conclusion, based on limited data, is that newly hatched larvae of marine fishes 

 are unlikely to suffer mortality as a direct effect of salintiy, but that older 

 larvae are more vulnerable and could be killed by physiological stresses caused 

 by salinity extremes. 



Oxygen uptake of anesthetized Atlantic herring eggs and newly hatched 

 larvae did not differ significantly at test salinities of 5, 1 5, 35 and 50*^/oo (38). 

 For larvae there was variable oxygen uptake, the rates sometimes being lOX 

 the pre-transfer oxygen consumption rate. For example, for a transfer from 35 

 to 5°/oo at 8°C, larval oxygen consumption went from about 0.07 jul 

 02/larva/hour to as high as 0.7 ijlI O^ /larva/hour within one hour after 

 transfer. Oxygen uptake then fluctuated before returning to normal about five 

 hours after transfer to the test saUnity. Such fluctuations occurred for six-eight 

 hours following transfer and were believed caused by osmoregulatory 

 imbalance prior to acclimation to the treatment salinity. 



Dissolved Oxygen 



Vernberg (97) remarked that effects of low oxygen levels on animals are not 

 easily determined under natural conditions because anoxic situations are 

 always accompanied by other factors such as increased carbon dioxide and 

 hydrogen sulfide concentrations. The effects of temperature and salinity on the 

 solubility of oxygen also complicate the analysis of direct oxygen effect. 



Dissolved oxygen requirements of developing eggs and larvae of Salmonidae 

 and other freshwater or estuarine species have been investigated many times 

 (e.g. 35, 91, 98). There are few studies on marine fish larvae to determine their 

 tolerances to low oxygen tensions (30, 85). De Silva and Tytler (30) found that 

 the incipient lethal oxygen level (LD^q) for Atlantic herring and plaice varied 

 with development from the yolk-sac stage to metamorphosis. At 10°C the 

 LDrQ for yolk-sac larvae was 1.93 ml/1 for herring and 2.73 ml/1 for plaice. 

 After larvae had been feeding for two weeks the LD^q was 3.08 and 2.66 ml/1 

 respectively. At 56-63 days after hatching for herring and 4249 days after 

 hatching for plaice, gills developed and the LDcq levels fell to 2.91 and 2.52 

 ml/1, respectively. At metamorphosis, 70-80 days after hatching for herring 

 and 77-84 days after hatching for plaice, the LD^q was 2.17 and 1.69 ml/1, 

 respectively. De Silva and Tytler (30) also measured routine metabolism of 

 herring larvae from 7-62 days after hatching and plaice larvae from 5-75 days 

 after hatching at lO^C. For the relationship between oxygen consumption and 

 weight, they obtained exponential coefficients of 0.82 for herring and 0.65 for 

 plaice. These values are higher than the values 0.49-0.56 obtained by Laurence 

 (59) for winter flounder from hatching through metamorphosis, although 

 Laurence obtained a coefficient of 0.80 when he excluded metamorphosed 

 individuals from his analysis. 



192 



