Respiratory characters of three species of haplochromine cichlids: Implications for use of wetland refugia

Respiratory characters of three east African haplochromine cichlid species that differ in their use of hypoxic wetlands were examined to consider the potential of dissolved oxygen as one factor affecting habitat use. All three species had a large gill surface area, ranging from the 67th ( Pseudocrenilabrus multicolor victoriae ) to 98th ( Astatotilapia velifer ) percentile of the known gill size range for freshwater fishes. Pseudocrenilabrus multicolor victoriae was the most tolerant to hypoxia exhibiting the lowest aquatic surface respiration ( Rs )thresholds and lowest critical oxygen tension of the three species. Astatotilapia velifer had the highest ASR thresholds, gill ventilation rates, and level of surface activity of the three species, indicating a relatively low tolerance to hypoxia. Prognathochromis venator was intermediate in its response to hypoxia. These findings are discussed in light of survivorship and distribution patterns of these species following Nile perch introduction into Lake Nabugabo.     

Interactions Between Nile Perch, Lates niloticus, and Other Fishes in Lake Nabugabo, Uganda

The introduction of the predatory Nile perch, Lates niloticus, into the Lake Victoria basin coincided with a dramatic decline in fish species richness and diversity. This study focused on interactions between Nile perch and indigenous fishes in Lake Nabugabo, Uganda, a small satellite lake of Lake Victoria. We evaluated how the foraging impact of juvenile Nile perch on prey fishes varied with the size of the predator. We also evaluated the role of wetland ecotones in minimizing interaction between Nile perch and indigenous fishes. Wetland ecotones in Lake Nabugabo were characterized by complex structure (e.g., dense vegetation) and lower dissolved oxygen levels than non-wetland (exposed) areas. Nile perch (8.6–42.2cm, TL) were 3.7 times more abundant in offshore exposed areas than in inshore areas near wetland ecotones, and the proportion of Nile perch using wetland and exposed areas was independent of their body size. However, species richness was higher in waters at wetland ecotones than in exposed areas. Nile perch (5–35cm, TL) exhibited a shift in diet at approximately 30cm TL from feeding primarily on invertebrates to piscivory. Although the shift to piscivory occurred at approximately the same body size for Nile perch from both wetland and exposed habitats, the shift to piscivory was less abrupt in Nile perch captured near wetland ecotones. Nile perch from wetland areas consumed a greater diversity and a larger percentage of fish prey than those from exposed sites. However, the low abundance of Nile perch in wetland ecotones suggested that interaction between predator and prey in these areas is much reduced.     

Divergence in aerobic scope and thermal tolerance is related to local thermal regime in two populations of introduced Nile perch (Lates niloticus)

We tested whether thermal tolerance and aerobic performance differed between two populations of Nile perch (Lates niloticus) originating from the same source population six decades after their introduction into two lakes in the Lake Victoria basin in East Africa. We used short-term acclimation of juvenile fish to a range of temperatures from ambient to +6°C, and performed critical thermal maximum (CT_max) and respirometry tests to measure upper thermal tolerance, resting and maximum metabolic rates, and aerobic scope (AS). Across acclimation temperatures, Nile perch from the cooler lake (Lake Nabugabo, Uganda) tended to have lower thermal tolerance (i.e., CT_max) and lower aerobic performance (i.e., AS) than Nile perch from the warmer waters of Lake Victoria (Bugonga region, Uganda). Effects of temperature acclimation were more pronounced in the Lake Victoria population, with the Lake Nabugabo fish showing less thermal plasticity in most metabolic traits. Our results suggest phenotypic divergence in thermal tolerance between these two introduced populations in a direction consistent with an adaptive response to local thermal regimes.     

Intraspecific variation in gill morphology of juvenile Nile perch, Lates niloticus, in Lake Nabugabo, Uganda

Several studies have demonstrated intraspecific variation in fish gill size that relates to variation in dissolved oxygen (DO) availability across habitats. In Lake Nabugabo, East Africa, ecological change over the past 12 years has coincided with a shift in the distribution of introduced Nile perch such that a larger proportion of the population now inhabits waters in or near wetland ecotones where DO is lower than in open waters of the lake. In this study, we compared gill size of juvenile Nile perch between wetland and exposed (open-water) habitats of Lake Nabugabo in 2007, as well as between Nile perch collected in 1996 and 2007. For Nile perch of Lake Nabugabo [<20 cm total length (TL)], there was a significant habitat effect on some gill traits. In general, fish from wetland habitats were characterized by a longer total gill filament length and average gill filament length than conspecifics from exposed habitats. Nile perch collected from wetland areas in 2007 had significantly larger gills (total gill filament length) than Nile perch collected in 1996, but there was no difference detected between Nile perch collected from exposed sites in 2007 and conspecifics collected in 1996.     

