Age and Growth of Bluegills, Lepomis macrochirus Ra:finesque, from Selected Central Iowa Farm Ponds

Population estimates were made on ~ve .farm ponds in central Iowa during 1965. Age and growth determmat10ns were ma.de from scales of 638 bluegills collected from these ponds. Bluegills standing crops averaged 248 pounds per acre, ranging from 201 to 3 21 pounds per acre. Growth rates of bluegms from the balan~ed ponds were average when compared with growth rates of bluegills from other Iowa lakes and ponds. On June 1, 1964, the Iowa Cooperative Fisheries Research Unit began studies relating to the physiology of fish in farm ponds with different population densities. As part of this program, collections of bluegills were made from five study ponds in central Iowa to estimate populations and gather data on bluegill age and growth. These data were later correlated with physiological parameters. MATERIALS AND METHODS Preliminary observations to determine the species present, and whether the ponds were balanced or unbalanced (Swingle, 19 5 6), were made with a minnow swine or 25-foot bag seine. Seining for population estimation was done with a 60-foot bag seine (one-fourth-inch bar mesh bag and one-half-inch bar mesh wings) . Fish larger than 2 .5 inches were marked by removing one or two pelvic fins. It was necessary to remove two fins from fish that had been marked the previous year. Additional seining was delayed for at least 48 hours to allow marked fish to become redistributed. In the initial seinings, bluegills were not randomly distributed throughout the ponds but were found in schools. Therefore, an attempt was made to sample as much of each pond as possible on each sampling day to capture bluegills from as many areas as possible. Populations were estimated by Schnabel ( 1938) and Schumacher-Eschmeyer ( 1943) formulas. Age and growth determinations were made for 638 bluegills from the five study ponds. Scales for study were removed from the right side of each fish at a point adjacent to the posterior margin of the depressed pectoral fin. Total lengths were measured to the nearest 0.1 inch, and weights obtained to the nearest gram. Bluegills were classified as immature if the gonads were poorly developed and the fish could not be sexed without microscopic examination. They were lJournal Paper No. J-5953 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project )lo. 1372, Iowa Cooperative Fishery Unit, sponsored by the Iowa State Conservation Commission, Iowa State University and Bureau of Sport Fisheries and Wildlife, U. S. Department of Interior" ' 2Assistant Professor, University of Southwestern Louisiana, Lafayette, Louisiana. 170 1 Shireman: Age and Growth of Bluegills, Lepomis macrochirus Ra:finesque, fro Published by UNI ScholarWorks, 1968 1968] AGE A?\iD GROWTH OF BLUEGILL 171 classified as mature if the gonads were large and the fish could be easily sexed. Impressions of uncleaned scales were made on dear plastic strips by using a roller press (Smith, 1954). Impressions were analyzed at a magnification of 44X with a scale projector similar to that described by Van Oosten, Deason, and Jobes (1934). The positions of the focus, annuli, and anterior margin of the scale were marked on paper tab strips. These tab strips were used on a nomograph as described by Carlander and Smith ( 1944). Each scale was read at least twice without knowing the length of the fish; some scales were read more than twice or until agreement as to the position of the annulus was reached. POPULATION ESTIMATES Using the same data, both the Schnabel and Schumacher-Eschmeyer formulas gave similar population estimates. Since there is a greater chance to underestimate fish populations (Buck and Thoits, 1965), the formula giving the greater estimate was used. Bluegill standing crops averaged 248 pounds per acre, ranging from 201 to 321 pounds per acre (Table 1). The mean standing crop is greater than that reported by Carlander and Moorman (1956), but within the range of standing crops reported by them. McLain Pond, which was overcrowded with bluegill and green sunfish (Lepomis cyanellus, Rafinesque), had standing crops of 2 54 pounds of bluegill and 129 pounds of green sunfish per acre. Carlander and Moorman ( 1956) found that ponds overcrowded with bluegill usually had small standing crops and attributed this to the fact that bluegill in these ponds were usually less than 3 inches in total length and thus not included in their estimates. Bluegill and green sunfish in McLain Pond were stunted; however, all were larger than 2.5 inches and were included in the estimates. BODY SCALE RELATIONSHIPS The linear distan.ces between the focus and anterior scale edge were plotted against bluegill total lengths for each pond. A linear regression line fitted by the least-square method was drawn through the plotted points. The general formula for the regression was: L =a+ bS where S = anterior scale radius in inches ( X 44) and L = total length in inches A straight line seemed to fit these data adequately, with the exception of Sparks Pond, where small numbers of bluegills of extreme lengths deviated from the regres.sion line slightly. Sprugel ( 1954) found that McFarland Pond bluegill growth data fit a curvilinear regression better than a linear regression. When growth calculations based on both 2 Proceedings of the Iowa Academy of Science, Vol. 75 [1968], No. 1, Art. 27 http://scholarworks.uni.edu/pias/vol75/iss1/27