The
Influence of Dissolved Oxygen on Winter Habitat
Selection by Largemouth Bass: An Integration of
Field Biotelemetry Studies and Laboratory
Experiments
In this study, field biotelemetry and
laboratory physiology approaches were coupled to
allow understanding of the behavioral and
physiological responses of fish to winter
hypoxia. The biotelemetry study compared
dissolved oxygen levels measured throughout the
winter period with continually tracked locations
of nine adult largemouth bass obtained from a
whole-lake submerged telemetry array. Fish
habitat usage was compared with habitat
availability to assess whether fish were
selecting for specific dissolved oxygen
concentrations. The laboratory study examined
behavioral and physiological responses to
progressive hypoxia in juvenile largemouth bass
acclimated to winter temperatures. Results from
the dissolved oxygen measurements made during
the biotelemetry study showed high variance in
under-ice dissolved oxygen levels. Avoidance of
water with dissolved oxygen <2.0 mg/L by
telemetered fish was demonstrated, but
significant use of water with intermediate
dissolved oxygen levels was also found. Results
from the lab experiments showed marked changes
in behavior (i.e., yawning and vertical
movement) at <2.0 mg/L of dissolved oxygen
but no change in tissue lactate, an indicator of
anaerobic metabolism. Combined results of the
biotelemetry and laboratory studies demonstrate
that a dissolved oxygen content of 2.0 mg/L may
be a critical threshold that induces behavioral
responses by largemouth bass during the winter.
In addition, the use by fish of areas with
intermediate levels of dissolved oxygen suggests
that there are multiple environmental factors
influencing winter behavior.
Introduction
Aquatic systems are spatially heterogeneous
for a number of variables across a range of
scales. Environmental heterogeneity exists
because of spatial and temporal variations of
different abiotic (e.g., temperature, dissolved
oxygen, wave action, sunlight, and salinity) and
biotic (e.g., prey abundance, vegetation cover,
and conspecific location) factors that result in
patches of optimal habitat mixed with patches of
suboptimal and intermediate habitat. For
example, lakes have some patches that are warmer
than others, and these temperatures change daily
and seasonally for a variety of reasons. Thus,
aquatic organisms must continually seek out and
compete for habitats that optimize their
requirement for a suite of environmental
resources based on a lake's abiotic and biotic
factors (Hutchinson 1957, 1965; Fretwell and
Lucas 1970). One important environmental
resource that influences fish habitat selection
and many physiological processes is dissolved
oxygen (Hughes 1973; Kramer 1987).
Fish respond to reduced levels of dissolved
oxygen in a variety of ways. Typically,
chemoreceptors sense a decrease in ambient
oxygen, and some physiological responses occur
(e.g., increases in ventilation rate and
amplitude; Perry and Gilmour 2002). Next, a
behavioral response occurs, which may include
habitat shifts; it is followed by the use of
air-breathing organs (if present) and an
increased use of surface respiration (Kramer
1987). After these behavioral responses, fish
often exhibit more physiological responses to
reduced levels of dissolved oxygen, including a
decrease in cardiac output (Furimsky et al.
2003). Finally, if no other response is
adequate, fish will switch to relying on
anaerobic metabolism to meet energy demands.
Anaerobic metabolism produces far less ATP
relative to aerobic mechanisms, and negative
consequences of anaerobic metabolism include the
production of lactate and a decrease in blood
pH, both of which must be actively cleared on
return to an oxygenated environment (Bennett
1978; Wendelaar Bonga 1997; Furimsky et al.
2003; Martinez et al. 2006). The behavioral,
physiological, and biochemical responses of fish
to hypoxia during winter conditions are not
fully understood, and cold temperature may be an
important covariable that influences behavioral
and/or physiological outcomes because of its
importance in determining rates of reactions.
During winter in lakes located at high
latitudes, dissolved oxygen is often less
abundant than in summer, and this reduction in
dissolved oxygen can influence fish behavior and
habitat selection (Suski and Ridgway 2009).
