Born to Spawn: Supplementation of Wild Steelhead in Scott Creek Via Partial Spawning of Hatchery Broodstock

Conservation hatcheries are challenged by minimizing collections from depleted populations while maintaining genetic diversity.  A conservation hatchery in Scott Creek (California, USA) annually harvested 5,000 eggs from 1-3 wild-reared threatened steelhead to enhance the local population. In 2011, the hatchery began spawning five females, collecting 1000 eggs from each.  This approach increased genetic diversity and gave individuals the opportunity to spawn naturally after release. To determine the potential for partially-spawned females to contribute to the naturally spawning population from 2011-2013, we radio-tagged and compared daily movement and spawning behavior of individuals partially-spawned in the hatchery to a control group that was not spawned. Partially-spawned females (2 of 9) dug one redd each, whereas control females (6 of 11) dug 1-4 redds each. There was a positive linear relationship between female size (pre-spawning mass) and total redd area (r² = 0.4426, p = 0.04), but not redd number. Visual observations of recaptured females indicated that at least seven control and three experimental females spawned naturally in Scott Creek. We conclude that female steelhead are capable of contributing to hatchery and natural production in a given year. Thus partial spawning of a greater number of females may contribute to preserving genetic diversity.