Size heterogeneity prevails over kinship in shaping cannibalism among larvae of sharptooth catfish Clarias gariepinus

Because preying on close relatives may cause a loss of inclusive fitness, c annibalistic animals are generally deemed to cannibalise non-related conspe cifics preferentially to kins. Whether this basic principle also applies to fish exerting intracohort cannibalism at an early developmental stage is u ncertain, and more intense cannibalism among mixed progenies might just be a consequence of greater size heterogeneity. To investigate this, progenies of sharptooth catfish of equivalent initial body weights but with contrast ing size heterogeneity were reared separately or in mixed groups containing equal proportions of each progeny (27 +/- 1 degreesC, 12 h light: 12 h nig ht, 2.5 L aquaria, 100 or 200 fish(.)L(-1)). Cannibalism was monitored on a daily basis until the end of the larval stage (3-15 days after hatching). Cannibalism in mixed groups was intermediate between those in pure progenie s, and was positively correlated (R-2 =0.803, P <0.01) with initial size he terogeneity, irrespectively of fish origin. This finding does not exclude t hat catfish larvae were able to discriminate between siblings and non-relat ed fish, but this obviously had very little influence on cannibalism. The i mplications of this finding are discussed, as they apply to prey selection in fast growing larvae, and aquaculture of catfish. (C) 2001 Ifremer/CNRS/I nra/IRD/Cemagref/Editions scientifiques et medicales Elsevier SAS.