SPATIAL-DISTRIBUTION, SURVIVAL, AND GROWTH OF SIBLING GROUPS OF JUVENILE COHO SALMON (ONCORHYNCHUS-KISUTCH) IN AN EXPERIMENTAL STREAM CHANNEL

The spatial distribution, survival, and growth of two full-sibling fam ilies of coho salmon (Oncorhynchus kisutch) in an experimental outdoor stream channel were compared between the families and with the perfor mance of one of the families in an allopatric control channel after 75 d of rearing. No difference in survival was observed between families (81.2% over all), but their spatial distributions in the channel diff ered markedly. One family or the other numerically dominated 5 of 7 ri ffle-pool habitat units within the sympatric treatment channel. The fa mily that was larger at the beginning of the experiment (0.52 vs. 0.37 g mean mass) was also larger at the end (2.03 vs. 1.58 g). While ther e was little evidence of density-dependent growth overall, the growth of each family was correlated with the density of siblings in the unit . However, the larger bodied family showed negative density-dependent growth, whereas the smaller bodied family showed positive density-depe ndent growth. These patterns of distribution and growth may have arise n from sibling recognition or some other proximate mechanism such as a ssortative distribution based on size or interfamily variation in aggr ession. Regardless of the mechanism, intrapopulation variation in dist ribution and growth may lead to considerable variation in fitness amon g families because these factors affect success in subsequent freshwat er and marine life history stages.