Interaction between the introduced eastern mosquitofish and two autochthonous Spanish toothcarps

In aquaria, adults and young-of-the-year (YOY) of both Spanish toothcarp Aphanius iberus and Valencia toothcarp Valencia hispanica received more aggression in the presence of eastern mosquitofish Gambusia holbrooki . YOY of both species also showed decreased feeding rates, as did V. hispanica adults. Adult V. hispanica also were more aggressive themselves and had their reproductive behaviour disrupted. Cases of eastern mosquitofish predation on small A. iberus were also observed. The occurrence and intensity of these potentially harmful effects, however, seemed to be modulated by water temperature, reproductive condition, relative size of the interacting fishes and species-specific habitat preferences. Thus, aggression almost disappeared at low water temperatures. Valencia hispanica and eastern mosquitofish used similar vertical positions in the water column and this apparently increased the frequency of their interactions. In contrast, A. iberus preferred positions closer to the bottom than the other two species and this vertical segregation appeared to lead to less direct interaction with eastern mosquitofish. In an outdoor mesocosm experiment, under semi-natural conditions, the presence of eastern mosquitofish resulted in an almost 70% reduction in the number of o.spring produced by A. iberus . The addition of extra, non-breeding males of A. iberus had no significant effect. YOY A. iberus from cages with eastern mosquitofish or extra males were significantly smaller (c. 1 mm, or 8% shorter) than those from cages without additional fish. Results for V. hispanica were not conclusive, as very few YOY were recovered. In contrast, eastern mosquitofish did not affect the survivorship of breeding adults of both native species.     

Temperature-Specific Competition between Invasive Mosquitofish and an Endangered Cyprinodontid Fish

Condition-specific competition is widespread in nature. Species inhabiting heterogeneous environments tend to differ in competitive abilities depending on environmental stressors. Interactions between these factors can allow coexistence of competing species, which may be particularly important between invasive and native species. Here, we examine the effects of temperature on competitive interactions between invasive mosquitofish, Gambusia holbrooki, and an endemic Iberian toothcarp, Aphanius iberus. We compare the tendency to approach heterospecifics and food capture rates between these two species, and examine differences between sexes and species in aggressive interactions, at three different temperatures (19, 24 and 29°C) in three laboratory experiments. Mosquitofish exhibit much more aggression than toothcarp. We show that mosquitofish have the capacity to competitively displace toothcarp through interference competition and this outcome is more likely at higher temperatures. We also show a reversal in the competitive hierarchy through reduced food capture rate by mosquitofish at lower temperatures and suggest that these two types of competition may act synergistically to deprive toothcarp of food at higher temperatures. Males of both species carry out more overtly aggressive acts than females, which is probably related to the marked sexual dimorphism and associated mating systems of these two species. Mosquitofish may thus impact heavily on toothcarp, and competition from mosquitofish, especially in warmer summer months, may lead to changes in abundance of the native species and displacement to non-preferred habitats. Globally increasing temperatures mean that highly invasive, warm-water mosquitofish may be able to colonize environments from which they are currently excluded through reduced physiological tolerance to low temperatures. Research into the effects of temperature on interactions between native and invasive species is thus of fundamental importance.     

Responses of a wetland ecosystem to the controlled introduction of invasive fish

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Feeding ecology of the endangered Valencia toothcarp,Valencia hispanica (Actinopterygii: Valenciidae)

Patterns of food resource utilisation by Valencia toothcarp were studied in a small coastal lagoon in the Iberian Peninsula, its most northern population. Emphasis was placed on feeding activity, diet composition, seasonal and intraspecific variation in diet and feeding strategy. Important features of the overall feeding pattern of the Valencia toothcarp are decreased feeding activity during winter, concentration on benthic invertebrates and narrow dietary breadth, with a focus on gammarid amphipods. The feeding of V. hispanica was independent of seasonal variation in food availability, because its main prey were much the same throughout the year. There were no differences in seasonal niche width and niche overlap between seasons was high. There are also no differences in diets between sexes, although diet composition, niche width and niche overlap differ between juvenile and adult fish. The Valencia toothcarp population from Santes Creus lagoon was formed by specialist individuals, with narrow niche widths, that fed on four preferential prey types, but also consumed some occasional prey.     