Many northern temperate lakes can experience
hypoxia (or even anoxia) during winter; ice
cover, low light intensity, reduction in
photosynthetic biomass, benthic decomposition,
and crowding of fish can combine to reduce
dissolved oxygen concentrations in localized
areas (Greenbank 1945; Cooper and Washburn
1949). Winter hypoxia has previously been shown
to influence fish behavior, movement, activity,
species richness, species ranges, and population
structure (Shuter and Post 1990; Fox and Keast
1991; Gent et al. 1995; Nu¨ rnberg 1995;
Raibley et al. 1997; Tonn and Magnuson 1982;
Farwell et al. 2007). However, few studies have
measured the impact of winter hypoxia on
seasonal fish habitat selection or linked
fieldand laboratory-based responses to winter
hypoxia. To quantify the impacts of hypoxia on
winter fish responses and populations, many past
studies have used controlled laboratory settings
to reproduce conditions that can occur in nature
(e.g., Petrosky and Magnuson 1973; Furimsky et
al. 2003).
Results obtained from these studies are
valuable because they allow the isolation of
individual environmental variables on habitat
selection to be determined in a controlled
setting; however, they do not consider the suite
of factors that can influence habitat selection
for free-swimming fish. Biotelemetry, or the
remote monitoring of free-ranging individuals,
provides clues about why fish choose particular
habitats over others and allows for an
assessment of environmentaland individual-level
variables (Lucas and Baras 2000; Cooke et al.
2004). The goal of this study was to use a
combined approach involving both biotelemetryand
laboratory-based experiments to better
understand the influence of winter hypoxia on
the behavior, physiology, and ecology of
temperate fishes. Largemouth bass (Micropterus
salmoides) was chosen as the study species
because it is abundant in northern lakes that
frequently experience winter hypoxia and
winterkill (Scott and Crossman 1973).
For the biotelemetry portion of the study, a
whole-lake acoustic telemetry array was used to
compare the locations of fish in the winter with
lakewide dissolved oxygen concentrations. For
the laboratory study, the behavioral and
physiological responses of fish to progressive
hypoxia at winter temperatures were quantified.
The combined results from these two studies will
improve our understanding of how environmental
variables influence habitat selection in fishes
and will also allow a direct coupling between
fieldand laboratory-based observations.
Discussion
Eastern Ontario's Warner Lake experiences
considerable variation in concentrations of
dissolved oxygen available to largemouth bass
over the winter. During this study's sampling
dates when ice was not present on the lake, mean
dissolved oxygen concentrations ranged from 10.2
to 11.3 mg/L; the twenty-fifth and seventy-fifth
percentiles for dissolved oxygen concentration
on these dates were 10.1 and 11.4 mg/L,
respectively. Hence, fish were exposed only to
water with dissolved oxygen concentrations
greater than 10 mg/L. In contrast, during the
icecover sample dates, mean dissolved oxygen
concentrations across all of Warner Lake ranged
from 2.7 to 3.8 mg/L, with the twenty-fifth and
seventy-fifth percentiles on these dates 0.6 and
6.5 mg/L, respectively. Consequently, fish had
access to water with a greater range of
concentrations of dissolved oxygen available at
these times, and the mean dissolved oxygen
concentration was lower than during ice-free
periods.
In addition, there was significant variation
in dissolved oxygen concentration laterally
across different sampling sites in the lake,
providing a patchwork of available oxygen
concentrations for largemouth bass to inhabit.
The variance across sample dates in dissolved
oxygen, as well as the overall reduction in
dissolved oxygen concentration during periods of
ice cover, is a result of a variety of inherent
characteristics of temperate lakes at high
latitudes. These include ice cover, organic
decomposition, and The combined results of the
biotelemetry and the laboratory studies
demonstrate that largemouth bass show behavioral
changes when exposed to water with dissolved
oxygen concentration below 2 mg/L during winter.
Sites in Warner Lake that had dissolved oxygen
concentrations !2 mg/L contained significantly
fewer largemouth bass than would be expected if
fish were uniformly distributed with respect to
oxygen concentration, while significantly more
largemouth bass than expected were found in
areas with 12 and !6 mg/L of dissolved oxygen.
