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| Florida Integrated Science Center |
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Understanding Everglades Invaders: Abstract A dozen species of non-indigenous fishes have colonized the wetlands of south Florida. Although some of these species continue to spread geographically, others remain local. Information relating to their biology, ecology, and environmental tolerances has been accumulating, but data gaps remain for several common species. These data are needed to understand and model the ultimate range expansion, habitat occupation and dynamics of non-native fishes, and are particularly important for risk assessments. In this series of studies, we are experimentally determining the environmental tolerances of African jewelfish Hemichromis letourneuxi to salinity, low dissolved oxygen (hypoxia) and low temperatures. The species is presently expanding its range rapidly throughout the freshwater wetlands of south Florida, where it may compete with native fishes for food and nesting sites (Fuller and others 1999; W.F. Loftus, pers. comm., 2007). These data will be used to estimate the habitats that this species may ultimately colonize in the Florida ecosystem. African jewelfish in South Florida The African jewelfish Hemichromis letourneuxi is native to the north and northwestern regions of Africa. Although the species has been present in the canals of south Florida since the 1950s, its geographic range has expanded greatly in recent years and continues to spread throughout south Florida habitats, from Everglades National Park to Big Cypress National Park (Loftus and others 2006). The species competes with native fishes for food and nesting sites and preys directly on native fishes (Fuller and others, 1999; W.F. Loftus, pers. comm., 2007). It is unclear what the ultimate distribution of H. letourneuxi will be, but tolerance to environmental stress is a limiting factor.
Studies on African jewelfish:
References Fuller, P. L., Nico, L.G. and J.D. Williams. 1999. Nonindigenous Fishes Introduced into Inland Waters of the United States. Special Publication 27. Bethesda, MD: American Fisheries Society. Loftus, W. F., Trexler J. C., Dunker, K., Liston, S. E. and J. S. Rehage. 2006. Introduced Fishes in Short-Hydroperiod Wetlands: Evaluation of Sampling, Status, and Potential Effects. Final Report from USGS to Everglades NP for Agreement # CESI IA F5284-04-0039. 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]).
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.
During aquatic surface respiration (ASR), fish use the uppermost layer of water that is generally richer in oxygen than deeper in the water column. This widespread adaptation to hypoxia is present in many different fish families. Contrary to popular belief, ASR does not involve air-breathing.
During experimental trials, the scientist sits quietly behind a blind while recording data. The fish are viewed through a small opening in the blind. 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.
ASR thresholds for the three fish species tested in this experiment. For reference, 10 mm Hg is approximately equivalent to 0.5 mg/L, and 20 mm Hg is approximately 1.0 mg/L.
Gill ventilation rates for the three fishes tested in this study. Boxes shaded in grey are for readings taken before the onset of ASR; unshaded boxes are for readings taken after ASR onset. The dark line inside the box is the median, the box bounds 50% of the data, and the whiskers show the data limits.
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. For more information on this study: Schofield, P. J., W. F. Loftus and M. E. Brown. 2007. Hypoxia tolerance of native centrarchid sunfishes (Lepomis gulosus Cuvier, 1829] and L. marginatus [Holbrook, 1855]) and an introduced cichlid, African jewelfish (Hemichromis letourneuxi Sauvage, 1880), in karstic Everglades wetlands of southern Florida, U.S.A. Accepted: Journal of Fish Biology. |
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