By Laura Lopez
The Eastern Mosquitofish, Gambusia holbrooki, is one of the most widespread- and reviled- of all invasive species to inhabit Australia (Pyke, 2008). Originally introduced as a biocontrol for mosquitos in the 1920’s, it has turned out that Mosquitofish aren’t any more efficient at consuming the insect’s larvae than many other native fish (Pyke, 2008). So, in a rather familiar story to Australians (ahem, cane toads), we are now tasked with somehow managing a species listed as one of the worst invaders in the world (Pyke, 2008). To continue on this rather depressing note, there is every indication that the consequences of failing to do so are severe, as it has been associated with the decline of nine fish species, amphibians and insects (Howe et al., 1997).
When we consider the geographic range of Mosquitofish in Australia, it’s tolerance to environmental variability is clear. Less clear, however, is whether and how its impacts on native species vary with context. While some research overseas has focused on the effects of Mosquitofish density, temperature and salinity on key interactions, such as predation and competition, with native species (Alcaraz et al., 2008; Mills et al., 2004), less has been done in Australia. Ultimately, identifying the influence of context on the impact of Mosquitofish in Australian waterways will aid their management, particularly when it comes to working out the conditions at which Mosquitofish are most problematic and need to be targeted.
A common and fair assumption about invaders is that with their increasing density a higher impact on the environment occurs. However, in the context of predation, an increase in competitive interactions between invasive predators can accompany a hike in density (Pintor et al., 2009). This means that predators may spend more time interacting with each other than hunting and attacking prey. In the lab, we set out to explore the relationship between Mosquitofish density and time of day (diel cycle) on the lethal and nonlethal effects experienced by a prey, the Glass shrimp, Paratya australiensis.
Berried Glass shrimp tagged with an elastomer, allowing for individual identification
In regards to lethal effects, we observed Mosquitofish to prey upon shrimp, yet the actual rate of predation did not vary between low (1 fish) and high (5 fish) fish densities, or diel cycle, which suggests that interference occurs between fish. When it came to non-lethal effects, shrimp drastically reduced the time they spent swimming (most likely to reduce their vulnerability to predation), and occupied shelters far more. The shift in these behaviors suggests that shrimp may also forage and interact less with reproductive mates in the presence of Mosquitofish, which could significantly lower their fitness. However, the shift in these activities was the same regardless of fish density and diel cycle, indicating that they were responding in proportion to the risk imposed by Mosquitofish. Interestingly, shrimp exposed to both a high density of fish and a high density of shrimp, the control, both increased their foraging activities. Ultimately, what is most notable is that even low densities of Mosquitofish have multiple negative effects on shrimp from direct consumption, causing changes in activity levels and potentially even by competing for food.
(Above) Experimental setup with lurking Mosquitofish and fearful shrimp hiding in shelters.
Tagged shrimp hanging out next to shelters
Along with density, we were also keen to look at the effect of abiotic factors on interactions between Mosquitofish and native species. One of the key mechanisms by which Mosquitofish are supposed to out-compete native species is by aggression, and their fondness for fin nipping is particularly well known (Pyke, 2008). With this in mind, we decided to look at how aggression between Mosquitofish and Australian Bass, Macquaria novemaculeata, fingerlings is influenced by a combination of temperature and salinity – two key factors in freshwater systems. Bass are stocked throughout the Eastern Drainage system as fingerlings to support recreational fishing (Cameron et al., 2012). While adults are predatory, stocked juveniles are often no larger than an adult Mosquitofish and being hatchery-bred, could be naïve to the behaviour of other fish species.
Weighing and tagging an Australian Bass fingerling
We were particularly interested in how a combination of temperature and salinity would influence aggression compared to each stressor alone. While the effect of two stressors can be the sum of both together, it can also be antagonistic (less than additive) or synergistic (greater than additive), which are much harder to predict (Sih et al., 2004). Again in the lab, we measured aggression between Mosquitofish and Bass at four different combinations of 21 °C or 28 °C temperature and 15 ppt or 35 ppt salinity. Like many fish species, both Bass and Mosquitofish were much more aggressive when exposed to 28 °C and low salinity levels of 15 ppt. Interestingly, when we combined elevated temperature with elevated salinity, 35 ppt, their aggression decreased markedly, which suggests that the effect of temperature is dependent on salinity. For both Bass and Mosquitofish, the interaction between temperature and salinity was non-additive, specifically antagonistic. This result confirms the value of considering multiple stressor effects, not just because it provides greater realism in lab experiments, but also because the outcome can be unpredictable.
Pregnant Mosquitofish tagged with an elastomer
Ultimately the impacts of Mosquitofish are complex. Our work provides no simple answers, but instead emphasises the importance of considering context, particularly in lab studies, when trying to understand the mechanisms behind this species’ success.
Alcaraz C, Bisazza A, Garcia-Berthou E, 2008. Salinity mediates the competitive interactions between invasive mosquitofish and an endangered fish. Oecologia 155:205-213. doi: 10.1007/s00442-007-0899-4.
Cameron LM, Baumgartner LJ, Bucher DJ, Robinson W, 2012. Critical Thermal Minima of age-0 Australian bass, Macquaria novemaculeata, fingerlings: implications for stocking programmes. Fisheries Management and Ecology 19:344-351. doi: 10.1111/j.1365-2400.2012.00850.x.
Howe E, Howe C, Lim R, Burchett M, 1997. Impact of the introduced poeciliid Gambusia holbrooki (Girard, 1859) on the growth and reproduction of Pseudomugil signifer (Kner, 1865) in Australia. Marine and Freshwater Research 48:425-433. doi: 10.1071/mf96114.
Mills MD, Rader RB, Belk MC, 2004. Complex interactions between native and invasive fish: the simultaneous effects of multiple negative interactions. Oecologia 141:713-721. doi: 10.1007/s00442-004-1695-z.
Pintor LM, Sih A, Kerby JL, 2009. Behavioral correlations provide a mechanism for explaining high invader densities and increased impacts on native prey. Ecology 90:581-587. doi: 10.1890/08-0552.1.
Pyke GH, 2008. Plague Minnow or Mosquito Fish? A Review of the Biology and Impacts of Introduced Gambusia Species. Annual Review of Ecology Evolution and Systematics 39:171-191. doi: 10.1146/annurev.ecolsys.39.110707.173451.
Sih A, Bell AM, Kerby JL, 2004. Two stressors are far deadlier than one. Trends in Ecology & Evolution 19:274-276. doi: 10.1016/j.tree.2004.02.010.