Genetic Data Inform Reintroduction and Experimental Hybridization of Coho Salmon In Central California

Coho salmon (Oncorhynchus kisutch) populations at the southern extent of their range, in the Central California Coast (CCC) ESU, are in steep decline and are ESA-listed as Endangered. In recent years, the species has disappeared from numerous basins and streams, including most tributaries of the Russian River, and fish that remain show a high level of inbreeding.  In spite of this overall decline, some CCC streams, such as Lagunitas Creek, continue to support a population with relatively high genetic diversity.  A coho salmon captive breeding program was established in 2002 at Warm Springs Hatchery, a mitigation facility on the Russian River, with the goal of repopulating the Russian River and other area streams with self-sustaining coho salmon populations.  Toward that goal, hatchery program fish are spawned using a breeding matrix designed to avoid inbreeding and maximize genetic variation in the resulting offspring, which are then released into streams where coho salmon have either vanished or nearly so. Most of these fish are released as juveniles, but a novel strategy of releasing captive-raised reproductively maturing adults directly into spawning habitat has also been employed. Here we report on the outcomes of the breeding and adult release strategies for coho salmon populations in the Russian River and vicinity. These results include relative in-stream survival of released juveniles with different levels of inbreeding and the outcome of releasing adults from different streams together in a stream where they had been extirpated. We analyzed the genetic composition of wild-born juveniles in Salmon and Walker Creeks, two area streams from which coho salmon had previously vanished, and found evidence of successful natural spawning by the adults released there. We also found larger numbers of hybrid crosses between fish from Lagunitas Creek and the Russian River than non-hybrid crosses, as well as larger numbers of juveniles in the hybrid families than the pure families when adult fish from both populations were released together in Salmon Creek. Concordantly, hybrid juveniles from experimental crosses in the hatchery had significantly higher post-release, in-stream survival than non-hybrid juveniles. The combination of genetically-managed hatchery spawning, including experimental outbreeding, and the successful adult release strategy are likely to play a key role in the survival and recovery of these critically endangered fishes.