Shoaling preference and evidence for maintenance of sibling groups by juvenile black perch Embiotoca jacksoni

Group sizes in free‐living juvenile black perch Embiotoca jacksoni were quantified and predictions of the hypothesis that such groups comprise sibling brood‐mates were tested. Group sizes in the field were within the range of female brood sizes and often occurred close to each other but did not merge. In captivity, juveniles formed groups immediately after birth. In laboratory experiments, they also associated significantly more with chambers containing familiar members of their own brood than empty chambers but did not associate more with chambers containing similar‐sized juveniles from a different brood. Juvenile E. jacksoni also associated significantly more with chambers containing familiar brood‐mates than with chambers containing unfamiliar members of a different brood. The strength of this preference increased with the number of days fish had been together since birth. When two broods were placed in a large outdoor tank, all individuals from both broods directed significantly more aggressive acts towards members of the other brood than towards members of their own brood. While the relative effects of familiarity and relatedness cannot be completely separated in this viviparous species, associating with familiar individuals would facilitate the maintenance of sibling groups in the field.