Is High Rearing Density Driving Domestication Selection on Steelhead in Hatchery Culture?

Pacific salmon and steelhead (Oncorhynchus spp.) that are reared in hatchery culture have lower lifetime fitness than wild born individuals.  Rapid domestication selection is the most likely cause of fitness loss, yet the mechanism driving domestication remains unknown.  In this study we tested if high rearing density could be exacerbating domestication selection.  First, we asked if high rearing density increases variance in family performance (body size) compared to low density (i.e. increased opportunity for selection).  Second, we asked if a tradeoff occurs in which the best performing families in high density do not perform as well in low density.  In addition, we determined if metabolic rate or behavior correlated with performance in either the high or low density treatments.  The experiment was repeated two times, first with a F1 generation broodstock under a short duration (4 months) and then with a wild broodstock for 12 months in the second year.  Preliminary results indicate high rearing density may be increasing the ability for domestication to act.  By determining what is driving domestication in captive culture we may be able to modify hatchery practices to reduce fitness loss and make conservation fish that are not a threat to wild fish populations.