Does temperature affect the life history of Eastern Mosquitofish (Gambusia holbrooki) in FIU’s Hennington Pond?
Abstract
The measurement in millimeters of the Eastern Mosquitofish (Gambusia Holbrooki) were taken at the Florida International University Hennington Pond. Five groups of researchers collected thirty females each. The fish were collected using large nets and were contained in buckets. Fish grow continuously throughout life, allowing us to estimate their ages using their length and perform a vertical life table analysis The objective of the study was to test whether temperature affects the life history of Eastern Mosquitofish (Gambusia holbrooki) in FIU’s Hennington Pond,A statistical test was ran to compare the net reproductive rate(R0), mean generation time(G) and intrinsic growth rate(r) in seasons, summer and winter. The statistical test for each gave a p-value of less than 0.05. Concluding that there was a significant difference in the net reproductive rate, mean generation time and intrinsic growth rate between the two temperatures.
Introduction
Gambusia Holbrooki is an invasive freshwater fish species which is mostly native to the Eastern part of the United states of America. One of the main reasons they are so widespread is because they do very well in a wide variety of environmental conditions. Interestingly, there is some evidence to support that they earned the name Mosquitofish due to their ability to mitigate mosquito population and diseases.(Pyke,2008) . Typically, Eastern mosquitofish reproduce through internal fertilization, a male will fertilize up to six broods of females when mating.
Temperature has been known to play a significant role in their reproductive processes. An Australian study found that breeding amongst G.Holbrooki took place when temperatures were above 15-16 degrees Celsius. (Pen & Potter,1991). On the other hand, it has also been found that an increase in temperature raises predation by other organisms. Which, in turn, lowers the population of Eastern mosquitofish (Meffe,2011). , Even though, Eastern mosquitofish are a notoriously invasive species, temperature is not the only factor that can limit their invasion. It’s been found that salinity also restricts the invasive success of the fish(Alcaraz, C. & García-Berthou, 2007). Specifically, higher salinity than average ambient freshwater salinities caused a later reproduction and embryonic development than seen in mosquitofish from freshwater or brackish water salinities, where G.Holbrooki typically reside . (Nordlie & Mirandi, 1996). Despite, these environmental exceptions G.Holbrooki are usually invasive and prefer still, shallow, as well as warmer waters. (Murphy, C. A., et al., 2015).
The goal of this study was to explore the question “Does temperature affect the life history of Eastern Mosquitofish (Gambusia holbrooki) in FIU’s Hennington Pond? In order to determine this we evaluated the population structure of G. Holbrooki by taking the data of 150 individuals and performing a vertical life table which included the survival rate, fecundity, and age distribution. As well as the, aforementioned intrinsic growth rate, net reproductive rate, and mean generation time. The vertical life table conducted by our research group was compared to winter season data using statistical tests with respect to R0, r and G. The null hypothesis was rejected in each test, concluding that there was a significant difference in net reproductive rate (R0), intrinsic growth rate (r), and Mean generation time (G), between both temperatures.
Methods
Field Sampling
The G.Holbrooki population was sampled at the Florida International University, Hennington Pond in Miami, Florida. The pond was shallow with still water and a moderate amount of vegetation. Only female G.Holbrooki were sampled and were required to be at least 9 mm in length. The fish were acquired using long nets, measured on flat surfaces, such as a clipboard then placed in a bucket, which would be later used to transfer the mosquitofish back to the water. The fish were distinguished as male or female depending on the presence of a gonopodium, which indicated a male. If the fish were found to be male, they were immediately returned to the pond. Standard lengths were measured from the beginning of the snout to the body, excluding the tail. Mosquitofish smaller than 9 millimeters were also to be returned. Mosquitofish larger than 9 millimeters were recorded. A total number of 150 Mosquitofish were collected for the data.
Calculations
After collecting the length data from all 150 female G. Holbrooki, A vertical life table was then built, since this data was collected from the population in one short-term occurrence. The age of the Mosquitofish was calculated using the following formula: age= 8*(standard length) -68, standard length being measured from snout to end of body, excluding tail. The mosquitofish were then categorized in age groups depending on their computed ages. It was necessary to calculate the number of individuals within each age class, which was computed using (s(x)). To compute the number of living individuals with each age class n(x)) was computed. The number of living individuals within the first age class was calculated using N0 = ∑nx. Every other age group was computed using Nx = (Nx-1) – (nx-1), which is essentially, the number of individuals in a previous age group subtracted from the number of individuals sampled in the age group. Survival rate was represented as (l(x)) and was computed as the number of individuals in an age group divided by the number of individuals alive in the first age class, which is represented by n(0). The formula used to compute fecundity is as follows (l(x)*b(x)) and it represents the amount of offspring produced by each particular age group. Highest age weighted fecundity was given by (l(x)*(b(x)*x). The net reproductive rate (R0) was computed using by the sum of the entire column of (l(x)*b(x)). Intrinsic growth rate (r) was computed using the division of the natural log and the net reproductive rate (R0) by the mean generation time (G). Mean generation time (G) was computed by taking the sum of the (l(x)*(b(x)*x) column and its division by the net reproductive rate (R0). The highest age-weighted fecundity given by (l(x)*(b(x)*x) age group was used to determine the optimal age of sexual maturity.