Genetic affinities of an introduced predator: Nile perch in Lake Victoria, East Africa

Although the introduction of Nile perch, Lates niloticus , to Lake Victoria has received intense global attention, especially in relation to its impact on endemic cichlid species and on fishery yields, fundamental information on its taxonomy and population genetics is lacking. Most importantly, the introduced fish originated from two lakes (Lakes Albert and Turkana) containing three Lates species, and it has never been entirely clear which of these became established in Lake Victoria, or indeed whether the Lake Victoria population is derived from hybridization between Lates species. In addition, genetic drift caused by the relatively small founder population (≈ 400), the initially slow population increase followed by a period of explosive population growth, and selection pressures in the new environment may have resulted in substantial genetic changes. Allozyme data indicated that the introduced Nile perch of Lake Victoria were mainly L. niloticus from Lake Albert, although maximum likelihood estimates of stock contributions (GSI) suggested the presence of L. macrophthalmus. In contrast, introduced Nile perch in adjacent smaller lakes (Lakes Kyoga and Nabugabo) appeared to be entirely L. niloticus . The effect of the introductions on allozyme diversity varied among lakes and appeared to be uncorrelated to the number of fish introduced.     

Lake Victoria fisheries: the Kenyan reality and environmental implications

Synopsis The introduction of the Nile perch into Lake Victoria has dramatically altered the fishery in that lake and contributed to the decline of the fishery for indigenous tilapias. One sector of the fishery in Lake Victoria has benefitted from the Nile perch introduction, although catches have declined in recent years. Inefficient enforcement of fisheries regulations has had a detrimental effect on indigenous species but may also have contributed to the recent decline in Nile perch catches. Fisheries development plans have tended to favour capital-intensive fisheries and to ignore small scale subsistence fisheries. A case study at Wichlum Beach on the Kenyan shores of Lake Victoria has revealed the efficiency of traditional fishing and fish drying methods as well as the high ecological costs of the practice of kiln-drying Nile perch. Forty-five tons of firewood are used per month at Wichlum Beach alone for kiln-drying perch. The increased economic viability of the fishery has attracted professionals into the industry and resulted in the development of an export-oriented trade. The Yala Swamp adjacent to Lake Victoria has been extensively drained as part of a large land reclamation scheme and more draining is planned. Increased environmental awareness in Kenya, and Kenya's membership in the Convention on Wetlands of International Importance, has resulted in a critical review of these plans.     

Comparative age and growth of Nile tilapia (<Emphasis Type="Italic">Oreochromis niloticus</Emphasis> L.) in lakes Nabugabo and Wamala,Uganda

Age and growth of Nile tilapia (Oreochromis niloticus) from Lake Nabugabo and Lake Wamala, Uganda, were determined using cross-sectioned sagittal otoliths. Marginal-increment and edge analyses of Nile tilapia otoliths from Lake Nabugabo indicated formation of two annuli per 12-month period. Opaque zones associated with faster growth were observed between April and June and between September and December, coincident with the two rainy seasons of the year. Within both lakes, males were larger at age than females. Nile tilapia from Lake Nabugabo, however, had faster growth rates than Nile tilapia from Lake Wamala, and fish >3 years old from Lake Nabugabo were larger at age than those from Lake Wamala. Ages ranged from 0 to 8.0 years for Nile tilapia from Lake Nabugabo, and from 0.5 to 6.5 years for tilapia from Lake Wamala. Differences in the patterns of growth in Nile tilapia between lakes may reflect, at least in part, the relatively energy-rich omnivorous diet of Nile tilapia in Lake Nabugabo versus a phytoplanktivorous diet in Lake Wamala. Diet differences of Nile tilapia between the two lakes are ascribed to trophic changes in the lakes due to the introduction of Nile perch (Lates niloticus) into Lake Nabugabo but not Lake Wamala. Alternatively, the greater exploitation of Nile tilapia in Lake Nabugabo may have resulted in increased growth rates, whereas Nile tilapia in Lake Wamala may be subject to slower, density-dependent growth. Handling editor: J. Cambray     

