Anweiler et al.: Effects of temperature and hypoxia on the metabolic performance of A lorone saxatilis 
345 
limit. In this study, MMR increased with temperature all 
the way up to 32°C (Table 1, Fig. 3). The rise in MMR, in 
concurrence with the smaller increase in SMR, resulted in 
a steady rise in AMS across all temperatures. In effect, the 
decrease in MMR and, therefore, in AMS at high tempera¬ 
tures that should signal the onset of thermal intolerance 
(Portner, 2001, 2002) was absent in our sample of juvenile 
striped bass. 
Although high temperature did not decrease the AMS 
available for striped bass, low DO levels did decrease 
it, with both MMR and AMS declining at the lower DO 
levels tested. These results support our third prediction. 
In fact, DO concentration clearly played an important 
role in limiting MMR and AMS. Most notably, the sharp 
differences between the values of AMS at the DO level 
of 4.0 mg/L and the values of AMS at DO levels of 2.5 
or 3.0 mg/L (Fig. 3, B-D) indicate that DO level has a 
greater influence on AMS than the range of water tem¬ 
peratures used in our study. Even when DO concentra¬ 
tion remained above a level that would directly cause 
fish mortality (e.g., 2 mg/L), the reduction in AMS that 
was observed in this study with low environmental DO 
concentration could limit the capacity of striped bass for 
locomotion, foraging, growth, or reproduction (Coutant, 
1990; Neill and Bryan, 1991). These conclusions are fur¬ 
ther supported by the results of our analyses based on 
P0 2 because, in models that used it, P0 2 was signifi¬ 
cantly positively related to AMS and MMR but tempera¬ 
ture was not (Table 2). 
Fish that were exercised at lower DO levels had shorter 
exhaustion times, further supporting the findings that 
low DO concentration plays a larger role in limiting met¬ 
abolic capacities than temperature. At every temperature 
tested, fish exercised at a DO level of 4.0 mg/L had sig¬ 
nificantly longer exhaustion times (Table 1). Although 
there was no statistically significant effect of tempera¬ 
ture on exhaustion time, fish that were acclimated and 
exercised at 25°C had slightly longer exhaustion times on 
average, and swam to greater speeds, than their counter¬ 
parts at other temperatures (Table 1, Fig. 2). This trend is 
more pronounced when P0 2 is substituted in the model, 
with exhaustion time being significantly longer at 25°C 
(Table 2). This result indicates that, although AMS may 
not be optimal at 25°C, swimming performance may be 
higher at this intermediate temperature. 
The overall effects of high temperature on survival of 
southern populations of striped bass remain unknown; 
however, the striped bass used in this study were not 
likely to have been metabolically limited in the Ashley 
River. Temperatures above 32°C are rare and brief in this 
river. Although DO levels between 3.0 and 4.0 mg/L are 
common, DO concentrations below 3.0 mg/L account for 
only 1% of all values recorded at water-quality stations 
during June-August (M. Denson, unpubl. data). Several 
caveats, however, should be noted. 
First, because of the decline in AMS with decreasing 
DO concentration, occupancy of hypoxic habitats (DO 
levels <3 mg/L) may have long-term metabolic costs, 
which could eventually affect growth and reproduction 
depending on the amount of time spent under such con¬ 
ditions. Second, temperatures in the southeastern United 
States are projected to increase by 2.2-4.4°C by the end 
of this century, with an additional 10-40 d over 35°C per 
year in South Carolina (Carter et al., 2014). The maxi¬ 
mum temperature that fish will be exposed to and the 
amount of time spent at high temperatures will increase. 
If 32°C is near the optimum for AMS, an increase in 
temperature by a few degrees could drastically decrease 
AMS, limiting available summer habitat for wild striped 
bass in rivers of the southeastern United States. Lastly, 
metabolic tolerances in relation to temperature and DO 
concentration may change as fish grow and age. In north¬ 
ern populations, adult striped bass have a lower tem¬ 
perature tolerance than juveniles. Additionally, Johnson 
(2015) found that age-0 striped bass in the Ashley River 
have positive growth rate potential over a wider range of 
temperatures than older fish. It is reasonable to conclude 
that in southern populations, AMS of adult striped bass 
will decrease at a lower temperature than it would for 
juveniles. Lower AMS at high temperatures will likely 
increase mortality and reduce the reproductive potential 
of a population. 
Larger fish had greater raw MMR and AMS and lon¬ 
ger exhaustion times, a finding similar to those of several 
previous studies (Clarke and Johnston, 1999; Killen et al., 
2006; Norin and Malte, 2011). This observation reflects 
the greater abilities of larger individuals. Although raw 
AMS is greater in larger fish, larger fish will also face 
absolutely greater metabolic demands from the physio¬ 
logical systems that support swimming. Therefore, the 
larger AMS of larger fish may not convey a greater abil¬ 
ity to deal with variable environmental conditions. Con¬ 
versely, heavier fish had lower relative AMS and MMR 
(i.e., size-adjusted metabolic rates). The negative rela¬ 
tionship between weight and size-adjusted active metab¬ 
olism can be explained by the supposition that heavier 
fish contain more stored reserves that are not involved 
in active metabolism (Brett, 1964). Lapointe et al. (2014) 
measured the AMS of larger Chesapeake Bay striped 
bass (mean weight: 1300g) under hypoxic conditions with 
a DO level of 3 mg/L. At 20°C, the striped bass used in 
our study had 80% greater AMS than the larger striped 
bass used by Lapointe et al. (2014), although a portion of 
this difference could be due to differences between north¬ 
ern and southern populations. We note that the aim of 
our study was to determine the AMS of striped bass that 
were most comparable to large age-1 or small age-2 wild 
fish during the summer in South Carolina. Therefore, a 
relatively small range of weights and Kn were intention¬ 
ally used in this study. The consequence of this narrow 
size range is that the significant relationship between 
weight and AMS may not hold for striped bass of other 
sizes in South Carolina. 
Given trends in the recapture of striped bass by the 
SCDNR in the Ashley River and knowledge of the physical 
conditions within this river, juvenile striped bass are occu¬ 
pying habitats that, during the warmest days of summer, 
are near the edge of their thermal tolerance. We predict 
