Effects of kinship on growth of a live-bearing reef fish

The influence of genetic relatedness on the individual performance (e.g. growth, development) of animals is often tied to agonistic or cooperative behaviors among conspecifics, and studies of the effects of kinship have produced mixed results. To explore genetic relatedness independent of these behaviors, we investigated the effects of kinship on the growth of the kelp perch Brachyistius frenatus, a live-bearing, planktivorous marine reef fish that is capable of only limited dispersal. Although juveniles occur in aggregations and compete for food resources, they do not exhibit overt aggressive or cooperative behavioral interactions. We hypothesized that under competition and in the absence of these behaviors, sibling and non-sibling groups of juvenile B. frenatus raised at the same densities in the field would not differ in average growth, but that siblings would exhibit lower variation in growth, simply due to genetic similarities in inherent growth rates. Pregnant, female kelp perch were collected and placed in cages until parturition was complete. Groups of young, recently born from the same mother or from different mothers, were then raised in the field for 9 wk. Our results revealed that average growth rates were similar between sibling and non-sibling treatments. While variation in growth increased initially in non-siblings, siblings showed little such variation. This divergence, however, was not consistent over the duration of the experiment, and variation in the growth of siblings ultimately converged with that of non-siblings. Effects of genetic relatedness would be most likely to manifest themselves early after birth, before environmental factors exert their influence, and this may explain the initial separation but eventual convergence in variation in growth between sibling and non-sibling treatments. For B. frenatus and other organisms that will encounter relatives and compete for resources without overt behavioral interactions, the degree of kinship may play a minor role in the demography of local populations. Received: 26 January 1998 / Accepted: 30 September 1998