In-Stream Egg Incubators Produce Hatchery Chinook Salmon with Similarities to and Differences from Natural Juveniles

Abstract

Supplementing fish populations at the egg stage is a low-cost alternative to hatchery rearing that is presumed to improve adaptation to local natural conditions. The Shoshone-Bannock Tribes began supplementing Chinook Salmon Onchorynchus tschwytscha in Panther Creek, Idaho, at the eyed egg stage in 2014. The Chinook Salmon eggs were artificially fertilized and reared to eye-up in the hatchery and then planted in custom-made in-stream incubators (egg boxes) for volitional release and natural rearing. Using data from three brood years, we evaluated the efficacy of this supplementation program solely in terms of juvenile production. We related juvenile production to the placement and retrieval of the egg boxes, assessed the relative contributions of offspring (parr and emigrants) from the egg boxes (HOR) to overall juvenile abundance, and compared the performance (length, condition, dispersal distance, and survival) of HOR versus natural-origin (NOR) juveniles. Brood year and box placement within the stream (distance upstream) were the best predictors of whether or not an egg box was retrieved from its original location. Meanwhile, the condition of the box (i.e., intact, damaged, or missing) was the best single predictor of juvenile production. Supplemented eggs represented an estimated 42, 50, and 42% of the total egg deposition in Panther Creek in brood years 2014, 2015, and 2016, respectively. A parentage analysis revealed that the egg boxes accounted for 6, 22, and 35%, respectively, of the parr production for the respective brood years when the data were normalized to the estimated egg deposition—less than the egg-to-parr production that was estimated for natural redds. As fall parr, the HOR fish differed from the NOR fish in terms of their length and dispersal behavior, but they were of similar length and condition at their emigration from Panther Creek and exhibited no significant difference in downstream survival through the Federal Columbia River Power System (FCRPS). Collectively, our results provide useful insights to fisheries managers who are interested in initiating or refining egg supplementation programs.