In addition, during the laboratory study,
largemouth bass significantly increased
yawning (or gill flaring) and vertical
movement behaviors when exposed to water with
dissolved oxygen concentrations of 1.99 mg/L or
less. Previous laboratory studies with yellow
perch (Perca flavescens) and bluegill (Lepomis
macrochirus) have shown that activity levels
increase when fish are exposed to less than 2
mg/L of dissolved oxygen, presumably as they
attempt to seek out habitat that is more
oxygenated (Scherer 1971; Petrosky and Magnuson
1973). In our laboratory study, yawning
activity (or gill flaring) increased at 1.99
mg/L, probably in order to increase water flow
across gill lamellae in an effort to increase
the amount of dissolved oxygen entering the
bloodstream (Hughes 1973; Randall 1982; Perry
and Gilmour 2002).
In winter field studies, Raibley et al.
(1997) measured the dissolved oxygen at specific
locations for multiple telemetered largemouth
bass and found the fish to be consistently in
water with dissolved oxygen concentrations 12
mg/L; however, they did not consistently record
the dissolved oxygen concentrations at fish
locations, and they did not quantify dissolved
oxygen throughout the river system. Largemouth
bass avoiding water with poor levels of
dissolved oxygen would be advantageous because
prolonged exposure to these waters could lead to
costly, inefficient anaerobic metabolism,
suffocation, and even death (Greenbank 1945;
Cooper and Washburn 1949).
The combined results of our biotelemetry and
laboratory studies suggest that a minimal level
of dissolved oxygen near 2 mg/ L is a threshold
below which behavioral changes in overwintering
largemouth bass are induced. Interestingly, even
though telemetered largemouth bass in our study
showed an aversion to water that contained less
than 2 mg/L dissolved oxygen, they did not
choose to inhabit the most oxygenated water
available. Specifically, during the entire
period of ice cover, fish were found to inhabit
sites with intermediate levels of dissolved
oxygen (concentrations between 6 and 2 mg/L),
even though water with 7, 8, and 9 mg/L of
dissolved oxygen was available.
Previous studies have documented an animal's
niche to be a combination of physical and biotic
interactions, with the two types of interaction
not always acting independently in space and
time (Hutchinson 1957, 1965; Chapman 1966; Tracy
and Christian 1986). Fry (1971) suggested that
there are at least seven important factors in a
fish's niche (temperature, dissolved oxygen,
toxicity, metabolites, food, salinity, and
carbon dioxide), while more recent work suggests
that other physical and biotic characteristics
(such as cover, water velocity, depth,
territories, and aggregations) are equally
important to niche generation (Stott and Cross
1973; Suthers and Gee 1986; Kramer 1987; Spoor
1990; Heggenes et al. 1999; Hasler et al. 2007).
If some or all of these parameters are assessed
by fish in Warner Lake when they make habitat
decisions, the choice of intermediate oxygen
patches must have benefits that outweigh the
advantages associated with inhabiting more
oxygenated water.
Specifically, in addition to oxygen, factors
such as proximity to conspecifics (Breder and
Nigrelli 1935; Hasler et al. 2007), proximity to
cover, and/or prey abundance may affect
selection of habitat by largemouth bass. Fish in
this study chose not to inhabit areas with the
highest dissolved oxygen concentrations and
selected areas with lower amounts of dissolved
oxygen, which suggest that fish are using other
environmental variables in conjunction with
dissolved oxygen to make habitat choices.
Although there was an avoidance of areas with
dissolved oxygen !2 mg/L, two of nine fish spent
some time in such water during the March
sampling period (although only for a few hours
per day).
Moreover, during the laboratory study, fish
did not exhibit increased lactate concentrations
in white muscle, despite 1 h exposure to
dissolved oxygen concentrations !2 mg/L,
indicating that they were still respiring
aerobically, despite this low oxygen
concentration. It is important to consider,
however, that oxygen requirements in winter will
be low; two separate studies have concluded that
the metabolic rate of largemouth bass during the
winter months is greatly reduced and that fish
are essentially dormant at that time (Beamish
1970; Crawshaw 1984; Lemons and Crawshaw 1985).
