Effects of social interaction on growth of juvenile hybrid sunfish held attwo densities

The effects of competitive social interaction on cumulative food consumptio n (CC), absolute growth rate (AGR), gross growth efficiency (GGE), and deve lopment of interindividual weight variation were quantified for juvenile hy brid sunfish (F-1: female green sunfish Lepomis cyanellus x male bluegill L . macrochirus) held in low-density (5 fish in 25 L) and high-density (20 fi sh in 25 L) groups for 50 d. Individually held hybrid sunfish without socia l interaction were used as controls. All groups were fed a commercial diet to apparent satiation three times daily. Mean CC, AGR, and GGE declined 7% and 24%. 21% and 34%, and 14% and 15%, in the low- and high-density groups, respectively, relative to controls. Patterns of declining CC and GGE with increasing fish density indicated that elevated activity and stress from so cial interaction caused much of the growth decline at the low density, whil e reduced food consumption caused the additional growth loss at the high de nsity. Coefficients of weight variation (100 x SD/mean) increased 77% on av erage among fsh in the low- and high-density groups but remained constant a mong the controls. Results demonstrate that substantial growth loss and siz e variation development can occur in grouped hybrid sunfish from social int eraction. Amelioration of social interaction effects would probably be bene ficial to hybrid sunfish culture. Comparison to a similar study of bluegill s suggests that growth loss from social interaction may be less for hybrid sunfish than for the parent species.