Temperature and Hypoxia Tolerances of the Indigenous Dollar Sunfish (Lepomis
marginatus) and the Non-indigenous Black Acara (Cichlasoma bimaculatum)
Sean W. Kirkpatrick and John C. Volin
Florida Atlantic University, Davie, FL
William Loftus
U. S. Geological Survey, Center for Water and Restoration Studies, Miami, FL
Wetlands that are being restored as a part of the Big Cypress Seminole Indian
Reservation Water Conservation Plan (WCP) provide ideal sites for studying native and
non-native fish population dynamics. The WCP, a designated Critical Project of the
Comprehensive Everglades Restoration Plan, involves construction of a network of
surface water management structures designed to improve hydrology and water quality
within the Reservation. The wetland restoration sites are bordered by agricultural and
stormwater drainage canals which provide the initial influx of water and aquatic fauna
into the newly created wetlands. Along with various native fish species, these canals
contain several non-indigenous Cichlidae species (such as red oscar, black acara, blue
tilapia, Mayan cichlid and spotted tilapia). The objective of this study was to compare the
temperature and hypoxia tolerances of two similar trophic-level fish, the indigenous
centrarchid, the dollar sunfish (Lepomis marginatus) with the non-indigenous cichlid, the
black acara (Cichlasoma bimaculatum).
We hypothesized that the non-native black acara would be more tolerant of low dissolved
oxygen levels than the native dollar sunfish. We also hypothesized that the black acara
would be less tolerant of exposure to cold temperatures in the range that occurs
commonly in South Florida waterways. To address these questions, we measured the
behavioral responses to various hypoxia regimes for both species at three different
acclimation temperatures and quantified the temperature tolerances at normal oxygen
levels.
Juvenile specimens were collected from canals and borrow ponds located on the Big
Cypress Seminole Indian Reservation. Each trial set of fish (n=20) were acclimated for at
least seven days prior to the trial date at 20, 25, or 30°C. For the temperature tolerance
trials, dissolved oxygen levels were kept constant while temperatures were
lowered/raised from the acclimation temperature at a constant rate of 0.15°C min-1 until
loss of equilibrium (LOE-failure to maintain dorsal-ventral equilibrium for 30 seconds).
To investigate hypoxia tolerance, ventilation rate and proportion of time spent
undergoing aquatic surface respiration were recorded for specimens exposed to both
gradually induced hypoxia and acute anoxia. During the gradually induced hypoxia trials,
dissolved oxygen levels were lowered to anoxia at a constant rate over four hours. For the
acute anoxia trials, dissolved oxygen levels were abruptly lowered to anoxic levels.
Temperature were maintained at the specimens’ acclimation temperature throughout the
hypoxia tolerance trials.
We found that for all acclimation temperatures, dollar sunfish and black acara differed in
their tolerance to extreme temperatures. When temperatures were raised above 35oC, the
LOE temperature for black acara was significantly higher than for dollar sunfish
(p<0.01). Conversely, when temperatures were dropped to below 10oC, dollar sunfish had
a significantly lower LOE than black acara (p<0.01). These results suggest that, as
observed for several other non-indigenous tropical fish in South Florida, the northern
expansion of black acara will be significantly limited by cold intolerance.
The hypoxia trials showed black acara to be more tolerant of anoxic conditions. During
gradually induced hypoxia, dollar sunfish significantly increased ventilation rates while
black acara ventilation rates were not significantly different. During acute anoxia, dollar
sunfish increased ventilation rate and lost equilibrium before the end of the trial period.
In contrast, black acara exposed to acute anoxia decreased ventilation rate and were able
to maintain equilibrium. These results may help to explain why cichlids are able to
maintain viable populations despite hypoxic conditions frequently encountered in South
Florida during the summertime diel cycle of dissolved oxygen.
John, Volin, Florida Atlantic University Environmental Sciences, 2912 College Avenue,
Davie, FL 33314, Phone: 954-236-1115, Fax: 954-236-1099, E-mail: jvolin@fau.edu,
Invasive and Exotic Species.