Secor and Gunderson: Effects of hypoxia and temperature on Acipenser oxyrinchus 
61 1 
to 0.3 mg Og/Cg-h) and fish sizes (12 to 69 g) in our ex- 
periments were both intermediate within this range. 
Surface behavior 
In the nested growth and survival experiment, sur- 
face access influenced both growth and survival rates. 
Eliminating surface accesses in tanks reduced growth 
rates by ca. 35%, and fivefold at high and low levels 
of DO, respectively. The effects of denying surface 
access under “hypoxia” at 26°C was fully lethal within 
a 30-h period. In the 26°C hypoxic treatments that 
allowed surface access, the majority of juveniles sur- 
vived the first 5 days of exposure, although they too 
eventually died. 
Many fishes surface in hypoxic environments to 
convey relatively oxygen-rich water, located at the 
air-water interface, across their gills. In laboratory 
experiment, access to surface waters may have in- 
creased the effective level of DO above nominal lev- 
els, resulting in improved growth and survival. Al- 
ternatively, aerial respiration cannot be ruled out for 
sturgeon that are physostomous. Histological stud- 
ies should be undertaken to investigate whether the 
swim bladder of Atlantic sturgeon contains a vascu- 
lar structure, apart from the gas gland, which meets 
the criteria for an aerial respiratory organ. No such 
structure has been identified in any sturgeon species. 
Reasons for decline of Atlantic sturgeon 
in Chesapeake Bay 
We conclude that increased frequency of hypoxia in 
Chesapeake Bay during this century (Officer et ah, 
1984; Cooper and Brush, 1993) was detrimental to 
Atlantic sturgeon production. Recent water quality 
monitoring has shown that during summer months 
(mid-June through mid-September), temperatures 
>25°C and DO levels <4.0 mg OJh are prevalent in 
Chesapeake Bay benthic habitats (Breitburg, 1990, 
1992; Phil et ah, 1991). Our laboratory experiments 
showed that juvenile Atlantic sturgeon were less tol- 
erant of summertime hypoxia than were other juve- 
nile estuarine species. Young-of-the-year spot, 
Leiostomus xanthurus (total length 10-20 cm), sur- 
vived long-term (>1 week) experimental exposure of 
2. 4-3.0 mg/L at 25°C, but 0. 8-1.0 mg/L DO was fully 
lethal (Phil et ah, 1991). Juvenile and adult hog- 
chokers, Trinectes maculatus (Phil et ah, 1991), and 
naked gobies, Gobiosoma bosc (Brietburg, 1992), can 
tolerate several-day periods of 0. 5-1.0 mg/L DO. 
Our laboratory experiments did not consider be- 
haviors that can 1) reduce exposure to hypoxic wa- 
ters and 2) compensate for reduced dissolved oxygen 
levels. Phil et ah (1991) and Brietburg (1992) have 
provided field evidence that fish will escape hypoxic 
conditions through local migrations. These include 
vertical or shoalward emigrations from hypoxic or 
anoxic bottom habitats. Following hypoxic events, 
bottom habitats are recolonized. Further, short-term 
episodic hypoxia may benefit bottom-feeding fish. 
Burrowing macrobenthic prey will emerge at DO lev- 
els <2 mg/L, increasing their vulnerability to preda- 
tion by fish that can tolerate short-term excursions 
into hypoxic waters (Phil et ah, 1992). If unable to 
escape hypoxic conditions, sturgeon may be able to 
compensate by either surfacing to exploit higher oxy- 
gen concentrations in surficial water or in the atmo- 
sphere or by adjusting their metabolic rate (e.g. 
through reduced swimming [Cech et ah, 19843). 
Hudson River “strain” Atlantic sturgeon, used in 
our experiment, might have exhibited a different 
response to hypoxia than a strain native to Chesa- 
peake Bay. The Hudson River rarely becomes hypoxic 
(Cooper et ah, 1988). Therefore, Hudson River At- 
lantic sturgeon may not have been adapted to hy- 
poxic conditions. An aquaculture study by Serov et 
ah (1988) on stellate sturgeon (A. steliatus) showed 
that heterozygosity in the LDH gene conferred sur- 
vival advantages to hypoxia and high temperature. 
Therefore, it is conceivable that selection of Chesa- 
peake Bay Atlantic sturgeon to hypoxic conditions 
could have occurred over several generations. How- 
ever, because generation time is extremely high in 
Atlantic sturgeon (c.a. 29 years [Stevenson and Secor, 
1996]) and because hypoxia increased rapidly dur- 
ing this century in the Chesapeake Bay, Chesapeake 
Bay Atlantic sturgeon may not have been able to re- 
coup historical abundances by dint of selection to low- 
oxygen conditions. In addition, Hudson River Atlan- 
tic sturgeon juveniles >80 cm TL are known to visit 
Chesapeake Bay during summer months (Dovei and 
Berggren, 1983). Presumably, these fish could have 
adapted to Chesapeake Bay conditions. Serov et al.’s 
(1988) observation that water quality experienced by 
juveniles in culture influences genotypic frequencies 
has important implications for the use of hatchery- 
produced sturgeon in restoration programs and mer- 
its additional research. 
Scientists and managers are now considering a 
restoration program for Atlantic sturgeon in Chesa- 
peake Bay and elsewhere (St. Pierre, 1994; Secor, 
1995). The feasibility of a sturgeon restoration pro- 
gram must address the same issues that led to the 
sturgeon’s decline. If these conditions persist in 
Chesapeake Bay, a restoration program cannot be 
easily justified. Necessary conditions for population 
recovery must include increased population abun- 
dance, and improvement in the quality and size and 
number of essential habitats. Population abundance 
