Gambusia holbrooki or the eastern mosquito fish native to the Southeastern United States, and especially found in Everglades National Park where it is an important part in maintaining balance between algae, animal species, and water properties in accordance with its diet and behaviors. These fish specialized to harsh conditions and proliferating in hot temperatures have lengths associated with their life spans and maturity, exhibiting optimal sexual maturity at approximately 90 days. In this study, 150 G. Holbrooki were sampled in Henington pond where they were captured and measured by 5 groups with the use of nets, buckets, and rulers. These methods did not harm the species and fecundity, age class, and survival rates were calculated through the captured measurements of fish length in millimeters. 150 of these fish were utilized as the sample in the study and their data set showed a significant relationship between survival rates and sexual maturity. Survivals rates declined dramatically as optimal sexual maturity (age class: 90 days) was approached reaching a ratio of 0.03 and slowly approaching zero once ideal maturity past. This pattern in declining slope of survival rates was attributed to the reproductive costs associated with G. Holbrooki life history.
Introduction
An indispensable part of a freshwater ecosystem includes many small species of fish that are essential in maintaining a balance below the surface of the water. Although there are many a rather important one in Southern United States is Gambusia Holbrooki (G. Holbrooki). These fish are known as the eastern mosquitofish local to Florida and most abundantly found in Everglades’ National Park. These fish are native to Southeastern zones due to the climate and nutrients where they exhibit high fecundancy in high temperatures ranging from 35-39 C, or higher (Mulvey et al, 1994). They play a key role in maintaining balance between planktivorous species, algae, and even other species larvae like tadpoles (Blanco et al, 2004). The average lengths of the G. holbrooki fish range from 3.5-6.4 cm with the male mosquitofish exhibiting the smaller range and females exhibiting a larger range for the benefit of reproduction (Laidlaw, 2014).
In this study we analyzed Gambusia Holbrooki populations in Hennington pond located at Florida International University, by sampling and measuring lengths of 150 mosquitofish with the objective of observing age-weighted fecundity and survival rates based on maturity to determine if there are differences between populations before and after a cold snap. The relationship between survival rates of G. holbrooki and sexual maturity were further analyzed in the sampling data showing possible correlational results. It was inferred that survival rates would decline more dramatically in assuming optimal sexual maturity due to the reproductive costs associated with the age class and a significant difference between populations before and after a cold snap was observed.
Methods
A group of students from Florida International University, collected samples of Eastern mosquitofish within Hennington pond. The samples of G. holbrooki were collected through the process of shallow fishing using non-destructive methods. Specifically, students were instructed to catch the fish using either large pool nets or small soft mesh nets. Once each specimen was collected the fish were measured in length from head to tail with the use of rulers measured in millimeters. The fish were maintained in buckets with water during the sampling methods to avoid resampling throughout the process. This process of collecting a representative sample of the G. holbrooki population was split among groups of five, leaving each group responsible for the sampling of 30 specimens of G. holbrooki. Although the length of the fish was measured the age of each fish in days, was calculated using the following formula: Age = 8*(Standard Length)-68 [r2=0.702, P=0.001] (Hornbach 1986). Based on this formula in calculation of age based on length those fish measured to be 9 mm or less were regarded as aged less than zero days and excluded from the data. This left a remaining 150 mosquitofish for analysis.
Using the data collected a vertical life table was conducted to observe sampling data that was collected during one period rather than dynamically- requiring a horizontal life table. This life table was used to identify and quantify the static observation of the data set presenting age in days, sample size, number in age class, survival rate, fecundity, offspring, and age-weighted fecundity.
According to the data presented above, the higher frequencies of age presented in the study was that of peak sexual maturity at approximately 90 days as well as those of younger maturity such as 60 days and 30 days in age class. Those high frequencies were presented among both data sets in the example as well as in the conducted study. The differences in survival rates and frequencies presented among Spring 2018 table and Summer 2017 table could be due to the higher temperature of the water after the cold span. As seen in Spring 2018 in comparison to Summer 2017 table, the survival rates decreased in ratios as maturity rose and assumed a peak position at the sexual mature stage of age class 4 approximately encompassing the range of 90 days.
Age classes 1, 2, and 3 showed a high frequency of sampling which could be due to the optimal sexual maturity to be 90 days among females but ranges from 43-67 days among male eastern mosquito fish (Rajikumar, 1987). These could have been sampled in high quantity due to the males of this species known to be highly aggressive in looking for a mate and thus highly active in pursuit of females in which they attack shoals of females. This aggressive behavior in mating protocol could have made the males more visible and caused that high frequency in earlier mature age groups.
In this life table study a couple of values were obtain and use to compared populations before and after the cold snap; The Net reproductive rate (R0) was one of them and this is the sum of the offspring/individual column. It shows the number of offspring an individual will produce over its lifetime. If R0 is greater than 1, then population size increases. If R0 is less than 1, then population size decreases, and if R0 equals to 1, then population size does not change. A second important value is the mean generation time (G) which is the sum of the age-weighted fecundity column divided by the net reproductive rate (R0) and this one reflects the average time until a female gives birth to one offspring. The intrinsic growth rate (r) is an estimate of population growth; it is equal to the natural log of the net reproductive (R0) divided by the mean generation time (G): r = ln (R0) / G and it provides the rate at which a population is growing. Further comparisons between both tables show that both R0 values were greater than one, indicating that population size was increasing among both samples. R0 value for Summer 2017 table was 25.82 in comparison to the R0 value for Spring 2018 table being 42.50 offspring indicating a greater population increase within the study conducted. This could be in turn tied into the timing of the study being done in the summer time when the brooding season of the mosquito season is exceedingly higher and the higher temperatures allow for a greater reproductive rate as well for greater size in broods (Pen & Potter, 1991). An aspect that did not indicate a greater reproductive rate in the study conducted is the value presented by generation time (G) as a value of 114.48 days for Spring 2018 in comparison to the Summer 2017 generation time of 114.28 days. As well as presenting the similar trend among the intrinsic growth rate did not increase exponentially in this study.
In analyzing the survival rates and their decline among sexual maturity (age classes), Figure 1 presents evidence to support the hypothesis provided. Survival rates decline exponentially until reaching age class 90 which encompasses the optimal sexual maturity of mosquitofish (92 days) within this study. This decline in survival rates in succession with increasing sexual maturation and age among the G. holbrooki is possibly due to the several allocating and viability costs associated with reproductive preparation like temperature. Viability costs such as depletion of energy storage and physiological changes in response to future reproduction inhibit behavior and survival of mosquitofish (Laidlaw et al, 2014). Allocating costs are deemed for current reproduction such as difficulty swimming for pregnant female fish further inhibiting their growth. Thus, it is suitable that survival rates will decline prospectively in preparations for reproduction due to the high energy costs seen physically. Therefore, once the brooding season and optimal sexual maturation have past, energy stores may fluctuate and G. holbrooki may maintain a balance and stabilize the existing survival rate post-reproduction. Furthermore, the alternative hypothesis indicates a relationship between declining survival rates and maturation as seen in the figure as well as in the comparative tables showing the same decrease.
This study showed similar results to the expected example with a larger population, and thus shows precise and rather accurate results. The study may be replicated with a larger population to ensure unbiased and representative sampling. Issues in the current study that could show certain bias numerically include one group’s spilling of sampled fish, possibly causing resampling in the data set. This may have led to a few repetitive measures in length. Another parameter that should be tested in this study is the direct effect of water temperatures on the life histories. Although the temperature was implied as a possibility for the increased population growth in comparison with the example, explicit data on temperature changes would be an interesting finding to observer the behavior of G. holbrooki for further research.