The conservation of the fishes of Lake Victoria,Africa: an ecological perspective

Synopsis The African Great Lakes are considered to be dynamically fragile ecosystems that are relatively resistant to minor changes with which they have co-evolved but vulnerable to major perturbations such as overfishing, the introduction of alien species and pollution. These lakes are inhabited by large species flocks of cichlid fishes which are characterised by a complex structure of interaction both between and within species, as is typical of mature ecosystems. Major perturbations, such as the disruption of trophic interactions through the introduction of alien fishes, may reverse the domination of relatively precocial, specialised forms and result in the creation of conditions that are conducive to the survival of more altricial, generalised forms with strong colonising abilities. The introduction of Nile perch and Nile tilapia, as well as other alien fishes, into Lake Victoria, combined with overfishing for the indigenous cichlid species, has resulted in marked changes to the fish communities and the fisheries that depend on them. The most important impacts of the Nile perch appear to be predation and aggressive effects whereas those of the tilapias include hybridization, overcrowding, competition for food and possibly the introduction of parasites and diseases. While the three proposed methods of conserving the indigenous flocks of cichlid fishes (captive propagation, reducing Nile perch stocks and closure of the haplochromine trawl fishery) all have merit, the changes that are occurring in Lake Victoria are basically irreversible. The highest priority should be to assist the governments of the riparian countries (Tanzania, Uganda and Kenya) with monitoring and research programmes and to support their policies of non-introduction of further alien fishes into any of the African Great Lakes so as prevent the same cycle of events from occurring, for example, in Lakes Tanganyika and Malawi. The diverse animal and plant communities of the African Great Lakes are a heritage of all mankind and it is the duty of every country to play a role in their conservation. It is therefore proposed that an internationally funded research programme should be mounted on the African Great Lakes on the scale of the tropical forest biome project of the IUCN. Editorial     

The catfish fauna of Lake Victoria after the Nile perch upsurge

Stocks of the indigenous catfish species of Lake Victoria have decreased dramatically since the beginning of the 1980s. This decline coincided with the Nile perch boom and concomitant ecological changes in the lake. In deep water, where Nile perch densities were higher, the decline proceeded more quickly than in shallow water. In the former all catfishes eventually vanished. Of the two largest species, Clarias gariepinus and Bagrus docmak, juveniles disappeared faster than adults. This indicates that predation by Nile perch may have played an important role in their decline. Other possible impacts were the deoxygenation of deepwater areas and the decline of haplochromine cichlids which were an important food source for B. docmak, C. gariepinus and Schilbe intermedius. The various catfish species were not all affected to the same extent. The endemic Xenoclarias eupogon, which lived predominantly in deep water, may have become extinct. B. docmak currently seems to be mainly restricted to refugia in rocky habitats. Synodontis victoriae and S. afrofischeri are still present in small numbers in shallow littoral areas. Schilbe intermedius and C. gariepinus seem to be the least affected of the catfishes in littoral and sublittoral areas. This may be caused, among other reasons, by their smaller habitat overlap with Nile perch than the other species. S. intermedius is partly pelagic, and a considerable part of the C. gariepinus stock lives in bodies of water surrounding the lake. The patterns of decline of the catfishes are very similar to those observed for haplochromine cichlids in the lake. The importance of catfishes for the fisheries in the lake is currently negligible.     

Fish species and trophic diversity of haplochromine cichlids in the Kyoga satellite lakes (Uganda)

