The environment during early life history strongly impacts phenotypic development in all organisms, which further influences developmental trajectory and ecological fitness later in life. Depending on the developmental stage and magnitude of change in the environment, phenotypes may become irreversible and thus have a long-lasting effect later in life. This thesis was designed to better understand how changes in the environment may influence plasticity and variation of metabolic phenotypes of Lake Sturgeon (Acipenser fulvescens) within the first year of life. Broadly speaking, the thesis tested two hypotheses that 1) all measured phenotypes would be plastic; and 2) durations of environmental effects on phenotypic development would be correlated with distinct developmental windows. Studies were developed to examine 1) short-term effects of temperature or diet on metabolic phenotypes such as metabolic rate, energy density, fatty acid profiles, and growth (Chapters 2 and 3) and 2) longer-term effects of temperature or diet during early life on these metabolic phenotypes (Chapters 4, 5 and 6). The first experimental chapter (Chapter 2) examined ontogenetic development of metabolic rate and demonstrated that dietary shifts between Artemia to bloodworm resulted in cessation of growth with elevated routine metabolic rate. Chapter 3 examined how fatty acid profiles and plasma cortisol concentration were influenced by environmental temperature and showed that decreasing temperature led to increases in mono- and polyunsaturated fatty acids in both phospholipids and triglycerides, and food deprivation resulted in lack of difference between baseline and peak cortisol concentrations. Chapter 4 examined how temperature during early life influenced plasticity of growth and showed that temperature post-dietary transition resulted in a transient effect on growth and energy metabolism without long-term effects post-winter. Chapter 5 examined how temperature during early life could influence growth and fatty acid metabolism when fish were exposed to colder temperatures later in life and demonstrated that elevated temperatures resulted in a longer-term effect on growth but lack of transcriptional responses of desaturating fatty acids when exposed to a cold temperature (3.5°C) later in life. The final experimental chapter, Chapter 6 examined longer-term effects of diet at the onset of exogenous feeding on metabolism and growth and demonstrated that an enriched diet resulted in prolonged effects on growth, digestive enzyme activity and survival prior to a simulated overwintering. This doctoral thesis research revealed that all measured metabolic phenotypes were plastic, but subtle changes in temperature and diet during early life history resulted in transient or prolonged effects on growth and metabolism in age-0 lake sturgeon. Results will aid our understanding of cohort and population dynamics as well as contribute to the development of conservation strategies for lake sturgeon, a species at risk or endangered across its natural range.