The effect of Cercospora rodmanii,a biological control for waterhyacinth,on the fish,Gambusia affinis

The effect of Cercospora rodmanii, a fungal pathogen being developed as a biological control for waterhyacinth, was evaluated on Gambusia affinis in a standard 96-hour bioassay. Testing indicated that C. rodmanii had no adverse effects on treated fish and would not be a hazard to populations of these fish in nature.Florida Agricultural Experiment Station Journal Series No. 1238.     

An invasive fish species within its native range: community effects and population dynamics of Gambusia affinis in the central United States

1. Management of invasive species benefits from detailed information on the biology of the invaders, both from where they have already invaded, and from within their areas of origin. Western mosquitofish, Gambusia affinis, is a widely invasive and destructive freshwater fish. However, within its native range, G. affinis co‐exists with many other fish species in a wide variety of habitats without obvious harm. 2. In this study, we used data on fish communities within the native range of G. affinis at 154 sites across a broad spatial scale to examine the effects of G. affinis on species richness and diversity of residual (species other than G. affinis) fish assemblages. We further used data based on annual samples at eight fixed river sites over 18 summers to examine temporal population dynamics of G. affinis and to test factors associated with population fluctuations. 3. Higher residual species richness occurred in the presence of G. affinis, but residual diversity did not differ. We found an inverse relationship between relative abundance of G. affinis and residual species richness (although effect size was extremely small), but no effect on residual diversity. 4. Gambusia affinis populations fluctuated markedly across summers at all eight fixed sites, but population sizes at a site over time were not autocorrelated. However, population fluctuations were highly correlated among sites across all years, suggesting that regional factors influenced population size. Regional abundance of G. affinis did not correlate with drought, rainfall or winter temperature, but varied with spring temperature. We suggest earlier onset of reproduction in warmer springs resulted in larger summer populations. 5. Overall, within its native range, G. affinis does not appear to impact negatively on the assemblages in which it occurs, possibly due to fluctuations in its density. These findings suggest that introduced Gambusia populations, and those of other invasive species, warrant careful monitoring over long periods of time where they have invaded. Long‐term monitoring of new populations can establish if they are prone to ‘boom and bust’ dynamics, in which case the invader may be less a threat than sometimes assumed. Population information from long‐term studies, either in their native ranges or at invaded sites, can thus help to form the basis of prudent, cost‐effective management strategies for invasive organisms.     

Regional ecological risk assessment for the introduction of Gambusia affinis (western mosquitofish) into Montana watersheds

Qualitative risk assessment methodologies were used to assess the risk of establishment and consequent impacts on native minnows and species of concern (SOC) associated with the intentional or unintentional introduction of the mosquito biological control agent, Gambusia affinis, to various Montana watersheds. Gambusia affinis introductions for mosquito control have been made throughout the world; some introductions have resulted in deleterious effects on native species. We used average January water temperatures, the presence of dams, and the presence of native minnows and SOC to define endpoints for our assessment. Our results suggest that a section of the Missouri River running between Wolf Creek and Landusky had the highest overall risk score, which corresponds to the highest likelihood of establishment and effects on native minnows and SOC. We also demonstrate how rivers with the highest temperatures are not necessarily at the highest risk of having deleterious effects on minnows and SOC; conversely, these rivers were found to be at highest risk of a population of G. affinis establishing.     

Influence of black spot disease on shoaling behaviour in female western mosquitofish, <Emphasis Type="Italic">Gambusia affinis</Emphasis> (Poeciliidae,Teleostei)

Parasites can fundamentally alter the cost–benefit ratio of living in a group, e.g. if infected individuals increase the predation risk of shoal mates. Here, the effect of an infection with a trematode, Uvulifer sp. (Diplostomatidae) on the shoaling behaviour of female western mosquitofish, Gambusia affinis, was investigated. The parasite examined causes a direct phenotypical change of the host by forming black spots on its body surface. When given a choice between a stimulus shoal and no shoal, we found shoaling tendencies to be significantly reduced in infected focal fish. In another experiment, we tested for association preferences relative to the infection status of the stimulus fish. Given the choice between an infected and a healthy stimulus fish, both infected and healthy focal fish preferred to associate with non-infected stimulus fish. Our results suggest that (1) the cost–benefit ratio of shoaling might be different for infected and non-infected individuals. Infected fish may be more affected by competition for food within a shoal. (2) Associating with infected conspecifics appears to be costly for female mosquitofish, maybe due to increased predation risk.     

