Domestication Selection and Loss of Fitness in Hatchery Salmon and Steelhead: Possible Mechanisms for Fitness Loss

It is well established that hatchery-origin Atlantic and Pacific salmon often have lower fitness (reproductive success) in the wild than natural-origin fish.  Data from six studies on four species of salmon and steelhead show that this phenomenon is general and occurs when hatchery fish are produced from local and predominantly wild broodstock.  Furthermore, there is evidence in steelhead that the fitness decline is a heritable effect resulting from rapid genetic adaptation to captivity (domestication).  What traits are under selection in hatcheries, and what aspects of hatchery culture exacerbate the rate of domestication?  Answers to those questions might identify ways in which hatchery practice could be modified to slow domestication.  Circumstantial evidence from Hood River steelhead suggests that high rearing densities (crowding) increase the rate of domestication.  If true, possible mechanisms include (1) that increased crowding increases the among-family variance in performance, and (2) that strong family-by-environment effects cause large changes in family rank-order performance across rearing densities.  However, in 2 years of experiments on varying rearing density in hatchery culture of steelhead, we found no evidence to support either hypothesis.  We therefore propose a new model of how high rearing density might exacerbate domestication in hatchery reared steelhead.