Results
A statistical test was ran for each respective life history aspect of R0, r, and G to test for statistical significance between the two temperatures. The net reproductive rate statistical test between the two temperatures gave a p value of much less than 0.05 (p-value = 1.97e-06). This signifies that the null hypothesis can be rejected. Concluding that net reproductive rate does have a significant difference between the two temperatures. For intrinsic growth rate the p value was also less than 0.05(1.778e-06) rejecting the null hypothesis. Intrinsic growth rate does exhibit a significant difference between the two temperatures. Mean generation time’s statistical test between the two seasons also resulted in a p-value of less than 0.05 (p-value < 2.2e-16). Concluding that mean generation time does exhibit a significant difference between the two temperatures. Intrinsic growth rate (r) and Net reproductive rate were found to have a strong POSITIVE linear correlation.
Figures:
Unit for each R0, r and G is Avg. number of offspring.
Figure 1. Bar Graph comparing Net Reproductive rate by Season in FIU Hennington Pond
Figure 2. Bar Graph comparing Intrinsic Growth Rate by season in FIU Hennington Pond.
Figure 3. Bar Graph comparing mean generation time by season in FIU Hennington Pond.
Figure 1. Scatter plot displaying POSITIVE LINEAR relationship between Net Reproductive (R0) and Intrinsic Growth Rate (r).
Discussion
The Gambusia Holbrooki population sampled and tested in our study supported a rejecting of our null hypothesis. It can be concluded that in our sampled population of G. Holbrooki temperature does have a significant effect on aspects of life history that were tested in our study i.e. Intrinsic growth rate (r), net reproductive rate (R0), and Mean Generation time(G). Net reproductive rate is the avg. number of offspring born to a female in her lifetime. In our study the net reproductive rate was found to exhibit a significant difference between the two temperatures of Summer and Winter. Some supporting evidence for this finding is reflected in a 28-year study where Gambusia Holbrooki were exposed to the abnormally high temperature of 32 degrees Celsius rather than normal conditions of 25 degrees Celsius. The mosquitofish in the higher temperatures were found to mature at younger ages than the fish in normal conditions.(Meffe,1992). It could be deduced that if fish were maturing earlier, they would in turn have an earlier sexual maturation age.
Intrinsic growth rate is defined as the rate that a population grows over a successive period of time. Our results showed that intrinsic growth rate did exhibit a significant difference between the two temperatures. A possible ecological explanation to support this could lie in the finding that food availability increases as temperatures rise. (Vondracek, Bruce, et. Al.,1988). One behavioral study observed that Eastern mosquitofish mating became very restricted in higher than normal temperatures (30-35 degrees Celsius). ((Wilson,2004)). These findings help provide possible explanations for our result of intrinsic growth rate exhibiting a significant difference between two different temperatures.
Mean generation time is average amount of time it takes for a female to give birth to her first offspring. In our results the mean generation time exhibited a significant difference between the two temperatures. A possible ecological explanation for this is displayed in a study taking place in Lake Herviz where the highest number of pregnant females were found to be from August to July when temperatures were warmer (Specziar,2004). There seems to be a consistent trend in the research provided in which, warmer temperatures lead to quicker developmental maturation. It’s also important to note that intrinsic growth rate (r) and net reproductive rate (R0) have a positive linear relationship. Meaning that as one increases so does the other and vice-versa.
Each one of our findings have a great degree of ecological importance. The finding that net reproductive rate, mean generation time, and intrinsic growth rate each held a significant difference in between the two differing temperatures may provide support by providing insight on obdurate ecological issues being faced today in relation to G.Holbrooki, such as, the introduction of mosquitofish decimating amphibian species (Hamer, et. Al,2002) or how introduction of the G.Holbrooki has also been found to cause algal blooms that are harmful to the environment, caused by their feeding on zooplankton ((Hurlbert et al. 1972) and how they’ve displaced certain topminnow fish with their invasive nature (Whittmore,1997).
27.2.2019