Annual Growth Cycle, Growth Potential, and Growth Compensation in the Bluegill Sunfish in Northern Indiana Lakes

Abstract

Eight lake populations of the bluegill sunfish (Lepomis macrochirus) with widely varying growth rates exhibited a surge of growth in the spring followed by a progressively declining growth rate in late summer and autumn. The growth rate declined as age increased; in no case did the growth of an older age group exceed that of a younger age group in the same year. Increase in length was estimated from marginal growth of scales collected at various times throughout 1962 and 1963.The beginning of the growing season, assumed to correspond in time with the formation of the annulus, was estimated to average April 8 in 1962 and April 26 in 1963. The time of annulus formation was observed in one lake by collecting scales when the mark was appearing, and in the others it was estimated by extrapolation of a regression between marginal scale growth and time. The end of the growing season, or the time when 90% of annual growth had been achieved, came on September 10 in 1962 and on September 27 in 1963. The length of the growing season varied among the lakes and between years in the same lake. The range was from 98 to 189 days with an average of 152 days. In one lake the season was a month longer in 1963 than in 1962, but this difference was not evident in the others.The annual rate of growth and the length of the growing season were related. Populations with a rapid growth rate had a longer growing season than those with a slow growth rate. A long season was typified by a somewhat earlier time of annulus formation than a short one and by a considerably longer period of growth in the late summer and autumn.All populations had the same growth potential as judged by weight increment of group III fish during the first month of the season. The first month's increments were 12.6, 12.9, and 13.9 g for populations with rapid, intermediate, and slow annual growth rates respectively. The population with the shortest growing season and the lowest annual growth had the greatest increment during the first month.The strong relationship between size and the time of annulus formation suggested a possible mechanism of growth compensation. Since the smaller fish of a year class begin to grow earlier than do their larger companions, growth compensation may result from the advantage in time gained by the smaller group, assuming that the growing seasons of it and the faster growing members of the cohort end at the same time. Most of the mechanisms offered heretofore have depended upon a difference in the time of hatching between two groups of fish.Consideration is given to an explanation of the annual growth cycle in terms of temperature, photoperiod, and the possible influence of growth hormone at different times of the year.