Several general conclusions may be drawn 

 regarding the dissolved oxygen requirements of 

 pink and chum salmon embryos and larvae. 

 First, the supply of dissolved oxygen made 

 available to an embryo or larva is both a function 

 of dissolved oxygen content and flow velocity of 

 intragravel water. Second, the rate of oxygen 

 consumption per unit mass of embryonic tissue 

 is little affected by growth over most of the 

 developmental period up to hatching. Hence, 

 the rate of oxygen consumption by a population 

 of embryos is possibly a simple function of the 

 biomass present. Third, oxygen levels limiting 

 metabolic processes and causing mortality ap- 

 proach a maximum shortly before hatching. After 

 hatching, there is a sharp decline in limiting levels 

 of dissolved oxygen. By considering only the 

 requirements of eggs and larvae and neglecting 

 changes in the environment, it would appear that 

 the dissolved oxygen requirements of eggs become 

 most critical at hatching. 



METABOLIC WASTE PRODUCTS 



Two metabolic waste products excreted by 

 salmon eggs and larvae are free carbon dioxide and 

 ammonia. Both are toxic to aquatic organisms. 



The effect of free carbon dioxide on the physiol- 

 ogy of blood has been studied exhaustively. 

 Jacobs (1920) showed that molecules of free 

 carbon dioxide passed readily through living cell 

 membranes. The ability of eggs and larvae to 

 respire is influenced by the blood's affinity for 

 oxygen, and there is a loss of affinity for oxygen in 

 the presence of free carbon dioxide (Bohr effect). 

 Salmonid blood in vitro lost half of its oxygen- 

 combining capacity in the presence of 150 mg./l. 

 of free carbon dioxide at 15° C. (Irving, Black, and 

 Safford, 1941). Since the oxygen tension equal 

 to one-half saturation is considered to be the mini- 

 mum compatible with exchange of oxygen to the 

 tissues, a salmonid having its blood oxygen- 

 combining capacity reduced 50 percent would die 

 theoretically of suffocation. 



Only a few investigators have investigated the 

 effect of free carbon dioxide on salmonid eggs and 

 larvae. Bishai (1962) induced a marked meta- 

 bolic stress on Atlantic salmon and brown trout 

 larvae by subjecting them to high free carbon 

 dioxide levels. High mortality among trout em- 

 bryos occurred at free carbon dioxide levels be- 



tween 55 and 80 mg./l. in hatchery water (Surber, 

 1935). Increased mortality of chum salmon em- 

 bryos was caused by 125 mg./l. of free carbon 

 dioxide (Alderdice and Wicket t, 1958). Addi- 

 tional information on the effect of high free carbon 

 dioxide content in conjunction with low dissolved 

 oxygen levels on growth, development, and sur- 

 vival of salmon eggs and larvae will be required 

 before relationships observed between mortality 

 and quality of intragravel water can be fully 

 evaluated. 



Ammonia is the most toxic metabolite. Am- 

 monia excreted by salmon eggs and larvae is 

 removed by the surrounding water, but it is 

 possible that toxic concentrations of ammonia 

 occur where the density of eggs and larvae is high 

 and the circulation of intragravel water is poor. 



The toxicity of ammonia is related directly to 

 the concentration of free ammonia (NH 3 ) or non- 

 ionized ammonium hydroxide (NH 3 -H 2 0) in solu- 

 tion. Ionization of NH 3 H 2 occurs according to 

 the equilibrium equation 



NH 3 H 2 O^NH ++OH- 



Ionization is nearly complete at pH 7.0 and lower, 

 and ammonia is least toxic in waters having high 

 concentrations of hydrogen ions (pH <C7.0). For- 

 mation of carbonic acid from respired free carbon 

 dioxide would, therefore, tend to decrease the 

 toxicity of ammonia. 



Reviews of influence of ammonia on fish have 

 been given by Doudoroff and Katz (1950) and 

 Doudoroff (1957). These authors concluded that 

 additions of 2 to 7 mg./l- of ammonia to natural 

 waters could kill fish. Experiments by Wuhr- 

 mann and Woker (1948) showed that concentra- 

 tions of only 1.2 mg./l. of NH 3 were lethal to 

 fresh-water fish of the genus Squalius. They also 

 found that 1.3 mg./l. of NH 3 killed rainbow trout 

 fry. 



According to Wolf (1957a, 1957b), blue-sac 

 disease was induced by subjecting salmonid em- 

 bryos to high concentrations of ammonia. The 

 incidence of disease was roughly proportional to 

 the contact period and the NH 3 concentration. 



SALINITY 



Pink and chum salmon spawn in intertidal areas 

 of streams, and in some streams more fry are 



SPAWNING BED ENVIRONMENT OF PINK AND CHUM SALMON 



499 