Antipredator Behaviour and Suppressed Aggression by Convict Cichlids in response to Injury-released Chemical Cues of Conspecifics but not to those of an Allopatric Heterospecific

In aquatic environments, chemical cues serve as an important source of information for the detection of predation risk. Here, we investigate the response of convict cichlids, Cichlasoma nigrofasciatum, to injury-released chemical cues. We exposed pairs of juvenile convict cichlids first to dechlorinated tap water (control), then later to one of two test stimuli: 1. chemical cues from injured convict cichlids; or 2. chemical cues from injured mosquito fish, Gambusia affinis. Gambusia are allopatric and phylogenetically unrelated to convict cichlids. Gambusia skin was used to control for a general response to injured fish. In response to conspecific cues, convict cichlids significantly increased time spent near the bottom of test aquaria and time under a shelter object. In response to Gambusia skin, convict cichlids tended to increase time spent near the tank bottom but did not increase use of the shelter object. There was a trade-off between antipredator and agonistic behaviours. In response to convict cichlid cues, there was a significant reduction in the frequency of approaches and bites. Gambusia skin extract had no significant effect on aggressive behaviour. These data suggest a species-specific antipredator response to conspecific alarm pheromones in a New World cichlid fish and demonstrate a trade-off between predator avoidance and intraspecific aggression. Further, the presence of an alarm response in this model species sets the stage for the use of chemical cues as a research tool to manipulate predation risk in studies of the interaction between predation risk and reproductive behaviour.     

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.     

Context-dependent effects of bluegill in experimental mesocosm communities

Most knowledge of direct and indirect effects of zooplanktivorous fish has come from studies in which a treatment with a zooplanktivore is compared to a fishless control. However, effects of a zooplanktivore may be different in the presence of other fish species because the other fish have direct and indirect effects that may alter the effects of the zooplanktivore in question. To test this hypothesis, we conducted a tank mesocosm experiment of 2×2 factorial design in which the presence and absence of bluegill (Lepomis macrochirus) were cross-classified with the presence and absence of a fish assemblage composed of western mosquitofish (Gambusia affinis) and common carp (Cyprinus carpio). The presence of bluegill decreased Daphnia, Ceriodaphnia, cyclopoid copepodids, calanoid copepodids, copepod nauplii, amphipods, gastropods, and notonectids. Daphnia, Ceriodaphnia, cyclopoid copepodids, copepod nauplii, gastropods, notonectids, Najas, and Chara were decreased and herbivorous rotifers, turbidity, chlorophyll a, total nitrogen and total phosphorus were increased in the presence of the fish assemblage. Significant bluegill×fish assemblage interaction effects were detected for Daphnia, Ceriodaphnia, cyclopoid copepodids, copepod nauplii, gastropods, and notonectids. Analysis of simple effects for these response variables revealed that all significant bluegill effects in the absence of the fish assemblage were not significant in the presence of the fish assemblage. Our results indicate that the effects of bluegill may be context dependent, or dependent upon the presence of other trophically similar fish species. Received: 3 November 1998 / Accepted: 24 September 1999     

A Review of the Biology of <Emphasis Type="Italic">Gambusia affinis</Emphasis> and <Emphasis Type="Italic">G. holbrooki</Emphasis>

Eastern and Western Gambusia (i.e., Gambusia holbrooki and G. affinis, respectively) are considered together here because these two fish species are very closely related, similar in appearance, similar in biology and often confused. Widely divergent attitudes have developed with regard to these fish with some viewing them as being highly beneficial to humans through controlling mosquitoes and the diseases they harbor, and others expressing concern about the negative impacts that these fish may have on other species with which they interact. Because of the widespread distribution, high levels of abundance, ease of capture and captive maintenance, and divergent attitudes, a very large and diffuse literature has developed with regard to these species. In fact, few fish species have been studied as much as or more than these two species combined. There has, however, been no comprehensive review of their biology published to date. As it is not possible to provide a comprehensive review of Gambusia biology in one reasonably sized document, I provide here a review of aspects of their biology at the level of species and individual. In another review I focused instead on the levels of population and species communities and consider the impacts that these fish have on mosquitoes and other organisms (Pyke, unpublished). As would be expected of such widespread and abundant species, Gambusia affinis and G. holbrooki are clearly very tolerant, adaptable and variable in their biology, at both an individual and population level. Both individuals and populations can tolerate, and often thrive within, a wide range of conditions and the abilities of individuals to do this are enhanced if they have time to acclimate to any changes. Populations can adapt through genetic or evolutionary changes in response to conditions that vary in space or time, and there is significant genetic variation within and between populations.     

