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.  In this study, conducted over six release years, hatchery spring Chinook salmon Oncorhynchus tshawytscha that were initially reared in a groundwater-supplied hatchery with seasonally stable water temperatures were transferred to acclimation ponds supplied with surface water either in November (November transfer; NT) or January (JT) for rearing at ambient water temperatures prior to release into the Umatilla River in early March.  After stream release, we monitored outmigration travel times and survival to John Day Dam using passive integrated transponder tags in a subsample of each release group.  Across all release groups, median travel time was significantly slower for NT groups (51 d) than for JT groups (46 d); however, average survival probabilities were 15% higher for NT groups.  We used recoveries of coded-wire tags to estimate smolt-to-adult survival (SAS) and straying between NT and JT strategies.  Average SAS over the study was 27% higher for NT groups (0.76%) than JT groups (0.60%) but little or no straying occurred for both strategies.  November transfer groups returned at a significantly older age compared to JT groups.  Our results suggest that providing salmon juveniles reared in groundwater supplied hatcheries an overwinter period of exposure to ambient water temperatures may optimize the development and timing of smolting, thereby increasing post-release survival.  We also discuss some risks and benefits to winter rearing in acclimation ponds for managers that are considering this approach.