Use of Ultrasound Imaging and Steroid Concentrations to Identify Maturational Status in Adult Steelhead

Abstract

We sought to develop a rapid, noninvasive, and accurate method for distinguishing prespawn (mature or maturing) adult steelhead (anadromous rainbow trout Oncorhynchus mykiss) from postspawn adults (kelts). Ultrasound images of gonads were collected, and levels of plasma testosterone (T), 11-ketotestosterone (11-KT), and 17α-hydroxy-20β-dihydroxyprogesterone (DHP) were determined for adult steelhead before and after spawning. Results demonstrated that ultrasound images provided quantifiable selection criteria (number of eggs or gonad size) for the identification of prespawn versus postspawn adults. Mature females were easily identifiable by the presence of numerous, well-developed eggs. Conversely, only a few mature eggs remained in the body cavities of female kelts. Ultrasound investigation of the maximum testis area demonstrated that testes undergo substantial size changes following spawning. Mean maximum testis area was 2.86 cm² in prespawn males and 0.62 cm² male kelts. Distributions of testis measurements between the two maturational types did not overlap, and results of a discriminant function analysis suggested that a classification criterion (threshold) of 1.25 cm² could accurately distinguish prespawn males from kelts. Median concentrations of T, 11-KT, and DHP were significantly higher in prespawn males (49.6, 78.4, and 13.0 ng/mL, respectively) than in kelts (<1.2, 6.6, and 1.6 ng/mL), providing an independent verification of the ultrasound technique. Despite the high degree of corroboration between ultrasound measurements of gonad size and plasma steroid levels, application of ultrasound imaging may result in some classification error (a few percentage points) if substantial gonad mass is retained in the body cavity after spawning. Use of ultrasound to accurately identify postspawn steelhead is an important first step towards development of effective kelt management practices in the Columbia River basin.