Variation in social information use: the influences of information reliability and mass on decision making in a group‐living fish Gambusia affinis

The effect of the reliability of available social information was assessed by examining whether the age of social information changes its effects on a foraging decision in a group‐living fish Gambusia affinis. Individuals switched their patch preference when faced with social information that conflicted with personal information in general; the age of the social information, however, did not significantly influence preference for feeding patch. The mass of decision makers was positively correlated with their use of available social information, with heavier individuals exhibiting a greater difference in patch preference than lighter individuals, suggesting that large and small G. affinis trade‐off the benefits of information acquisition and the costs of competition from conspecifics differently.     

Have introduced fish initiated piscivory among the long-fingered bat?

During the winter of 2003–2004 feces were collected from a winter colony of the long-fingered bat (Myotis capaccinii) in the Hazorea cave (north-west Israel). Of the 1913 feces that were examined, 234 contained scales of Gambusia affinis – a small fish that had been introduced to Israel around 1920 in order to control mosquito larvae. The remains of spiders and five insect orders were also represented in the feces. This is the first report of a piscivorous bat in the Middle East and the first finding of fish remains in the feces of M. capaccinii. The findings show that in the north of Israel this species does not hibernate but remains active throughout the winter. It appears that the consumption of G. affinis reflects a change in the diet of these bats from insectivory to semi-piscivory.     

Epidermal club cells and the innate immune system of minnows

Thousands of fish species belonging to the Superorder Ostariophysi possess specialized club cells in their epidermis. Damage to the cells, as would occur during a predator attack, releases chemical substances that evoke antipredator responses in nearby shoalmates. These chemical substances have often been referred to as alarm substances and the cells that release them as alarm cells. Understanding the evolution of the cells in an alarm context has been difficult. The fish needs to be captured prior to the chemicals being released, hence the benefit to the receiver is unclear. Recent studies have suggested that the club cells are part of the immune system and are maintained by natural selection owing to the benefits that they confer against pathogens, parasites, and general injury to the epidermis. In the present study, we gave fathead minnows intraperitoneal injections of cortisol, a known immunosuppressant, or injections of a control substance (corn oil). We found that fish exposed to cortisol had suppressed immune systems (as measured by a respiratory burst assay) and that they also reduced their investment in club cells. This is the best evidence to date indicating that the club cells of Ostariophysan fishes are part of the innate immune system and that the alarm function of the cells evolved secondarily. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 891–897.     

Frog breeding pond selection in response to predators and conspecific cues

Predators are a major influence on the breeding site selection decisions of anurans. Many species actively avoid breeding in habitat with predators when given the choice between predator and predator‐free sites. However, certain factors such as site fidelity or conflicting cues may preclude avoidance behavior. We conducted two experiments examining how western chorus frogs, Pseudacris triseriata, respond to predators, western mosquitofish, Gambusia affinis, using an array of artificial ponds located at two field sites. In one experiment, we added G. affinis to half of our experimental ponds and monitored subsequent colonization by frogs. We found that frogs laid significantly fewer eggs in ponds with fish compared to fishless ponds. In another experiment, we introduced an additional cue to complicate the decision‐making process and monitored colonization of ponds in response to treatments of conspecific breeding cues only (eggs), predators (G. affinis) only, and conspecific cues and predators. We found no significant differences in number of eggs deposited among these three treatments. Based on these results, P. triseriata does not always exhibit complete avoidance of fish predators, and avoidance may vary based on factors such as site fidelity or dispersal costs. This study represents a step toward understanding how multiple biotic factors at a breeding pond may influence anuran site selection behavior in the field.     

Role of olfaction and vision cues in feeding behavior and alarm reaction in the catfish pintado, Pseudoplatystoma corruscans

Experiments were performed to investigate senses that are essential for mediating fright reaction and food behavior in Pseudoplatystoma corruscans, pintado. The dilemma “to feed or to flee” was also analyzed in fishes with intact and sectioned olfactory tracts, stimulated by alarm substance extracts and food. Fishes were arranged into five groups: fish with intact lateral olfactory tracts (LOT), fish with intact medial olfactory tract (MOT), fish with tracts totally sectioned (TOTAL, both LOT and MOT), sham operated, and nonoperated fish. The five groups were submitted to either alarm substance extract and food stimulus or to distilled water (control) and food stimulus. Fish reacted to food independently of which tract (LOT, MOT or TOTAL) was sectioned; vision seems necessary and elemental to detect and deflagrate food response. Latency of the responses to each reaction was different between groups. None of the fish with sectioned tracts reacted to alarm substance extract, while sham- and nonoperated fish showed the typical alarm behavior response, leading to the conclusion that olfaction is essential for mediating alarm response. These results indicate that others sense systems (e.g., vision) are sufficient to trigger and elicit feeding behavior and that olfaction is not necessary to fully maintain food detection to qualitative and quantitative extent. However, olfactory tract integrity seems to be required for mediation of alarm reaction in P. corruscans.     

