Effects of Social Interaction on Growth of Juvenile Hybrid Sunfish Held at Two Densities

The effects of competitive social interaction on cumulative food consumption (CC), absolute growth rate (AGR), gross growth efficiency (GGE), and development of interindividual weight variation were quantified for juvenile hybrid sunfish (F1: Female green sunfish Lepomis cyanellus × male bluegill L. macrochirus) held in low-density (5 fish in 25 L) and high-density (20 fish in 25 L) groups for 50 d. Individually held hybrid sunfish without social 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 social interaction caused much of the growth decline at the low density, while reduced food consumption caused the additional growth loss at the high density. Coefficients of weight variation (100 × SD/mean) increased 77% on average among fish in the low- and high-density groups but remained constant among the controls. Results demonstrate that substantial growth loss and size variation development can occur in grouped hybrid sunfish from social interaction. Amelioration of social interaction effects would probably be beneficial to hybrid sunfish culture. Comparison to a similar study of bluegills suggests that growth loss from social interaction may be less for hybrid sunfish than for the parent species.