In surface waters of the euphotic zone dissolved oxygen usually ranges from 

 4-8 ml/l with supersaturation (> 6-9 ml/1) possible in higlily 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). MortaHty 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 tyranmis 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 

 over exposure times of 1-20 minutes. The estimated median lethal shear (LS^q) 

 that could kiU 50 percent of the embryos and larvae ranged from 120 

 dynes/cm for a 20 minute exposure, to 785 dynes/cm for a one minute 



193 