Responses to Light in Epigean and Hypogean Populations of Gambusia Affinis (Cyprinodontiformes: Poeciliidae)

The responses to light among the fish Gambusia affinis, permanent residents of hypogean brackish water basins of the Spunnulate dolinas system of Torre Castiglione, in southeastern Italy, were studied at light intensities of 180 and 430 lux. These fish showed, on the average, a negative phototactic behaviour (scotophilia) with a low coefficient of variability. On the other hand, epigean, spring-dwelling individuals of G. affinis from the same region showed a positive phototactic behaviour with a greater variability in the responses when compared with cave-dwelling conspecifics. Possible causes for the detachment of hypogean from epigean forms are discussed.     

Competitive Interactions between the Australian Native Fish Galaxiasmaculatus and the Exotic Mosquitofish Gambusiaholbrooki,in a Series of Laboratory Experiments

Competitive interactions between Galaxias maculatus, native to southeastern Australia, and Gambusia holbrooki, an exotic pest, were examined in relation to two types of artificial cover and a food source. Experiments were performed in an 800 l tank using photographic techniques. The relative distance of G. maculatus from cover or food source before and after the introduction of G. holbrooki were compared. Also, the proportions of the species were altered to examine the effects of unequal numbers on dominance behaviour. It was found that G. holbrooki was unable to out compete G. maculatus for either cover or food. The distance of G. maculatus from a food source was seen to increase following the introduction of G. holbrooki, when they outnumbered G. maculatus by 3 to 1. Although the distance from the food of␣G.␣maculatus increased following the introduction of G. holbrooki, in all cases the mean distance of G.␣maculatus from cover or food was less than that of G. holbrooki. Significant intra-species competition appeared to occur between G. maculatus and it may be that this competition had a greater effect than the competitive pressure G. holbrooki was able to place on the natives. This study revealed that the exotic pest species, G. holbrooki, could not out compete a small native Australian fish species.     

Factors Associated with Fish Modify Life History Traits of the Cladoceran Ceriodaphnia cf. dubia

This study examined the effect of water that previously containedfish on several life history traits of the small-bodied cladoceranCeriodaphnia cf. dubia. Four fish density treatments were tested(i.e.16.7, 5.9, 2 and 0.9 1 fish-(1).^–1). A significant(P < 0.05) increase in the mean generation length occurredat a fish density of 16.7 l fish^–1. Fish densities ofat least 5.9 l fish^–1 had a significant(P < 0.05) positiveeffect on mean brood sizes, population growth rate and net reproductiverate. Increased fish density appeared to increase the lengthof time during which the cladocerans reproduced significantly(P < 0.05). None of the tested fish densities affected (P> 0.05) cladoceran survival. The observed effects are ascribedto the presence of unidentified substances in the water thatpreviously contained fish. These substances are thought to be‘fish kairomones’. While life history changes occurin the laboratory, it remains unclear whether similar changeswould occur in the field.     

Predator-dependent diel migration by <Emphasis Type="Italic">Halocaridina rubra</Emphasis> shrimp (Malacostraca: Atyidae) in Hawaiian anchialine pools

Diel migration is a common predator avoidance mechanism commonly found in temperate water bodies and increasingly in tropical systems. Previous research with only single day and night samples suggested that the endemic shrimp, Halocaridina rubra, may exhibit diel migration in Hawaiian anchialine pools to avoid predation by introduced mosquito fish, Gambusia affinis, and perhaps reverse migration to avoid the predatory invasive Tahitian prawn, Macrobrachium lar. To examine this phenomenon in greater detail, we conducted a diel study of H. rubra relative abundance and size at 2-h intervals in three anchialine pools that varied in predation regime on the Kona-Kohala Coast of Hawai‘i Island. We found two distinct patterns of diel migration. In two pools dominated by visually feeding G. affinis, the abundance of H. rubra present on the pool bottom or swimming in the water column was very low during the day, increased markedly at sunset and remained high until dawn. In contrast, in a pool dominated by the nocturnal predator M. lar, H. rubra density was significantly lower during the night than during the day (i.e., a pattern opposite to that of shrimp in pools containing fish). In addition, we observed that the mean body size of the shrimp populations varied among pools depending upon predator type and abundance, but did not vary between day and night in any pools. Our results are consistent with the hypothesis that H. rubra diel migratory behavior and size distributions are influenced by predation regime and suggest that diel migration may be a flexible strategy for predator avoidance in tropical pools where it may be a significant adaptive response of endemic species to introduced predators.     

