Abstract
Steelhead trout (Oncorhynchus mykiss, hereafter referred to as steelhead) display diverse life histories after initial or maiden spawning, with some individuals spawning again after 1 year (consecutive spawning) and others after two or more years (skip spawning). This study seeks to understand how reproductive performance compares between maiden and repeat spawners, how physiological condition is involved in the development of consecutive and skip spawning life histories, and how and when resources are allocated within and between reproductive events. An experimental model employing female, hatchery-origin steelhead was used to test for: (1) physiological trade-offs between maiden reproductive effort and future reproduction, (2) a conditional vs. fixed maturation strategy, (3) a critical period after maiden spawning for rematuration initiation, and (4) the sensitivity of ovarian development in relation to physiological condition. Female steelhead were captured following their upstream spawning migration, manually spawned, fed, sampled, and repeat spawned in captivity. This experimental model represents one of extreme energy depletion associated with an approximately 6 month-long, 800 km freshwater fasting spawning migration. Repeat spawners were larger and had greater absolute fecundity than maiden spawners. Skip spawners had larger eggs and greater total reproductive effort than maiden and consecutive spawners. When standardized for mass, consecutive spawners had more but smaller eggs than maiden spawners, indicating that egg size can vary and is determined early in oogenesis based on energy reserves. These observations imply that excess energy gained during the vitellogenic period is allocated to somatic growth. Mass growth and plasma triglyceride (TG) levels were greater in consecutive than skip spawners 10 weeks after maiden spawning, and muscle lipids and Fulton’s condition factor were greater in consecutive spawners 20 weeks after maiden spawning. These findings suggest that consecutive spawners consumed more food than skip spawners during the period after spawning. Mass growth and TG were positively related to reproductive performance at repeat spawning in consecutive spawners, with the maximal correlation with egg size at 10 weeks, and the maximal correlation with total reproductive investment at 20 weeks. Consecutive maturation was not significantly associated with any measure of physiological condition at maiden spawning. However, post-spawning survival was positively associated with maiden spawning TG, representing the ability to access energy, and plasma osmolality, indicating the ability to maintain homeostasis. Surprisingly, consecutive maturation was positively related to maiden reproductive effort, which is not consistent with a theorized tradeoff between current and future reproduction in animals. Instead, this result suggests that both maiden reproductive effort and consecutive maturation may be condition dependent at time points prior to maiden spawning. This dissertation provides insight into how and when consecutive and skip spawning life history tactics develop physiologically in steelhead. Additionally, this dissertation provides evidence that female steelhead can modulate reproductive investment based on physiological condition early during oogenesis.