In our study, the use of intermediate areas by
largemouth bass is not unexpected, since
previous studies have shown that hypoxia is not
an absolute barrier to fish movements and that
fish will use hypoxic zones for opportunistic
feeding (Pihl et al. 1992; Rahel and Nutzman
1994).
One possible reason is that during winter
conditions, slightly higher temperatures that
are present in areas with low amounts of
dissolved oxygen (because of decomposition of
organic material) would allow for increased
metabolism and activity (Fry 1971; Gee et al.
1978; Burleson et al. 2001). In winter, it may
be beneficial for fish to tolerate lower
dissolved oxygen concentrations that might be
lethal during warmer periods, when the oxygen
requirements are relatively higher (Fry 1971).
However, our laboratory study did not find a
physiological change when fish were exposed to
hypoxia: tissue lactate, an indicator of
anaerobic respiration, did not change. It is
evident from the current biotelemetry and
laboratory studies that largemouth bass tolerate
low levels of dissolved oxygen during winter.
In addition, short-term laboratory exposure
to low amounts of dissolved oxygen did not
facilitate a metabolic change, suggesting that
the physiological consequence of winter habitat
selection is minimal. Age and size differences
among fish of the same species may influence
behavioral and physiological responses to
stressors such as exhaustive exercise or low
dissolved oxygen (Cech et al. 1979; Petersen and
Petersen 1990; Kieffer et al. 1996; Burleson et
al. 2001). For example, Kieffer et al. (1996)
found a significant positive relationship
between the accumulation of muscle lactate,
metabolic protons, and body size in brook trout
(Salvelinus fontinalis ) that were exhaustively
exercised (exercise that essentially results in
hypoxic conditions in the swimming muscle;
Kieffer et al. 1996) but did not find a
difference in the anaerobic response to exercise
between large and small largemouth bass. In our
study, larger fish were used in the biotelemetry
study than in the laboratory study because of
size limitations related to transmitter
implantation.
However, regardless of this size difference,
both experiments revealed a similar threshold at
which a response to hypoxia was
generated&emdash;approximately 2 mg/L.
Specifically, in our laboratory study,
yawning and vertical movement began to
happen at 1.99 mg/L, while in the telemetry
study, fish were rarely found in water with less
than 2 mg/L of dissolved oxygen. In a similar
result, Burleson et al. (2001) found that
regardless of fish size, largemouth bass
typically avoided water below 2.4 mg/L during
laboratory experiments. Also, they did not find
differences in short-term selection for
particular areas; larger fish were equally as
likely to venture for brief periods of time to
areas of low dissolved oxygen concentration when
compared to smaller fish (Burleson et al.
2001).
Thus, results from previous studies have not
documented a size-specific response to anaerobic
stressors for largemouth bass (Kieffer et al.
1996; Burleson et al. 2001). Likewise, fish in
our biotelemetry and laboratory studies,
although different in size, demonstrated similar
avoidance responses to cold water with
approximately 2 mg/L of dissolved oxygen.
Conclusion
Biotelemetry and laboratory studies
conducted to determine the factors affecting
behavior and physiology are most often performed
independently. In this study, these two
approaches were used synergistically to quantify
the effect of hypoxia on the behavior and
physiology of overwintering largemouth bass.
Ambient dissolved oxygen concentration was found
to influence not only individual behavioral
responses such as increased surface breathing
but also habitat selection in the wild.
More specifically, largemouth bass
telemetered in the field tended to avoid areas
with dissolved oxygen concentrations !2.0 mg/L,
and laboratory-tested largemouth bass exhibited
behavioral responses, such as yawning and
vertical movement, when exposed to water with
dissolved oxygen levels near 2.0 mg/L.
Large-mouth bass tended to choose water with
intermediate concentrations of dissolved oxygen
over the most oxygenated water available,
possibly because of multiple abiotic and biotic
variables. Overwintering largemouth bass appear
to avoid water with less than 2.0 mg/L of
dissolved oxygen, but further research is needed
to understand the extent to which prolonged
exposure to low amounts of dissolved oxygen
affects physiological processes.
