Investigating the Impact of Water Level Fluctuations on Largemouth Bass Populations in Lake Murray

Margaret Wise, Dr. Troy Farmer

Abstract

Variable springtime water levels in southeastern reservoirs may impact Largemouth Bass (LMB) reproduction and recruitment. LMB spawn in shallow water during spring and support economically important recreational fisheries that positively impact local economies and communities. The aim of this study was to investigate relationships between springtime water level and LMB year-class strength (each cohort’s relative contribution to the adult population) in Lake Murray, South Carolina. Using annual electrofishing surveys from the South Carolina Department of Natural Resources, we quantified a relative metric of LMB year-class strength for seven years from 1998-2008. We then related our metric to historical springtime USGS Lake Murray water level data. Results indicate that there is no correlation between springtime water level and LMB recruitment and fisheries managers may not need to maintain certain water levels during the spring spawning season.

Introduction

  • Largemouth bass are one of the most important recreational fish species
    • They occur throughout the United States outside of their native range due to stocking in many places
    • They spawn in the spring
    • LMB fisheries have positive impacts on local communities
    • They are an ecologically important part of reservoir fish communities
  • Lake Murray is located in central South Carolina, west of Columbia
    • The lake is owned and operated by Dominion Energy
    • When Lake Murray Dam was completed in 1930, it was the largest man-made hydroelectric dam in the world
    • Lake Murray is a major habitat for Largemouth Bass and has countless bass fishermen visit it each year
  • The objective of this study was to see how spring water level fluctuations affect LMB reproduction and recruitment
    • We used April-May monthly average water level for each year along with electrofishing data to see if water level during LMB spawning months affects the number of fish that survive year to year.

Materials and Methods

The historical electrofishing datasets from Lake Murray were from the South Carolina Department of Natural Resources (SCDNR). We used data from the years 1999-2001 and from the years 2006-2008. Lake level data was from the United States Geological Survey (USGS).

An age-length key was developed to age the unaged fish in the dataset using R software. Once all fish had assigned ages, a catch-curve was developed. Then, a weighted least squares regression using the proportion of total catch in each year class as an index of relative abundance. Residuals were then determined and used as a measure of abundance and plotted against the average lake level during April and May.

Results

chart

Figure 1. This figure indicates the relationship between year-class strength (indicated by residuals) and estimated lake level during April and May for each cohort of Largemouth Bass in Lake Murray.

There was no correlation determined between spring lake levels and the year-class strength. This means that a higher lake level does not always mean that more LMB eggs will survive to adulthood. Each dot in Figure 1 indicates the average residuals (which tells us year-class strength) of each cohort of largemouth bass and the average lake level in April and May of the year that those fish were born. As in the figure, there is no relationship between the two variables.

Conclusions

As shown in the results, there is no correlation between lake level and largemouth bass recruitment/reproduction. This may be a good thing for fisheries managers who are concerned about low lake level years having an impact on fish populations. If Dominion Energy, who owns and operates the dam, chooses to let out more water in the springtime, fishery managers at Lake Murray may not have to worry about this negatively impacting the number of surviving fish that hatched that year.

Additionally, it may be a good thing for anglers in the future who may be concerned about the effects of climate change on lake level. Drier and hotter years may mean a lower lake level. If a lower lake level meant fewer surviving fish, climate change potentially would have impacted catch rates. However, since there is no correlation this may not be a concern.

It is important to note that while there was no correlation found, not many years were used in this study, and with more data points to look at, the trend may present itself differently. This study would benefit from incorporating more years of electrofishing data to get a better sense of the relationship between spring lake level and year-class strength of Largemouth Bass.

Acknowledgements

Thanks to South Carolina DNR for electrofishing datasets, USGS for monthly water level data, and Sport Fish Restoration for funding salaries of SCDNR scientists