Abstract
The eastern mosquitofish, Gambusia holbrooki, is a globally distributed species of viviparous freshwater fish, found in tropical and temperate climates. It is considered an invasive species on several continents, including Australia, and it may threaten the survival of certain critically endangered native species throughout the world. It is of great interest to ecosystem conservation to monitor and control their populations. To gain a better understanding of this non-native species’ life history with potential implications of other species, we used passive-capture methods to obtain a 365-specimen sample from a campus pond. The lengths of the specimens were used to estimate their ages, and the age-specific data was organized into a life table. From this, we calculated descriptive statistics about the population, including its net reproductive rate, mean generation time, optimal age for sexual maturity, and intrinsic growth rate. We compared these statistics to those of an example population, and drew inferences about the reasons for the measured differences. The life table calculations provide evidence that the observed differences between the populations were due to variations in the fishes’ environment, rather than to inherent genetic or developmental differences.
Introduction
Background
Mosquitofish is a globally distributed species of viviparous freshwater fish. The Eastern Mosquitofish, Gambusia holbrooki, can be found in tropical climates througout the world, from the freshwater ponds and marshes of the Florida Everglades to the coastal lagoons of the Iberian Peninsula. G. holbrooki subsist primarily on a diet of insect larvae, zooplankton, and other invertebrate animals. Easily mistaken for guppies, they grow to an average length of 2.5 inches (female) and 1.5 inches (male). Having a high reproductive rate and tending to thrive in nearly every habitat where it has been introduced, the eastern mosquitofish is considered an invasive species in multiple countries, including Australia (Cabral & Marquez, 1999). As a predator of larval and adult indigenous fish, and as a host to environmentally destructive parasites, its continued spread poses an extinction risk to many rare and endangered native species of fish throughout the world (Nicol et al., 2015). The monitoring and control of eastern mosquitofish populations is a matter of great interest to ecosystem management and conservation efforts.
Objective and Hypothesis
Methods
Choosing a field sampling method that was non-destructive and randomized was paramount. Furthermore, since the length of each specimen needed to be measured in order to make a determination of its age, it was decided that a capture method would be necessary. Several different fish capture methods are routinely used in fishery surveys, including electric fishing, netting, trapping, snaring, and settlement substrates (Pope et al., 2010). A passive trapping method using baited minnow traps was chosen for this research. Minnow traps were thought to be the most appropriate capturing medium, due to the small size of G. holbrooki, and due to the suitability of the lightweight traps to calm and shallow waters (Culp & Glozier, 1989).
The ages of the fish were derived from measurements of their length, recorded in millimeters. The equation that relates the age of G. holbrooki to its length is
Age = 8 × (Standard Length) – 68
The age-specific data was organized into a life table to make the calculations of the other variables more convenient. The survival rate, l(x), is the proportion of the total population that survives to each age category. The net reproductive rate, R0, is the expected number of offspring per individual per lifetime. The mean generation time, G, represents the average age difference between parent and offspring. The intrinsic growth rate, r, estimates the change in the size of the population, with units of individuals ∙ days-1. Because data was collected at a single point in time, rather than over a course of weeks or months, a vertical, or static, life table was constructed, rather than a horizontal, or dynamic, one.
Results
Unlike Table 1, Table 2 shows that the most numerous age class consisted of the group between 60 days and 90 days of age, with 240 individuals making up 31.6% of the sample. After 120-149 days, a sheer drop-off in the number of individuals occurs. Only 2.5% of the population is older than 150 days. This population seems to have a shorter lifespan than the one sampled, but it also seems to have more individuals that survive beyond 0-30 days.
The third, fourth, sixth, and seventh columns of the life table were calculated using the first two columns, which were created from field measurements. The fifth column, fecundity, was provided, and is the same that was used for the example population’s life table.
R0, the net reproductive rate, is the sum of the sixth column of the life table; it is the sum of l(x) * b(x) for each age class. G, the mean generation time, is the sum of the seventh column divided by R0. The intrinsic growth rate is given as the log of the net reproductive rate divided by the mean generation time, or r = ln (R0)/G. The optimal age for sexual maturity was found by looking in the sixth column to see which age class had the most offspring per individual. It can be seen that the optimal age is 90 days, at 7.1 offspring per individual.
The population features of the example population are summarized in Table 5 for convenience, since one of the objectives of this research is to compare the features of the two populations. These data were provided, not calculated.
Discussion
A review of Tables 4 and 5 demonstrates that the two populations of eastern mosquitofish were very similar, as predicted by the research hypothesis. Two of the population metrics were identical, while the other two population metrics were less close in value, but still comparable, being of the same order of magnitude. This evidence supports the hypothesis made at the beginning of this research that the population features would closely match.
The intrinsic growth rate, measured in terms of the average number of new individuals added to the population each day, was the same for both populations, or about 0.02. This indicates a highly stable population. This number signifies that approximately every fifty days, the total population grows by one individual. Since the populations are geographically isolated (being located in ponds), they do not lose or gain individuals due to migration, and so their stabilities must be explained by a birth rate that barely compensates for the death rate.
Both populations of eastern mosquitofish can be seen to become most fecund after maturing to an average age of 90 days. This result would seem to suggest that the fish are developmentally identical. These data provide evidence in support of the theory that the two populations are essentially the same, and that any differences between the populations arise from differences in their environment. The lower net reproductive rate for the example population, 7.19 offspring per individual to the sampled population’s 10.8 per individual, can be explained by environmental differences; for example, a smaller habitat, providing less food, would tend to limit the number of offspring from each fish over its lifetime. A shorter mean generation time for the example population compared to the smaller sampled population may also be explained by differences in the fishes’ environment. For example, there may be less selective pressure on the smaller school to breed quickly, because fewer natural predators are present.
We attempted to minimize and control for sampling bias with the field sampling method we chose. We believe selecting individuals from the population using minnow traps gave us an authentic random sample. The large difference in sample size between the example population and the one we sampled, however, may be a source of misleading information. To improve the quality of the research, samples that are of similar size should be compared.
Because data was collected at only one point in time, a vertical life table was chosen for the statistical modeling of the sampled population of G. holbrooki. Life tables, both horizontal and vertical, are important in population ecology because they allow researchers to understand the demography of the populations they are investigating. The demographic data that life tables yield reveals important information about the processes that affect these populations. The organized presentation of this data assists in the researchers’ analysis, enabling them to see patterns and make predictions about the future state of the population (Begon et al., 1996).
References
Begon, M. Harper., JL., and Townsend, CR. (1996). Ecology: Individuals, Populations and Communities. 3rd.–Blackwell Science oxford.
Cabral, J. A., & Marques, J. C. (1999). Life history, population dynamics and production of eastern mosquitofish, Gambusia holbrooki (Pisces, Poeciliidae), in rice fields of the lower Mondego River Valley, western Portugal. Acta Oecologica, 20(6), 607-620.
Culp, J. M., & Glozier, N. E. (1989). Experimental evaluation of a minnow trap for small lotic fish. Hydrobiologia, 175(1), 83-87.
Nicol, S., Haynes, T. B., Fensham, R., & Kerezsy, A. (2015). Quantifying the impact of Gambusia holbrooki on the extinction risk of the critically endangered red‐finned blue‐eye. Ecosphere, 6(3), 1-18.
Pope, K. L., Lochmann, S. E., & Young, M. K. (2010). Methods for assessing fish populations.
