Non-lethal Tools to Identify Mass Ovarian Follicular Atresia in Burbot Lota lota

Skipped spawning occurs in many annual spawning fish species and should be evaluated to effectively manage their populations.  We assessed plasma sex steroid concentrations, gonad size measured by ultrasonography, and ovarian follicle diameter as metrics to non-lethally identify mass ovarian follicular atresia (i.e., skipped spawning) in Burbot Lota lota.  We maintained wild fish in captivity and exposed them to increasing water temperatures during a three-week period prior to the spawning season to induce mass ovarian follicular atresia.  We collected ovarian follicles, blood plasma, and gonadal sonograms from fish weekly between January 28, 2018 to March 25, 2018.  We histologically analyzed ovarian follicles to confirm stage of maturity.  We measured plasma sex steroid concentrations, testosterone (T) and estradiol-17β (E2), by radioimmunoassay.  We measured gonad diameter and circumference by ultrasonography and ovarian follicle diameter by image analysis.  Mean plasma T concentration decreased from 8.94 ng/ml during late vitellogenesis to 1.83 ng/ml during atresia, suggesting that plasma T concentrations may be used to identify mass ovarian follicular atresia.  We do not recommend using plasma E2 concentrations to identify mass ovarian follicular atresia because concentrations rapidly decreased during the completion of vitellogenesis and the initiation of atresia in Burbot; therefore, plasma E2 may not accurately identify mass ovarian follicular atresia.  Mean gonad diameter measured by ultrasonography decreased from 4.05 cm during late vitellogenesis to 3.65 cm during atresia.  Mean diameter of ovarian follicles decreased during the last week of the study suggesting that ovarian follicle diameter may be used to identify advanced mass ovarian follicular atresia.  The non-lethal tools assessed in this paper (plasma sex steroid concentrations, ultrasonography, and ovarian follicle diameter) will enable fisheries biologists to determine the occurrence and frequency of mass ovarian follicular atresia among Burbot in Lake Roosevelt and may be applied to other Burbot populations.