Secor and Gunderson: Effects of hypoxia and temperature on Acipenser oxyrinchus 
605 
No surface Surface 
Figure 1 
Nested experimental design. Replicates are presented as layers. Dissolved oxygen rates (3 or 
7 mg/L) were nested within temperature and surface access levels. 
stand-pipes for flow-through water allowed sturgeon 
to use the entire bottom surface. During the experi- 
ments clear plexiglass lids were attached to the tanks 
with clamps and duct tape. A large hole (5-cm diam- 
eter) in each lid, fitted with a stopper, permitted ac- 
cess to the tank for feeding of fish, for periodic checks 
of wafer quality, and for removal of dead individu- 
als. Hoses for air and water supply also passed 
through the lid. In the tanks that permitted access 
to the air-water interface, water level was maintained 
at 5 cm below the lid. In treatments designed to limit 
access to the surface, lids were sealed to the tank 
and tanks were filled completely. Two head tanks 
delivered water at either 19°C or 26°C to experimen- 
tal tanks, each maintained at a rate of 1.2 -1.5 L of 
flow-through water/min. Dissolved oxygen was con- 
trolled by maintaining hypoxic conditions in head- 
tanks and by adding aeration directly to normoxic 
treatment tanks. Hypoxic levels (2. 5-3.0 mg/L) were 
maintained in head-tanks by mixing hypoxic well 
wafer (<2 mg/L) with oxygenated Patuxent estuary 
water (6 mg/L). Thus, salinity varied slightly between 
experiments but was always within the range 1.5-3 
ppt. When necessary, water was aerated to bring 
oxygen levels to 2.5 mg/L in the head-tank. Normoxic 
treatment water was a mixture of well water and 
Patuxent estuary water. Dissolved oxygen levels were 
well mixed (homogenous) in the tanks owing to their 
shallow design, flow-through water, and constant 
swimming of YOY sturgeon. Lighting was provided 
on a 12:12 h lightrdark cycle. 
With the exception of the 26°C and no-surface-ac- 
cess treatment, temperature and oxygen concentra- 
tions were maintained within 10% of their prescribed 
levels (19 or 26 C. 3 or 7 mg/L). In the 26 C sealed 
experiment, aeration supplied to the head-tank was 
insufficient to attain oxygen conditions close to 7 mg/L 
for the normoxic treatment { mean replicate dissolved 
oxygen concentrations were 5.10 and 5.25 mg/L). In 
the hypoxic sealed treatment (mean replicate dissolved 
oxygen concentrations were 3.76 and 4.44 mg/L), dis- 
solved oxygen was deliberately held above 3 mg/L 
because this level with high temperature had been 
observed previously to be lethal in unsealed tanks. 
Despite these elevated “hypoxic” conditions, the com- 
bination of high temperature and low oxygen was 
fully lethal in the sealed tanks (see below). 
Juveniles (8 to 30 grams wet weight) were accli- 
mated to experimental conditions over a 4-d period. 
On day 0 of each experiment, lengths and wet weights 
(juveniles weighed in water on a top-loading balance) 
were recorded and a dorsal scute(s) clipped to iden- 
tify each individual. In preliminary trials, we found 
that scute clips did not significantly affect growth 