Prior to the 1980s, lakes Kyoga and Victoria previously supported an exceptionally diverse haplochromine fish fauna comprising at least 11 trophic groups. The species and trophic diversity in these lakes decreased when the introduced Nile perch depleted haplochromine stocks. From December 1996 to October 1998, we studied species and trophic diversity of haplochromine fishes in six satellite lakes without Nile perch in the Kyoga basin and compared them with the Kyoga main lake against historical data from Lake Victoria where Nile perch were introduced. Forty‐one species were found in the study area, of which, the Kyoga satellite lakes contributed 37 species in comparison to only 14 from the Kyoga main lake. Analysis of trophic diversity based on 24 species that contained food material revealed seven haplochromine trophic groups (insectivores, peadophages, piscivores, algal eaters, higher plant eaters, molluscivores and detritivores) in the Kyoga satellite lakes in comparison to two trophic groups (insectivores and molluscivores) in the Kyoga main lake. Many of the species and trophic groups of haplochromines depleted by the introduced Nile perch in lakes Kyoga and Victoria still survive in the Kyoga satellite lakes. This is attributed to the absence of Nile perch in those lakes. Nile perch has been prevented from spreading into the satellite lakes by swamp vegetation that separate them from the main lakes. If these swamps prevent Nile perch from spreading into the lakes, it is possible to conserve fish species, especially haplochromines, which are threatened by introduction of Nile perch in the main lakes.     

Changes in the prey ingested and the variations in the Nile perch and other fish stocks of Lake Kyoga and the northern waters of Lake Victoria (Uganda)

The fish stocks of Lakes Kyoga and Victoria have changed since Nile perch, Lates niloticus (L.), was introduced, and this is reflected in the prey ingested by the predator. Initially, haplochromine cichlids constituted the main prey of most sizes of Nile perch. As the stocks of these have declined, Caridina nilotica (Roux) and Anisopteran nymphs have become the dominant food of the juveniles, while Rastrineobola argentea (Pellegrin), juvenile Nile perch and Oreochromis niloticus (L.) have become the main food of larger Nile perch. Apart from R. argentea , most of the native fish species of these lakes have disappeared. The stocks of Nile perch in Lake Kyoga, to which it was introduced earlier than to Lake Victoria, have declined after dominating the fishery since 1965. and have been superseded by O. niloricus . an introduced herbivore. Similar changes are now occurring in Lake Victoria. The Nile perch might not maintain the high yield realized in the two lakes when haplochromines were abundant. It is therefore necessary to exercise caution with high and long-term investments aimed specifically at developing the Nile perch fishery.     

Biodiversity of an interface zone of a nutrient-deficient lake (Nabugabo) in Uganda: macrophytes

Lake Nabugabo was a big bay of Lake Victoria before being separated from the latter by a sand bar about 4000 years ago. Whereas conductivity of Lake Victoria is 100 μScm⁻¹ that of Nabugabo has been less than 30 μ cm⁻² since the Cambridge Scientific Expedition, 1962. Dissolved calcium carbonate is below detectable levels although nitrate and phosphate are not significantly different from those of Lake Victoria. However, the total cations and anions are five times more in Lake Victoria than Nabugabo. Along the eastern, southern and south‐western margins of the lake is an emergent swamp composed of different types of plants. In most places, Miscanthidium violaceum forms a floating platform of matted roots and rhizomes which border the open lake; it is associated with Sphagnum, a type of moss which normally grows in acidic and low‐temperature conditions at very high altitudes and latitudes. Because of the presence of this moss, the Nabugabo swamps are sometimes called ‘Sphagnum swamps’. Vossia cuspidata (hippo grass) is yet another emergent swamp plant in Lake Nabugabo, occurring in a number of isolated narrow mats stretching from a few metres to 0.5 km or longer along the edge of the emergent swamp. These floating mats are firmly anchored to the Micanthidium platform. Cyperus latifolia is another emergent‐floating plant in Lake Nabugabo firmly anchored on to the Miscanthidium platform. Cyperus latifolia and V. cuspidata tend to grow side by side but in completely sheltered bays the former plant grows alone. Cyperus papyrus L. occurs in very few patches within the emergent swamp. In the interface zone along the emergent swamp, there are a number of euhydrophyte plant species: Nymphaea caerulea is the most dominant and widespread euhydrophyte species except in the eastern tip of the lake where it is replaced by Nymphaea lotus; interspersed among these lilies is Ceratophyllum demersum in certain bays which could either be water inlets or out‐flows; Utricularia andNymphoides indica are associated with monospecific stand of N. caerulea in the south‐western end of the zone.     