Experimental evidence for the use of artificial refugia to mitigate the impacts of invasive <Emphasis Type="Italic">Gambusia holbrooki</Emphasis> on an endangered fish

Habitat degradation has a major impact on freshwater ecosystems and also facilitates biological invasions thus intensifying the problem. The survival of native species under threat from invaders can be improved either by eradicating the invading species or by providing resources or conditions that benefit the native species. One such resource is shelter and in degraded habitats artificial refugia may be a viable option. However, the use of artificial refugia to promote coexistence between native and invasive species remains poorly understood. We assessed the potential for artificial refugia to ameliorate the disruption of social interactions in an endangered native Iberian toothcarp, Aphanius iberius, by the invasive mosquitofish, Gambusia holbrooki. We found that mosquitofish do not compete for access to refugia despite their higher level of aggression compared to native fish. Native fish use refugia more overall, particularly in the presence of mosquitofish. Despite this, the benefits of refugia are not clear cut: increases in refuge use by male toothcarp induced by mosquitofish aggression correspond to decreases in attention to conspecifics. However, changes in refugia use over time together with constant attention to conspecifics indicates that it is not refugia use itself that disrupts social interactions but the interrelated effects of mosquitofish aggression. Provision of artificial refugia in degraded freshwater ecosystems may thus be a viable management tool to protect native populations under imminent threat of invasion.     

Inter‐specific differences in invader and native fish functional responses illustrate neutral effects on prey but superior invader competitive ability

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Ground-truthing the impact of invasive species: spatio-temporal overlap between native least chub and introduced western mosquitofish

Western mosquitofish (Gambusia affinis) are one of the most destructive introduced species in the American West. The negative impact of introduced species on native taxa depends on their spatio-temporal overlap, which will determine the availability of refugia for native species. Experiments on the mechanisms underlying the interactions between introduced and native species rarely address habitat use, overlap, and refugia because individuals are confined to enclosures. In a previous study we used cages, microcosms, and aquaria to show that mosquitofish could prey on and out-compete native least chub (Iotichthys phlegethontis). In this study, we examined the spatio-temporal overlap between mosquitofish and least chub under natural conditions. We found periods of overlap and partitioning in the seasonal and diel habitat use of these species. Both species used shallow habitats during the day and night throughout the spring when least chub were spawning. Predation by adult mosquitofish on young least chub during the spring likely explains the reduction in least chub recruitment in the presence of mosquitofish. During the summer least chub avoided mosquitofish by exclusively using cooler habitats, or by occupying deeper, cooler habitats during the day when mosquitofish were active, and shallower, warmer habitats at night when mosquitofish were inactive. A shift to cooler habitats in the presence of mosquitofish may result in decreased growth and fecundity of least chub. However, we suggest that a reduction of warm shallow habitat may have a disproportionately greater negative impact on mosquitofish than species native to temperate regions, such as least chub. Habitat manipulations that reduce or eliminate warm habitats may ameliorate the harmful effects of mosquitofish and promote the long-term persistence of native taxa.     

