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Title: Use of ultrasonography to identify late-stage maturity in rainbow trout Oncorhynchus mykiss

item Weber, Gregory - Greg
item Leeds, Timothy - Tim

Submitted to: Aquaculture America Conference
Publication Type: Abstract Only
Publication Acceptance Date: 9/13/2014
Publication Date: 2/19/2015
Citation: Weber, G.M., Leeds, T.D. 2015. Use of ultrasonography to identify late-stage maturity in rainbow trout Oncorhynchus mykiss. Aquaculture America Conference. A568.

Interpretive Summary:

Technical Abstract: Morphometric measurements by ultrasonography has been used to determine gonad and follicle size in many species of fish for purposes of identifying sex and estimating stage of maturation. We have been using a portable ultrasound system (SonoSite MicroMaxx, L25e/13-6 MHz transducer) to identify female trout close to ovulation and to assess testis size in neomales treated with salmon pituitary extract to induce maturation at or shortly after 1-year of age. In each situation, required information to make an evaluation of the maturation status can be attained by a quick look at the sonogram screen image, allowing sorting in real-time. Ultrasonography can be used to identify female trout typically within about 3 days from ovulation based on a dramatic decrease in echogenicity in which the eggs turn from light gray to black in the sonogram. Estimation of testis size is hampered by the irregular shapes of testes in neomales, but fish sufficiently mature to yield viable milt can usually be identified. As part of an effort to mature rainbow trout at 1 year of age, 230 neomales began receiving thrice weekly injections of salmon pituitary extract (SPE, 1.5 mg/kg) at ~7 months post-hatching. Approximately 95% survived until testes removal or by the end of the project at month 11. Ultrasound was used at month 10 to identify potential breeders. Over 60% were identified as having developed testes, and >85% of those selected had motile sperm. Previously untreated 14-month-old neomales were also injected thrice weekly with SPE (1.5 mg/kg) to evaluate the effect of treatment on testis size. Control fish remained below 6 g testes weight and 0.6% gonadosomatic index (GSI). Treated fish reached 23 g testes weight and 2.4% GSI within 4 weeks of treatment but did not exceed 31g testes weight and 3.1% GSI after 8 weeks of treatment.