The tilapiine fish stock of Lake Victoria before and after the Nile perch upsurge

Since the beginning of fisheries in Lake Victoria, two native tilapiine species, Oreochromis esculentus and Oreochromis variabilis , were the main target of the local fishermen. A continuous increase in fishing pressure led initially to a declining catch per unit of effort, and a smaller average fish size; eventually, there was a reduced landing of tilapiines. To boost the fisheries, three alien tilapiine species and the Nile perch Lates niloticus were introduced. Thirty years after its introduction, Oreochromis niloticus appeared to be the most successful tilapiine species. It replaced the indigenous tilapiines almost completely before the Nile perch came to dominate the ecosystem of Lake Victoria. Reduced fishing pressure on the tilapiines in the 1980s, due to the shift of the local fishery towards the Nile perch, resulted in an increase in the stock of O. niloticus and an increase in average fish size. Subsequently, the total mass of O. niloticus landed increased. The stocks of the indigenous tilapiines did not recover but declined to extremely low levels, or vanished from the main lake. Currently, these species still occur in satellite lakes of Lake Victoria, from which O. niloticus is absent. Nile perch feed on O. niloticus; however, the limited overlap in distribution between piscivorous Nile perch and O. niloticus of consumable sizes is probably an important factor in explaining the coexistence of the two species. The main cause of the disappearance of the native tilapiine species is presumed to be competitive dominance by O. niloticus .     

Dwelling at the oxycline: does increased stratification provide a predation refugium for the Lake Victoria sardine Rastrineobola argentea?

1. We tested the hypothesis that increased hypoxia in Lake Victoria provides a refugium for the cyprinid dagaa ( Rastrineobola argentea ) against hypoxia-sensitive Nile perch ( Lates niloticus ).
2. Hypoxia in the main habitat of dagaa was rare during 1979–80, but lasted 3–5 months in 1987 and 1988, during which time adult dagaa spent the day just above the oxycline instead of near the bottom.
3. Dissolved oxygen concentrations in the layer just above the oxycline were not critical for Nile perch, but a mass kill of this species following a sudden upwelling of hypoxic water suggested that oxycline-dwelling is risky.
4. Our data suggest no difference between dagaa and Nile perch in the level at which dissolved oxygen starts to limit their vertical distribution in the water column, but dagaa seems to be even more sensitive to extreme hypoxia (<1–2 mg O2 L−1) than Nile perch.
5. We argue that oxycline-dwelling dagaa are not seeking a predation refugium but that they are limited by low oxygen levels in reaching their feeding areas near the bottom.     

The invasion of an introduced predator,Nile perch (<Emphasis Type="Italic">Lates niloticus</Emphasis>, L.) in Lake Victoria (East Africa): chronology and causes

Nile perch, a large predatory fish, was introduced into Lake Victoria in 1954. The upsurge of Nile perch in Lake Victoria was first observed in the Nyanza Gulf, Kenya, in 1979. In Ugandan waters this occurred 2–3 years later and in the Tanzanian Mwanza Gulf 4–5 years later. At the beginning of the upsurge in the Mwanza Gulf in 1983/1984 only sub-adult and adult fishes were found. The first juveniles appeared in 1985, suggesting that the initial increase of Nile perch was mainly caused by migration of sub-adults and adults. Shortly after the onset of trawl fishery in the area in 1973, haplochromines in the Mwanza Gulf started to decline. The final disappearance of the haplochromines, in 1987, only occurred after the Nile perch boom, and despite the abandoning of the haplochromine fishery in 1986. We hypothesize that the decline of haplochromines decreased predation on and competition with juvenile Nile perch and then facilitated survival of these juveniles. Consequently the immigration of sub-adult and adult Nile perch in an area may have paved the way for successful recruitment. Over-exploitation of haplochromine cichlids in the 1970s in the Nyanza Gulf, where the Nile perch upsurge was first observed, may have played a similar role.     

Biological diversity of the Yala Swamp lakes, with special emphasis on fish species composition, in relation to changes in the Lake Victoria Basin (Kenya): threats and conservation measures

During the second half of the last century, the Lake Victoria ecosystem has undergone drastic ecological changes. Most notable has been the decline in the populations of many endemic cichlid fishes. The lake has lost nearly 200 haplochromines and one tilapiine, Oreochromis esculentus. The above changes have been attributed to effects of species stocking and, in particular, from predation pressure by the introduced Nile perch, Lates niloticus. Other factors that have led to the decline of the endemic species include intensive non-selective fishing, extreme changes in the drainage basin, increased eutrophication, and the invasion of the lake by the water hyacinth, Eichhornia crassipes. However, the remnants of some species that had disappeared from Lake Victoria occur abundantly in the Yala Swamp lakes (Kanyaboli, Sare and Namboyo). This paper discusses the biodiversity of the swamp and the three lakes and gives suggestions for their conservation.     