Antipredator Responses by Native Mosquitofish to Non-Native Cichlids: An Examination of the Role of Prey Naiveté

The strong impact of non‐native predators in aquatic systems is thought to relate to the evolutionary naiveté of prey. Due to isolation and limited dispersal, this naiveté may be relatively high in freshwater systems. In this study, we tested this notion by examining the antipredator response of native mosquitofish, Gambusia holbrooki, to two non‐native predators found in the Everglades, the African jewelfish, Hemichromis letourneuxi, and the Mayan cichlid, Cichlasoma urophthalmus. We manipulated prey naiveté by using two mosquitofish populations that varied in their experience with the recent invader, the African jewelfish, but had similar levels of experience with the longer‐established Mayan cichlid. Specifically, we tested these predictions: (1) predator hunting modes differed between the two predators, (2) predation rates would be higher by the novel jewelfish predator, (3) particularly on the naive population living where jewelfish have not invaded yet, (4) antipredator responses would be stronger to Mayan cichlids due to greater experience and weaker and/or ineffective to jewelfish, and (5) especially weakest by the naive population. We assayed prey and predator behavior, and prey mortality in lab aquaria where both predators and prey were free‐ranging. Predator hunting modes and habitat domains differed, with jewelfish being more active search predators that used slightly higher parts of the water column and less of the habitat structure relative to Mayan cichlids. In disagreement with our predictions, predation rates were similar between the two predators, antipredator responses were stronger to African jewelfish (except for predator inspections), and there was no difference in response between jewelfish‐savvy and jewelfish‐naive populations. These results suggest that despite the novelty of introduced predators, prey may be able to respond appropriately if non‐native predator archetypes are similar enough to those of native predators, if prey rely on general antipredator responses or predation cues, and/or show neophobic responses.     

Differential survival of two minnow species under experimental sunfish predation: implications for re‐invasion of a species into its native range

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Friends in Low Places: Responses of a Benthic Stream Fish to Intra‐Prey‐Guild Alarm Cues

Many aquatic species produce chemical alarm cues that serve as a warning to nearby conspecifics. In mixed‐species aggregations, individuals may also benefit by ‘eavesdropping’ on the chemical alarm cues of other species that are in the same prey guild. Rainbow Darters (Etheostoma caeruleum) are benthic fish that co‐occur with native Ozark Minnows (Notropis nubilus), recently introduced Western Mosquitofish (Gambusia affinis), and native Oklahoma Salamanders (Eurycea tyrnerensis), all of whom are vulnerable to the same predators. We tested the responses of darters to the damage‐released alarm cues of conspecifics (positive control), minnows, and mosquitofish; alarm cues from Bumblebee Gobies (Brachygobius doriae) served as a negative (allopatric) control. We also tested the response of sympatric and allopatric darters to the damage‐released alarm cues of Oklahoma Salamander. Darters exhibited a fright response to conspecific and minnow alarm cues, but not to cues from mosquitofish or gobies. Lack of response to mosquitofish cues could be because they are introduced or because they typically occur higher in the water column than darters. Darters that were sympatric with the salamander exhibited a fright response to the alarm cues of the salamander, while allopatric darters did not. Rainbow Darters can develop responses to the alarm cues of syntopic species (minnows and Oklahoma Salamander) within their prey guild.     

Nonvisual feeding in a visual planktivore,Xenomelaniris venezuelae

Summary Studies of the diel feeding patterns of the planktivorous fish, Xenomelaniris venezuelae, in Lake Valencia, Venezuela, revealed that, although the fish is primarily a diurnal feeder, it consumes substantial numbers of Chaoborus larvae and pupae at night. A number of fish species are known which feed on plankton at night, but these fish are filter feeders and their diets largely consist of relatively small, nonevasive prey. Chaoborus, however, is large and agile. Predation by Xenomelaniris in the dark was also studied experimentally. Captured fish were placed in completely darkened aquaria with zooplankton from Lake Valencia. After several hours the plankton was removed and examined for evidence of feeding. The fish were found to consume Chaoborus pupae and fourth instar larvae but not other types of prey. The mode of feeding by Xenomelaniris in the dark is unknown.     

