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Hypoxia Tolerance of Hemichromis letourneuxi

Hypoxia tolerance

An important attribute of swamp-adapted fish is their ability to tolerate periods of low dissolved oxygen (hypoxia). In the Rocky Glades region of the Everglades, water levels recede during the dry season and drop below ground surface, leaving fish with few options for refuge. Fish can retreat to the deep, box-cut canals; however, canal habitats often contain numerous predators. Another refuge option is to remain in the marsh and use solution holes in the limestone matrix. Solution holes vary in size and depth, and can become harsh habitats that are frequently crowded and subject to periodic hypoxia.

Few fishes are capable of surviving during the dry season in solution holes, and it was not known whether H. letourneuxi would be able to colonize them. We tested the hypoxia tolerance of H. letourneuxi and two native centrarchid sunfishes that are abundant in solution holes (dollar sunfish [Lepomis marginatus] and warmouth [Lepomis gulosus]).

Dollar sunfish (Lepomis marginatus) Photo by Howard Jelks, USGS

Warmouth (Lepomis gulosus) Photo by Howard Jelks, USGS

Culvert pool where fishes were collected.

The flooded marsh is seasonally exploited by Rocky Glades fishes.

In the experimental trials, the dissolved oxygen content of water was gradually lowered (approximately 1 to 1.25 mg L-1 per hour) over several hours by bubbling nitrogen gas into an aquarium. Use of aquatic surface respiration (ASR), gill ventilation rates and frequency of agonistic interactions were recorded.

A wide variety of fishes use ASR when faced with low dissolved oxygen, and this behaviour increases the probability of their survival during hypoxic conditions (Kramer and McClure 1982). However, the efficiency of ASR varies by species. To compare the abilities of the fishes in our study to cope with hypoxia, ASR thresholds were compared. These values are the levels of oxygen at which 10% (ASR10), 50% (ASR50) and 90% (ASR90) of the fish performed ASR. Fishes from habitats with chronic hypoxia generally wait until dissolved oxygen levels are very low before initiating ASR.

We also compared gill ventilation rates among the three fish species. Lower gill ventilation rates generally reflect better adaptation to hypoxia. Additionally, the lowering of gill ventilation rates after the onset of ASR indicates that the species is efficient in its use of ASR.

Aquatic surface respiration (ASR) thresholds and gill ventilation rates for H. letourneuxi were lowest of the three species. Aquatic surface respiration thresholds for L. marginatus were typical of small, freshwater tropical fishes, whereas those of L. gulosus were similar to swamp-adapted fishes. For H. letourneuxi, ASR thresholds were some of the lowest reported in the literature. Overall, although all three species easily tolerated hypoxia, H. letourneuxi appeared to be the species best adapted to cope with hypoxia, followed by L. gulosus and then L. marginatus. Hemichromis letourneuxi also exhibited more aggressive behaviours than the two native sunfishes. These results suggest that hypoxia will not likely restrict H. letourneuxi from exploiting the seasonally-inundated marshes of south Florida and expanding its range there.

References

Kramer, D. L. and M. McClure. 1982. Aquatic surface respiration, a widespread adaptation to hypoxia in tropical freshwater fishes. Environmental Biology of Fishes 7: 47-55.

Full Citation

Schofield, P.J., W.F. Loftus & M.E. Brown. 2007. Hypoxia tolerance of two centrarchid sunfishes and an introduced cichlid in karstic Everglades wetlands of southern Florida, U.S.A. Journal of Fish Biology 71 D: 87-99.

Acknowledgements

Funding for this study was provided by the U.S. Geological Survey Invasive Species program and the U.S. Fish and Wildlife Service, Region 4. This study was conducted under International Animal Care and Use Committee (IACUC) approval protocol USGS/FISC 2006-02. Permission to collect fishes was granted through Everglades National Park Scientific Collectors’ Permit numbers EVER-2005-SCI-0106 and EVER-2007-SCI-0014. Carla Wieser provided technical assistance in the laboratory. Buck Albert assisted with photography.

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