The impact of the introduction of Nile perch, Lates niloticus (L.) on the fisheries of Lake Victoria

The piscivorous Nile perch was introduced into Lake Victoria some 30 years ago, since when it has completely transformed the fishing industry and the species composition of the fish fauna of the lake. The original multispecies fishery, based mostly on cichlids (haplochromines, tilapias), cyprinids ( Barbus, Labeo, Rastrineobola ) and siluroids ( Bagrus, Clarias, Synodontis, Schilbe ), has changed dramatically to one based on three species: the introduced Nile perch, the cyprinids, Rastrineobola argenrea (Pellegrin), and the introduced Nile tilapia, Oreochromis niloticus (Linnaeus).
Within 25 years of its introduction the Nile perch became ubiquitous and now occurs in virtually every habitat with the exception of swamps and affluent rivers. It has preyed on all other species with profound effects, especially on the stocks of haplochromines. These originally comprised 80% of the total fish biomass in Lake Victoria, but have now decreased to less than 1% offish catches from the Kenyan waters of the lake. The fishermen of Lake Victoria have adjusted to this ecological crisis by using large-meshed nets to catch Nile perch, which has become the most important commercial species. For the first time in the history of Lake Victoria, fish fillets are now being exported to several overseas countries: the fillets are all from Nile perch.     

Co-introduction of Dolicirroplectanum lacustre,a monogenean gill parasite of the invasive Nile perch Lates niloticus: intraspecific diversification and mitonuclear discordance in native versus introduced areas

The Nile perch (Lates niloticus) is a notorious invasive species. The introductions of Nile perch into several lakes and rivers in the Lake Victoria region led to the impoverishment of trophic food webs, particularly well documented in Lake Victoria. Additionally, its parasites were co-introduced, including Dolicirroplectanum lacustre (Monogenea, Diplectanidae). Dolicirroplectanum lacustre is the single monogenean gill parasite of latid fishes (Lates spp.) inhabiting several major African freshwater systems. We examined the intra-specific diversification of D. lacustre from Lates niloticus in Lake Albert, Uganda (native range) and Lake Victoria (introduced range) by assessing morphological and genetic differentiation, and microhabitat preference. We expected reduced morphological and genetic diversity for D. lacustre in Lake Victoria compared with Lake Albert, as a result of the historical introductions. We found that D. lacustre displayed high morphological variability within and between African freshwaters, with two morphotypes identified, as in former studies. The single shared morphotype between Lake Albert and Lake Victoria displayed similar levels of haplotype and nucleotide diversity between the lakes. Mitonuclear discordance within the morphotypes of D. lacustre indicates an incomplete reproductive barrier between the morphotypes. The diversification in the mitochondrial gene portion is directly linked with the morphotypes, while the nuclear gene portions indicate conspecificity. Based on our results, we reported reduced genetic and morphological diversity, potentially being a result of a founder effect in Lake Victoria.     

Hypoxia tolerance of two centrarchid sunfishes and an introduced cichlid from karstic Everglades wetlands of southern Florida, U.S.A.

In this study, the hypoxia tolerance of three Everglades fishes, two native centrarchids ( Lepomis gulosus and Lepomis marginatus ) and a recently introduced cichlid ( Hemichromis letourneuxi ), were documented. Aquatic surface respiration (ASR) thresholds were lowest for H. letourneuxi , followed by L. gulosus , then L. marginatus . The ASR thresholds for L. marginatus were within ranges reported for small, freshwater tropical fishes, while those for L. gulosus were similar to swamp-adapted fishes. For H. letourneuxi , ASR thresholds were some of the lowest reported. All three species showed excellent tolerance of low dissolved oxygen levels when allowed access to the surface. When denied surface access, L. marginatus lost equilibrium at a higher oxygen tension than the other species. Overall, although all species easily tolerated hypoxia, H. letourneuxi appeared to be best equipped to deal with hypoxia, followed by L. gulosus , then L. marginatus . Hemichromis letourneuxi also exhibited more aggressive behaviours than the centrarchids. These results suggest that hypoxia is not likely to prevent H. letourneuxi from exploiting the seasonally inundated wetlands of south Florida while expanding its range there.