Exotic fish species in a global biodiversity hotspot: observations from River Chalakudy,part of Western Ghats,Kerala, India

We record here the occurrence of five exotic fish species viz Oreochromis mossambicus, Gambusia affinis, Osphronemus goramy, Xiphophorus maculatus and Poecilia reticulata in the Chalakudy River, part of the Western Ghats, a global biodiversity hotspot in Kerala, India. O. mossambicus was ubiquitous in occurrence with large shoals being encountered at all sampling sites spread along the downstream-upstream gradient of the river, including at an altitude of 1050 m ASL. Osphronemus goramy was recorded from the downstream region of the river while Gambusia affinis was recorded from three sampling sites located downstream as well as midstream. Xiphophorus maculatus was collected from a second order stream flowing through a tea plantation at an altitude of 1050 m ASL. Samples of brooders and early fry of Poecilia reticulata indicate that the fish has already established a breeding population in the river. Possible threats to the indigenous fish fauna of the Chalakudy River as a result of the invasion and proliferation of these exotics is discussed.     

Species invasions and the changing biogeography of Australian freshwater fishes

Aim By dissolving natural physical barriers to movement, human‐mediated species introductions have dramatically reshuffled the present‐day biogeography of freshwater fishes. The present study investigates whether the antiquity of Australia's freshwater ichthyofauna has been altered by the widespread invasion of non‐indigenous fish species. Location Australia. Methods Using fish presence–absence data for historical and present‐day species pools, we quantified changes in faunal similarity among major Australian drainage divisions and among river basins of north‐eastern Australia according to the Sørensen index, and related these changes to major factors of catchment disturbance that significantly alter river processes. Results Human‐mediated fish introductions have increased faunal similarity among primary drainages by an average of 3.0% (from 17.1% to 20.1% similarity). Over three‐quarters of the pairwise changes in drainage similarity were positive, indicating a strong tendency for taxonomic homogenization caused primarily by the widespread introduction of Carassius auratus, Gambusia holbrooki, Oncorhynchus mykiss and Poecilia reticulata. Faunal homogenization was highest in drainages subjected to the greatest degree of disturbance associated with human settlement, infrastructure and change in land use. Scenarios of future species invasions and extinctions indicate the continued homogenization of Australian drainages. In contrast, highly idiosyncratic introductions of species in river basins of north‐eastern Australia have decreased fish faunal similarity by an average of 1.4%. Main conclusions We found that invasive species have significantly changed the present‐day biogeography of fish by homogenizing Australian drainages and differentiating north‐eastern river basins. Decreased faunal similarity at smaller spatial scales is a result of high historical similarity in this region and reflects the dynamic nature of the homogenization process whereby sporadic introductions of new species initially decrease faunal similarity across basins. Our study points to the importance of understanding the role of invasive species in defining patterns of present‐day biogeography and preserving the antiquity of Australia's freshwater biodiversity.     

Alarm response in larval western toads,Bufo boreas: release of larval chemicals by a natural predator and its effect on predator capture efficiency

An alarm response in larvae of the western toad, Bufo boreas, was examined in the laboratory. A natural predator, the giant water bug, Lethocerus americanus, while capturing and feeding on a tadpole, can cause sufficient damage for the release of toad alarm substance and subsequent alarm response in prey conspecifics. Larvae increased their activity and avoided the side of the tank that had a predator feeding on a conspecific tadpole in a visually isolated but interconnected container. Larvae did not increase activity or avoid the side of the tank that had a predator feeding on a heterospecific tadpole. Performance of the alarm response in conspecifics increased survivorship. The capture efficiency of predators (naiads of the dragonfly Aeshna umbrosa) hunting larvae that had been exposed to the alarm substance of conspecifics was reduced: predators required more capture attempts per successful attack compared to control tests.