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Inherent Growth Capacity and Social Costs of Bluegill and Hybrids of Bluegill and Green Sunfish: Which Fish Really Grows Faster?
R. S. Hayward

Department of Fisheries and Wildlife Sciences, School of Natural Resources, 302 Anheuser-Busch Natural Resources Building, University of Missouri, Columbia, Missouri 65211, USA

H. P. Wang

Fisheries College, Huazhong Agricultural University, Wuchang, Wuhan 430070, China

Abstract.—There is interest in knowing whether the bluegill Lepomis macrochirus or a hybrid of bluegill and green sunfish L. cyanellus (F1: male bluegill × female green sunfish; hereafter called B × G hybrids) can be grown faster to food market size (225–340 g). The predominant view is that the hybrid grows faster. In this study, the inherent growth capacities (IGCs) of age-1 bluegills and B × G hybrids were determined over four successive 25-d periods (May–August 2000) by holding them individually at 22°C and feeding them to apparent satiation three times daily. The hybrid's IGC was greater during period 1 but fell to only 33% of the bluegill's over the three subsequent periods; the consumption and gross growth efficiency (GGE) of the hybrid showed similar declining patterns. The growth-in-weight trajectory of bluegills crossed above that of the hybrids at about 30 g. Gonadosomatic index values suggested that hybrid growth rates declined below those of the bluegill because the former invested more energy in gonads as age-1 fish. A follow-up study, conducted under similar conditions from August 2000 to March 2001, found that individually held age-1 bluegills starting at 30 g reached more than 100 g in 200 d, gaining nearly twice the weight achieved by hybrids of similar starting weight that were reared in parallel. The costs of social interaction in terms of reduced consumption, growth, and GGE were also quantified for bluegills and hybrids by comparing individually held fish with group-held fish over periods 1–3. Social costs reduced the growth rates of grouped bluegills more than those of grouped hybrids over the 75-d period. Our findings indicate that while the IGC was higher for the hybrids as early age-1 fish, the long-term IGC (e.g., to food market weights) is higher for bluegills; however, this result may be obscured, in part, by the bluegill's higher social costs. Reduction of bluegill social costs in certain culture settings should result in growth rates that approach their higher IGC. Also, social costs varied in response to short-term shifts in the IGCs of both fishes, indicating a previously unknown influence on social interactions in fishes.

Received: December 22, 2000; Accepted: August 15, 2001