Development and application of steelhead (Oncorhynchus mykiss) kelt identification techniques Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/v692t9748

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  • The migrations upstream of pre-spawned adult steelhead (Oncorhynchus mykiss) and downstream of post-spawned steelhead (referred to as "kelts") can geographically and temporally overlap in the Snake River, leading to management challenges for populations listed under the Endangered Species Act (ESA). This thesis describes the development and application of a method to accurately identify kelts using ultrasound, which is viewed as the first step towards development of effective management practices. In order to develop classification criteria for pre- and post-spawn steelhead, ultrasound images of gonads were taken and plasma levels of testosterone (T), I 1-ketotestosterone (1I-KT), and 17a,20β- dihydroxyprogesterone (DHP) were determined in adult steelhead before and after spawning. Results indicated that ultrasound images provide quantifiable selection criteria - based on the size, number, location, and/or echogeneity of gonads - for the identification of pre-spawned versus post-spawned adults. Pre-spawn females were easily identifiable by the presence of numerous, well-developed eggs. Conversely, only a few mature eggs remained in the body cavity of female kelts. Males were readily identified by the presence of testicular tissue, but determining their maturation status (i.e., pre- or post-spawned) was more difficult compared to females because some males retained gonad mass after spawning. The average cross-sectional testis area in mature males was 2.86 cm² (SD = 0.74) relative to 0.62 cm² (SD = 0.24) in postspawned males, demonstrating a statistically significant difference in testis size before and after spawning. Distributions of testis measurements between the two maturational types did not overlap and results of a discriminant function analysis suggest a classification criteria of 1.25 cm² could accurately (Ca. 99%, based on percentiles) distinguish males as pre- or post-spawned. Concentrations of T, 1 1-KT, and DHP were significantly higher in pre-spawn males relative to post-spawned males, confirming an association between gonad size and reproductive hormones. Using classification criteria developed from steelhead of known sex and maturation type, ultrasound imaging of adult steelhead at Lower Granite Dam (LGR) juvenile bypass facility revealed that 94.6% of 3,968 adult steelhead sampled from 1 April to 10 June 2000 were kelts. This included approximately 2,050 wild kelts, or about 17% of the entire protected steelhead run above the dam. The majority of kelts examined were female (77.0%) and most were in good overall morphological condition (69.5%), as indicated by the lack of scars, infections, or other morphological damage. Based on the abundance of kelts at LGR, management initiatives aimed at improving the repeat spawning rate of steelhead may be considered an important part of a comprehensive plan to rebuild ESA-listed steelhead populations throughout the Snake River basin.
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