Gonad size (diameter and circumference) measured by ultrasound was assessed as a metric to assign stage of maturity in Burbot Lota lota from Lake Roosevelt, Washington.  Paired gonad tissue and ultrasound measurements were collected monthly from November 2017 to March 2018.  Gonad tissue was processed for histological analysis to confirm stage of maturity.  Gonad diameter and circumference were measured by ultrasound.  Excised gonad diameter (i.e., true gonad diameter) was measured by digital calipers, and excised gonad circumference (i.e., true gonad circumference) was measured by a measuring tape.  All late vitellogenic (stage 6) ovaries measured by ultrasound had a diameter greater than 3.90 cm, suggesting a value of 3.90 cm or greater may be used to characterize females capable of spawning in the current reproductive cycle.  One mid-spermatogenic (stage 3) and all ripe (stage 4) testes were too large to be measured and assigned diameter of 5.11 cm, the maximum value capable of being measured by our ultrasound transducer.  A value of 5.11 cm or greater may be used to characterize males capable of spawning in the current reproductive cycle.  Testis circumference measured by ultrasound is not reported because some testes were wider than the ultrasound transducer and could not be measured.  Measurements of testis diameter did not differ between measurement methods (ultrasound vs. true), but ultrasound measurements of ovary diameter and circumference were higher than true measurements.  We attributed the difference between measurement methods to flattening of ovary while applying the ultrasound transducer.  Gonad diameter and circumference measured by ultrasound were highly correlated with gonadosomatic index and ovarian follicle diameter indicating gonad size measured by ultrasound is an appropriate index of gonad development in Burbot.

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