Exploratory behaviour and novel predator recognition: behavioural correlations across contexts

It was hypothesized that the exploratory behaviour of an individual measured in a novel environment could predict its behaviour in response to a novel predator. This study examined novel predator recognition in the western mosquitofish Gambusia affinis, a species with individual differences in risk‐taking, activity and exploration in novel environments. Prey responded with characteristic shoaling and avoidance in response to native predators, but did not show characteristic antipredator behaviour towards novel predators. Furthermore, G. affinis exhibited individual‐level behavioural correlations across contexts but only when prey were tested with native predators. This could be the result of native predatory selection on behavioural correlations in the prey species.     

Why do eastern curlews Numenius madagascariensis feed on prey that lowers intake rate before migration?

Eastern curlews Numenius madagascariensis spending the nonbreeding season in eastern Australia foraged on three intertidal decapods: soldier crab Mictyris longicarpus, sentinel crab Macrophthalmus crassipes and ghost‐shrimp Trypaea australiensis. Due to their ecology, these crustaceans were spatially segregated (=distributed in ‘patches’) and the curlews intermittently consumed more than one prey type. It was predicted that if the curlews behaved as intake rate maximizers, the time spent foraging on a particular prey (patch) would reflect relative availabilities of the prey types and thus prey‐specific intake rates would be equal. During the mid‐nonbreeding period (November–December), Mictyris and Macrophthalmus were primarily consumed and prey‐specific intake rates were statistically indistinguishable (8.8 versus 10.1 kJ×min^?1). Prior to migration (February), Mictyris and Trypaea were hunted and the respective intake rates were significantly different (8.9 versus 2.3 kJ×min^?1). Time allocation to Trypaea‐hunting was independent of the availability of Mictyris. Thus, consumption of Trypaea depressed the overall intake rate. Six hypotheses for consuming Trypaea before migration were examined. Five hypotheses: the possible error by the predator, prey specialization, observer overestimation of time spent hunting Trypaea, supplementary prey and the choice of higher quality prey due to a digestive bottleneck, were deemed unsatisfactory. The explanation for consumption of a low intake‐rate but high quality prey (Trypaea) deemed plausible was diet optimisation by the curlews in response to the pre‐migratory modulation (decrease in size/processing capacity) of their digestive system. With a seasonal decrease in the average intake rate, the estimated intake per low tide increased from 1233 to 1508 kJ between the mid‐nonbreeding and pre‐migratory periods by increasing the overall time spent on the sandflats and the proportion of time spent foraging.     

Behavioral responses of American toad and bullfrog tadpoles to the presence of cues from the invasive fish, Gambusia affinis

The introduction of non-native predators is thought to have important negative effects on native prey populations. The susceptibility of native prey to non-native or introduced predators may depend on their ability to respond appropriately to the presence of these non-native predators. We conducted a laboratory based behavioral experiment to examine the response of American toad (Bufo americanus) and bullfrog (Rana catesbeiana) tadpoles to the presence of cues from the introduced mosquitofish (Gambusia affinis), a potential tadpole predator. Neither the American toad tadpoles nor the bullfrog tadpoles responded behaviorally to the presence of mosquitofish cues. If tadpoles are unable to respond to the presence of mosquitofish cues appropriately, then their ability to avoid predation by mosquitofish may be compromised and this may contribute to the impacts of mosquitofish on some tadpole populations.     

Identifying Environmental Conditions to Promote Species Coexistence: An Example with the Native Barrens Topminnow and Invasive Western Mosquitofish

Coexistence of a native and invasive species may be possible at certain conditions along an environmental gradient where the individual responses of each species are maximally apart. Water temperature may differentially affect the growth of a native cool-water species like the Barrens topminnow, Fundulus julisia, and an originally warm-water adapted western mosquitofish, Gambusia affinis, who is a recent invader in Barrens Plateau region of middle Tennessee. We measured the specific growth rate (SGR) of the two species separately in laboratory aquaria at 10, 15, 20 and 25 °C, representing a range of temperatures that occur in topminnow habitats throughout the year. Both species grew faster with increasing temperature and SGRs were highest at 25 °C. The interspecific difference in SGR was maximized at 15 °C. At this temperature, mean growth rate of topminnows was 0.78% per day, more than twice that of mosquitofish (0.38% per day). These results suggest that cool springhead habitats with a near-constant thermal environment of 15 °C throughout the year may provide a growth advantage to the Barrens topminnow over the mosquitofish. Other environmental, density-dependent, or behavioral factors not examined here may act along with temperature to mediate the coexistence of the topminnow and mosquitofish.     

