Performance of Spring Chinook Salmon Reared in Acclimation Ponds for Two and Four Months before Release

Seasonal temperature cues may play an important role in the parr–smolt transformation of Pacific salmon Oncorhynchus spp., but salmon reared in groundwater-supplied hatcheries with seasonally stable water temperatures do not experience these cues. In this study, conducted over six release years, hatchery spring Chinook salmon O. tshawytscha that were initially reared in a groundwater-supplied hatchery were transferred to acclimation ponds supplied with surface water either in November (November transfer [NT]) or January (January transfer [JT]) for rearing at ambient water temperatures prior to release into the Umatilla River, Oregon, in early March. After stream release, we monitored out-migration travel times and survival to John Day Dam (JDD) by using passive integrated transponder tags in a subsample of each release group. Across all release groups, median travel time was slower for NT groups (51 d) than for JT groups (46 d); significant differences were observed in five of six release years. Average survival probabilities were 15% higher for NT groups than for JT groups, although this difference was not significant. We used recoveries of coded wire tags to estimate smolt-to-adult survival (SAS) and straying for NT and JT groups. Average SAS over the study was 27% higher for NT groups (0.76%) than for JT groups (0.60%), but straying was minimal for both types of release group. We found no significant linear relationship between SAS and travel time or survival to JDD. Adult returns of NT groups had a slightly older age distribution than those of JT groups. Our results suggest that for salmon juveniles reared in groundwater-supplied hatcheries, providing an overwinter period of exposure to ambient water temperatures should optimize postrelease survival. We also discuss some risks and benefits to winter rearing in acclimation ponds for managers that are considering this approach.