In surface waters of the euphotic zone dissolved oxygen usually ranges from 
4-8 ml/1 with supersaturation (> 6-9 ml/1) possible in highly productive 
shallow coastal waters. Kalle (52) reported that in coastal areas with high 
primary production, oxygen super-saturations up to 120 percent are not 
unusual during periods of intensive solar illumination. In shallow waters, 
temporary super-saturations may approach 500 percent (52). Mortality or 
stress of fish larvae due to low oxygen tensions probably occurs only under 
unusual conditions in the sea. 
Miscellaneous Environmental Factors 
Environmental factors such as turbidity, mechanical stresses, and shear 
forces likely to be found in nature have not been studied experimentally with 
regard to effects on marine fish larvae. A few investigations of effects of these 
factors on embryonic stages indicate that embryos are resistant to high 
sediment suspensions and mechanical forces which are present in the 
environment (e.g. 71, 75, 88). 
Schubel et al (88) observed that striped bass Morone saxatilis eggs could 
tolerate silt loads up to 500 mg/1. They noted that turbidity in areas being 
dredged could be as high as 1000 mg/1, which would cause significant embryo 
mortality, but that such high concentrations rarely occurred. Hoss et al (40) 
tested larvae of seven estuarine species with three concentrations of sediment 
extracts (the supernatent from 500 g of Charleston Harbor sediment shaken in 
one liter of filtered seawater). Under their laboratory conditions, survival of 
larval pinfish and menhaden Brevoortia tyrannus was 25-0 percent at the 75 
and 100 percent test concentrations. The supernatent water and sediments 
were not analyzed for toxic substances by the authors, but they cited 
references to relatively high levels of lead, copper, zinc and chromium in 
Charleston Harbor sediments. 
Pommeranz (75) investigated mechanical properties of plaice eggs by 
deforming them with a lever. The force to burst the chorion varied with time 
from fertilization and ranged from about 1.5 g during the 30 minutes after 
fertilization to a mean of about 700 g and 600 g for gastrula and embryo 
stages, respectively. For comparison purposes, Pommeranz (75) cited one 
rough estimate of the pressures developed by a spilling breaker in the open sea 
as approximately 0.1 kg/cm". 
Morgan et al (71) subjected striped bass and white perch Morone americana 
embryos and yolk-sac larvae to experimental shear forces of 0-86 dynes/cm 2 
over exposure times of 1-20 minutes. The estimated median lethal shear (LS^q) 
that could kill 50 percent of the embryos and larvae^ ranged from 120 
dynes/cm 2 for a 20 minute exposure, to 785 dynes/cm 2 for a one minute 
193 