Can separation along the temperature niche axis promote coexistence between native and invasive species?

Aim Western mosquitofish (Gambusia affinis) have been linked with the decline of native fish and amphibians throughout the world. Separation along the temperature niche axis may promote the long‐term coexistence of introduced western mosquitofish, with native species in temperate regions. Recent research has shown that western mosquitofish can reduce the recruitment of native least chub (Iothichthys phlegethontis) endemic to the Bonneville Basin. We tested the hypotheses that cold temperatures (≤ 15 °C in the summer, freezing winters) would: (1) reduce the aggressive and predatory effects of western mosquitofish on least chub, and (2) eliminate the overwinter survival and recruitment of western mosquitofish while having little effect on least chub recruitment. Location Bonneville Basin of Utah, USA. Methods We used short‐term tests in the laboratory at the level of individuals and manipulated temperature (warm, cold and seasonal treatments) in long‐term experiments using mesocosms at the population level. Results Cold temperatures (≤ 15 °C) reduced the aggression and predation of western mosquitofish on least chub at the level of individuals. At the population level, however, cool summers (≤ 15 °C) eliminated recruitment in both species because they required warm summers (c. 20–30 °C) to survive freezing winters. Although least chub had an overwinter advantage in survival (75% least chub, 45% western mosquitofish), it was overwhelmed by the rapid reproduction of western mosquitofish as temperatures increased in the summer. Main conclusions Studies at the level of populations are necessary to understand the ultimate effects of introduced species on native taxa. Separation along the temperature niche axis was not sufficient to promote coexistence between these species in habitats with warm summers (c. 30 °C). Although coexistence may be possible in habitats with cool summers (≤ 20 °C) and freezing winters, the ability of niche separation to promote long‐term coexistence between native and introduced species may ultimately depend on their respective rates of evolution. Long‐term coexistence may not be possible if introduced species can adapt to new environmental conditions faster than native species can evolve mechanisms to reduce their harmful effects.     

Salinity mediates the competitive interactions between invasive mosquitofish and an endangered fish

The interplay of abiotic factors and competition has a long history in ecology, although there are very few studies on the interaction of salinity and competition in fish. Mosquitofish (Gambusia holbrooki) are among the most invasive fish worldwide, with well documented ecological impacts on several taxa such as amphibians and small native fish. It has been previously hypothesized, based on field observations, that salinity limits the invasive success of mosquitofish and provides a competitive refuge for Mediterranean cyprinodonts. We experimentally tested this hypothesis by examining the agonistic behaviour and food competition between mosquitofish and an endangered native cyprinodont (Aphanius fasciatus) at three salinities (0, 15, 25‰). Intraspecific aggressive behaviour for both species was not significantly affected by salinity. As salinity increased, mosquitofish decreased their aggressive behaviour towards cyprinodonts and captured less prey. In contrast, the cyprinodonts did not change their behaviour with different salinity treatments, with the possible exception of increased defensive acts in higher salinities, but captured more prey with increasing salinity because of the reduced efficiency of mosquitofish. Our study confirms previous field observations that salinity limits the invasive success of mosquitofish and provides one of the few experimental demonstrations that it may mediate behavioural and competitive interactions between fish species. Condition-specific competition of mosquitofish might be expected with other species and ecosystems worldwide and illustrates the importance of integrating biotic and abiotic factors in the study of interspecific interactions.     

