation with ammonia, probably not only reduced 

 survival and caused premature emergence of fry 

 but may have also reduced the ability of alevins to 

 use yolk for growth. In incubation studies at simi- 

 lar temperatures, the developmental indices de- 

 creased and lengths increased just before 

 emergence of pink salmon fry (Bams 1972; Bailey 

 and Taylor 1974). A decrease of 0.01 in /C^ corre- 

 sponds to about 0.2 mm increase in length (Bams 

 1972; Bailey and Taylor 1974). In our study, the 

 developmental index decreased to about 1.96 at 

 emergence in the incubator seeded with 1 ,600 eggs 

 (Figure 11). The fry in the incubator seeded with 

 25,600 eggs were 28.44 mm, mean length, at 50% 

 emergence and had a mean developmental index 

 of 2.07. If these fry had remained in the incubator 

 until their developmental index decreased to 1.96, 

 they would have been only 30.64 mm long, 1.52 

 mm shorter than the fry emerging from the in- 

 cubator seeded with 1,600 eggs. 



Survival from egg to fry exceeded 90% in in- 



656 



