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, 15, 35 and 50 /oo (38). 
For larvae there was variable oxygen uptake, the rates sometimes being 10X 
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 M1 
0-,/larva/hour to as high as 0.7 jul 0->/larva/hour within one hour after 
transfer. Oxygen uptake then fluctuated before returning to normal about five 
hours after transfer to the test salinity. 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 
LD^q 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 LD^q 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 10°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 
