Defining Life Histories of Precocious Male Parr, Minijack, and Jack Chinook Salmon Using Scale Patterns

Abstract

AbstractMale Chinook salmon Oncorhynchus tshawytscha have multiple life history strategies tied to smolting, age, and size at maturation. Precocious strategies include mature parr, minjacks, and jacks; however, there are conflicting definitions of what constitutes a minijack. We describe the variety of growth and life history patterns in precocious spring–summer Chinook salmon in the Snake River basin as inferred from scale patterns. We found six repeatable patterns on the scales collected that differed in average number of freshwater and saltwater circuli and average spacing of circuli during the period of fastest growth. This represented four life histories with two variations. These life histories were differentiated by length distribution: mature headwater parr that never left their natal stream (8–15‐cm fork length), mature river parr (17–24 cm), minijacks that spent 3–4 months in the estuary or ocean (24–37 cm), and jacks that spent one winter in the ocean (≥36 cm). Thus, male Chinook salmon have a continuum of life history strategies. Mature parr spend their entire life in freshwater, mature headwater parr have parr marks, and the mature river parr are more silver in appearance. Neither of these stages have scale resorption. Some parr stay in headwater streams while others make short‐distance migrations to larger rivers. Minijacks make long‐distance migrations to the estuary or ocean but only stay a few months before returning to the spawning grounds. Jacks spend 1 year in the ocean and have many saltwater circuli. Minijacks and jacks are spotted similar to full‐size spawning adults and have scale resorption consistent with a long spawning migration. Finally, our data demonstrate that minijacks are capable of successfully traveling from Idaho to the Columbia River estuary and back (a minimum of 2,600 river kilometers from the Pahsimeroi River) within approximately 4 months.