Application of ultrasound-based maturational status assessment in the management of female rainbow trout broodstock

Authors: Weber, Gregory; Iwanowicz, Luke; Hostuttler, Mark; Birkett, Jill; Leeds, Timothy

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2026
Publication Date: 2/26/2026
Citation: Weber, G.M., Iwanowicz, L.R., Hostuttler, M.A., Birkett, J.E., Leeds, T.D. 2026. Application of ultrasound-based maturational status assessment in the management of female rainbow trout broodstock. Aquaculture. 618: Article Number 743828. https://doi.org/10.1016/j.aquaculture.2026.743828.
DOI: https://doi.org/10.1016/j.aquaculture.2026.743828

Interpretive Summary: As female rainbow trout approach final sexual maturation their eggs undergo a rapid increase in hydration. This change in hydration can be detected via ultrasound, with eggs appearing gray (before hydration) or black (after hydration) in sonograms. In this study, scientists evaluated female rainbow trout via ultrasound on a weekly basis throughout the approximately 3-month spawning season (mid-December through mid-March) to determine when hydration occurred, and if the time between hydration and spawning affected viability of the fertilized eggs. The data suggest that females release their eggs from the ovary (ovulate) approximately three days after hydration and demonstrate that egg viability decreases as time from hydration to spawning increases. Ultrasound is a readily-available, easy-to-use, non-invasive technology that can be used by hatchery managers to predict when rainbow trout females will ovulate and inform the optimal time to spawn females to maximize egg viability.

Technical Abstract: As female rainbow trout (Oncorhynchus mykiss) approach spawning and initiate oocyte maturation the cytoplasm clears and echogenicity increases causing an unambiguous transition from gray to black in the sonograms as the oocytes mature. The cytoplasm begins clearing at the center of the oocytes. For ten years we used ultrasonography to identify late maturing fish at weekly broodstock spawn-checking events to characterize their reproductive performance. Over 95% of non-ovulated fish with maturing and black oocytes in the sonogram are ovulated when spawn-checked one week later. On average about 44% of the fish that are ovulated one week after spawn checking appeared maturing or black in the previous week’s sonogram, and 56% appeared gray. This ratio indicates our rainbow trout ovulate about three days after first exhibiting black oocytes. The proportion of females that ovulated a week after sonography was conducted, to those identified as clearing or black in the sonograms, was about 2.4:1. Females that were black in the sonograms for the first time the week before ovulation was detected, have lower eyeing rates compared with those that were gray, 39.3% (SE = 4.59%) compared with 52.5% (SE = 4.11%), respectively. Egg quality diminishes rapidly in the small proportion of females that fail to release eggs within a week of transitioning to black, with almost no eyeing achieved by three weeks. Ultrasonography is inexpensive and easily adapted to spawn-checking activities providing useful information to guide hatchery decisions including when to strip fish of eggs to maximize eyeing rates and short-term predictions of broodstock ovulation rates.