Conservation hatchery programs designed to increase the abundance of Chinook salmon (Oncorhynchus tshawytscha) have reported high rates of precociously maturing males at age 2, referred to as minijacks. The negative consequences of releasing large numbers of minijacks into a population have driven the development of methods for reducing the frequency of this life history strategy. In this two-trial study, the effect of rearing juvenile Chinook salmon under varying continuous light regimes to reduce the number of minijacks was tested. At the end of the trials, the effect of these light regimes was measured by comparing the ratio of males that were maturing at 1+ age to non-maturing males (%MT) between treatments. Twenty-four-hour light treatment (LL) start/end dates were selected based on solstice/equinox dates which are hypothesized to queue physiological factors related to reproduction. Controls were maintained on a simulated natural photoperiod for both trials. For trial 1, the start date for one LL treatment began in June 2019 on the summer solstice and was maintained through late March 2020 (LL-Jun-Mar). The second LL group began on the fall equinox in September 2019 and ended in late March 2020 (LL-Sep-Mar). A significant difference in %MT was observed between the control (28.4%) and both LL groups (LL-Jun-Mar = 5.4% and LL-Sep-Apr = 9.4%) at the end of the trial. Trial 2 was undertaken to evaluate the length of LL treatment necessary to significantly reduce %MT relative to controls. It consisted of a control group, an LL group maintained from June 2020 on the summer solstice to April 2021 (LL-June-Apr) and an LL group beginning in June 2020 and ending in December 2020 (winter solstice), and then was returned to a simulated natural photoperiod until April 2021 (LL-Jun-Dec). Large differences in %MT was observed between control and LL groups. %MT in the control group (66%) was significantly higher than LL-Jun-Apr (10.3%) and LL-Jun-Dec (11.6%) by the end of the trial. For both trials, treatment and maturation category showed an effect on size parameters. Minijacks had higher body weights, were longer, and had increased condition factor when compared to female/immature fish (no treatment effect for female or immature fish was observed weight/length measurements). Weight and length amongst minijacks in the LL groups were larger than minijacks in the control groups and female/immature fish in LL groups had increased condition factor compared to the controls. The conclusion is that exposing male juvenile Chinook salmon to long term LL significantly reduces the number of fish precociously maturing at age 1+ relative to fish exposed to a natural photoperiod.