BAILEY ET AL.: EFFECTS OF SEEDING DENSITY OF PINK SALMON 



cubators seeded with < 25,600 eggs, but survival 

 was only 5(y/( in the incubator seeded with 25,600 

 eggs. We attribute this poor survival to crowding, 

 possibly low dissolved oxygen concentrations, or 

 the combined effects of these and elevated NH^ 

 concentrations. Ammonia, as a single factor, is 

 unlikely to reach levels harmful to pink salmon 

 alevins in gravel incubators supplied with slightly 

 acidic water, as the water in Auke Creek. However, 

 if the pH were 7.75 rather than 6.4, then the high- 

 est concentration of total ammonia, 0.32 mg 1, 

 would be equivalent to 2.1 ppb of NHg and would 

 reduce survival. 



Much remains to be learned before we can define 

 combinations of seeding density and water flow for 

 efficient production of healthy, unfed fry. Seeding 

 densities of 1,200-1,800 eggs/0.015 m^ of gravel 

 and an apparent water velocity of 70-300 cm^/h 

 per cm^ can be used (Bams 1972; Bailey et al. 

 1976). Bams and Simpson (1977) suggested that 

 1,965 eggs/0.015 m^ with a water velocity of 200 

 cm/'h is safe. In our study, increasing seeding den- 

 sity from 1,600 eggs 0.015 m=^ to 6,400 eggs/0.015 

 m^ at a water flow of 53 cm /h apparently increased 

 swimming activity of the alevins and also in- 

 creased oxygen consumption and ammonia pro- 

 duction. However, the average length, weight, de- 

 velopmental index, emergence time, and survival 

 of these alevins were not importantly affected. 



Under our experimental conditions, a seeding 

 density of 6,400 eggs /0. 015 m^ appears to be ac- 

 ceptable, although perhaps a nearly maximum 

 seeding density. Our test incubators were operated 

 at a water flow of only 0.81/ min ( apparent velocity, 

 53 cm^/h per cm^). If an apparent water velocity of 

 200 cm^/h per cm^ were used as recommended by 

 Bams and Simpson (1977), acceptable seeding 

 densities might be higher. 



SUMMARY 



Pink salmon eggs were seeded in gravel in- 

 cubators at four different densities (from 1,600 to 

 25,600 eggs/0.015 m^ of gravel) and incubated 

 until fry voluntarily emerged. Dissolved oxygen 

 and total ammonia concentrations of the in- 

 cubator effluent were monitored periodically, and 

 the emerged fry were counted, sampled, and mea- 

 sured. The rate of total ammonia production per 

 egg or alevin increased with time after seeding at 

 all densities. At seeding densities of 6,400 eggs/ 

 0.015 m^ the rate of total ammonia production 

 increased from 2 x 10 "* mgh per egg 3 wk before 



hatching, to 4 x 10^ mg/h per alevin at hatching, 

 to 6 X 10"* mg/h per alevin at emergence. The rate 

 of total ammonia production per individual also 

 increased with seeding density. Because of low pH 

 and low temperature, NHg concentrations did not 

 reach toxic or lethal concentrations in any in- 

 cubator; however, NH3 concentrations would have 

 become toxic in the incubator seeded with the most 

 eggs (25,600 eggs/0.015 m"^) shortly after hatch- 

 ing if the pH had been 7.75 rather than 6.4. 



Rate of oxygen consumption per egg or alevin 

 increased during incubation. In the incubator 

 seeded with 6,400 eggs/ 0.0 15 m^, it increased from 

 0.003 mg/h per egg 3 wk before hatching, to 0.007 

 mg/h per egg at hatching, to 0.028 mg/h per ale- 

 vin at emergence. Probably because of increased 

 interaction between adjacent alevins, rates of 

 oxygen consumption per hour per alevin in- 

 creased, until emergence, with increased seeding 

 density. In incubators seeded with >6,400 eggs/ 

 0.015 m^, dissolved oxygen concentrations 

 dropped to stressful levels (<6 mg/1) that limited 

 metabolism. At seeding densities >6,400 eggs/ 

 0.015 m^, stressful conditions caused early 

 emergence of premature fry and reduced the abil- 

 ity of alevins to convert yolk for growth. Addition- 

 ally, survival was reduced at 25,600 eggs '0.015 

 m^. At an apparent water velocity of 53 cm^/ h per 

 cm^, a seeding density of 6,400 eggs appeared to be 

 marginally acceptable for the production of 

 healthy pink salmon fry. 



ACKNOWLEDGMENTS 



The pH determinations were made by James 

 Knull, oceanographer at the Auke Bay Labora- 

 tory. We wish to thank Don Alderdice, Envi- 

 ronment Canada, for his helpful review of this 

 manuscript. 



LITERATURE CITED 



Bailey, J. E., and W. R. Heard. 



1973. An improved incubator for salmonids and results of 

 preliminary tests of its use. U.S. Dep. Commer, 

 NOAA Tech. Memo. NMFS ABFL-1, 7 p. 



Bailey, J. E., J. J. Pella, and S. G. T.'XYLOR. 



1976. Production of fry and adults of the 1972 brood of pink 

 salmon, Oncorhynchus gorbuscha, from gravel incuba- 

 tors and natural spawning at Auke Creek, Alaska. Fish. 

 Bull., U.S. 74:961-971. 

 BAILEY, J. E., AND S. G. TA\T0R. 



1974. Salmon fry production in a gravel incubator hatch- 

 ery, Auke Creek, Alaska. 1971-72. U.S. Dep. Commer, 

 NOAA Tech. Memo. NMFS ABFL-3, 13 p. 



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