Monitoring largemouth bass and bluegill populations over time can help pond managers accomplish their goals. A good way to monitor these trends is to conduct hook and line or electrofishing surveys. However, once surveys are conducted, interpreting the data can be difficult. Creating bar graphs of fish size and calculating proportional stock density are two tools that pond managers can use to help them interpret data from fish population surveys.
Plotting length and weight of bass and bluegill in a bar graph is one way to interpret survey results. By plotting data, a pond manager can visualize the distribution of fish in length and weight classes or monitor the average weight of fish in certain size classes. If changes occur over time, management decisions can be made accordingly. For instance, if surveys are conducted for several years in a pond and they indicate 12-inch bass averaging 1 pound, then the bass are in good shape. However, if the average weight of 12-inch bass drops to 0.6 pound over time, something is affecting weight for that size class. Changes such as this can be gradual and hard to detect if catch records are not kept.
Proportional stock density (PSD) (see calculation 1), also called the percentage size distribution, is another tool. PSD is a calculation that can give a pond manager an idea of the proportion of quality-size bass or bluegill to stock-size bass or bluegill. Stock-size fish are those that are sexually mature. Bass are sexually mature at around 8 inches and bluegill at around 3 inches. The standard quality length for bass and bluegill is > 12 inches and > 6 inches, respectively. In balanced fish populations, an accepted PSD for bass is 40-70 and 20-60 for bluegill. A PSD of 50-80 for bass is desirable for a pond manager with trophy bass goals.
A pond manager should be aware that all surveys have biases and limitations associated with the equipment used. Pond managers using the hook and line sampling technique need to understand there is a bias toward larger bluegill since it is hard to catch 3- to 4-inch bluegill. Therefore, when using this technique, PSD for bluegill will be higher (40-60) than the actual population because the denominator in the calculation will be a smaller number. However, if a pond manager uses electrofishing, the PSD for bluegill will be lower (20-40) because electrofishing more effectively catches smaller bluegill, resulting in a larger denominator in the calculation. Bass have less bias associated with hook and line because it is not as difficult to catch 8- to 9-inch bass as it is to catch 3- to 4-inch bluegill.
Remember that these techniques are surveys, not censuses. Survey sampling error could cause a balanced population to appear unbalanced on any one day. Ponds are also dynamic and constantly changing. That's why it's important to sample multiple times and evaluate long-term trends, not just the results of one survey. If you are interested in learning more about monitoring fish population and other pond management topics, attend the Noble Research Institute Pond Management Workshop on June 10, 2010. For workshop information and registration, see our events page or contact Tracy Cumbie at 580.224.6292; firstname.lastname@example.org.
For more information on hook and line surveys, see Hook and Line Fish Sampling by Mike Porter in the June 1995 Ag News and Views.
Murphy, B. R. and D. W. Willis, editors. 1996. Fisheries Techniques, 2nd Edition. American Fisheries Society. Bethesda, Maryland.
Anderson, R. O. 1976. Management of Small Water Impoundments. Fisheries. Vol. 1, No 6. p. 5