Recognition and Response to Native and Novel Predators in the Largespring mosquitofish,Gambusia geiseri

The introduction of predator species into new habitats is an increasingly common consequence of human activities, and the persistence of native prey species depends upon their response to these novel predators. In this study, we examined whether the Largespring mosquitofish, Gambusia geiseri exhibited antipredator behavior and/or an elevation of circulating stress hormones (cortisol) to visual and chemical cues from a native predator, a novel predator, or a non‐predatory control fish. Prey showed the most pronounced antipredator response to the native predator treatment, by moving away from the stimulus, while the prey showed no significant changes in their vertical or horizontal position in response to the novel or non‐predator treatments. We also found no significant difference in water‐borne cortisol release rates following any of the treatments. Our results suggest the prey did not recognize and exhibit antipredator behavior to the novel predator, and we infer that this predator species could be detrimental if it expands into the range of this prey species. Further, our study demonstrates prey may not respond to an invasive predator that is phylogenetically, behaviorally, and morphologically dissimilar from the prey species' native predators.     

No evidence of nonlinear effects of predator density,refuge availability,or body size of prey on prey mortality rates

Predator density, refuge availability, and body size of prey can all affect the mortality rate of prey. We assume that more predators will lead to an increase in prey mortality rate, but behavioral interactions between predators and prey, and availability of refuge, may lead to nonlinear effects of increased number of predators on prey mortality rates. We tested for nonlinear effects in prey mortality rates in a mesocosm experiment with different size classes of western mosquitofish (Gambusia affinis) as the prey, different numbers of green sunfish (Lepomis cyanellus) as the predators, and different levels of refuge. Predator number and size class of prey, but not refuge availability, had significant effects on the mortality rate of prey. Change in mortality rate of prey was linear and equal across the range of predator numbers. Each new predator increased the mortality rate by about 10% overall, and mortality rates were higher for smaller size classes. Predator–prey interactions at the individual level may not scale up to create nonlinearity in prey mortality rates with increasing predator density at the population level.     

The effect of photosynthetically active radiation and temperature on the photosynthesis of two Vietnamese species of Sargassum,S. mcclurei and S. oligocystum,based on the field and laboratory measurements

SUMMARY The effects of photosynthetically active radiation (PAR) and temperature on the photosynthesis of two Vietnamese brown algae, Sargassum mcclurei and S. oligocystum (Fucales), were determined by field and laboratory measurements. Dissolved oxygen sensors and pulse‐amplitude modulated (PAM) fluorometry were used for the measurements of photosynthetic efficiency. A Diving‐PAM revealed that underwater measurements of the effective quantum yield (Φ _PSII ) of both species declined with increasing incident PAR, with minimum Φ _PSII occurring during noon to early afternoon. Φ _PSII recovered in the evening, indicating photo‐adaptation to excessive PAR. In laboratory experiments, Φ _PSII also decreased under continuous exposure to 1000 μmol photons m^?2 s^?1; and full recovery occurred after 12 h of dark acclimatization. The net photosynthesis – PAR experiments of S. mcclurei and S. oligocystum conducted at 28°C revealed that the net photosynthetic rate quickly increased at PAR below the saturation irradiance of 361 and 301 μmol photons m^?2 s^?1 and nearly saturated to maximum net photosynthetic rates of 385 and 292 μg O₂ g_ww ^{? 1} min^?1 without photoinhibition, respectively. Gross photosynthesis and dark respiration experiments determined over a range of temperatures (12–40°C), revealed that the maximum gross photosynthetic rates of 201 and 147 μg O₂ g_ww ^{? 1} min^?1 occurred at 32.9 and 30.7°C for S. mcclurei and S. oligocystum, respectively. The dark respiration rates increased exponentially over the temperature ranges examined. The estimated maximum value of the maximum quantum yield occurred at 19.3 and 20.0°C and was 0.76 and 0.74, respectively. Similar to the natural habitat of the study site, these two species tolerated the relatively high temperatures and broad range of PAR. The ability of these species to recover from exposure to high PAR is one of the mechanisms that allow them to flourish in the shallow water environment.