Book excerpt: Species with complex life histories - where individuals undergo a rapid ontogenetic shift followed by a change in habitat - may respond to environmental and ecological drivers in unanticipated ways. For example, traits shaped by conditions in one habitat could carry over to the next habitat leading to unpredicted consequences. Steelhead trout are anadromous with plastic, complex life-histories that display broad variation in both freshwater and marine phases of their life cycle. Here, I linked environmental variation (e.g., nutrient subsidies, marine climate) to life-history traits (e.g., smolt size, size at maturity) and carry-over effects to understand how these factors may affect population production in the depressed Keogh River, BC population. Beginning with a 40-year timeseries of steelhead abundance and juvenile life-history data, I examined how smolt production and traits (length and age) vary with freshwater rearing conditions. Steelhead smolt production, length-at-age, and age covary with temperature, artificial nutrient addition, and pink salmon spawning abundance (spawners generate a nutrient subsidy via eggs and carcass tissues that young steelhead eat). Next, I investigated how pink salmon egg abundance (and thus pink salmon spawning abundance) translated to egg consumption by young steelhead across varying fish communities. Using experimental egg additions, I discovered that increasing egg availability disrupted size- and species-based dominance hierarchies, allowing small juvenile steelhead and less competitive fishes to access eggs. In Chapter 4, I developed and validated improvements to scale-based fish length back-calculation to estimate smolt length from adult scales for use in Chapter 5. Finally, using three decades of archived adult steelhead scales, I examined how smolt traits (i.e., sex and back-calculated length) combine with marine environmental conditions to affect adult life-history traits and female steelhead egg production across three decades. I discovered carry-over effects of the freshwater environment on adult traits via relationships between smolt length and age- and length-at-maturity. However, the opposing effects of smolt length on age-at-maturity (which affects length-at-maturity) and length-at-maturity dampened the positive effect of smolt length on egg production. Collectively, this thesis informs steelhead management and demonstrates the complicated inter-relationships between environment and traits across complex and plastic life-histories.