Human activities that fragment fish habitat have isolated inland salmonid populations. Trout populations across the intermountain west are losing migratory life histories, with more fish staying in their natal streams for their entire lives. These “resident” fish are smaller and lay fewer eggs than their “migratory” counterparts. Isolated populations exhibiting resident life histories may be more likely to persist if individuals can increase lifetime reproductive success by maturing at smaller sizes or earlier ages. Therefore, understanding how early resident trout reach maturity would improve estimates of population viability.

Commonly used methods for assessing maturity such as dissection, endoscopy, and hormone analysis are invasive and may disturb vulnerable populations. Ultrasound imaging is a noninvasive method that has been used to measure reproductive status across fish taxa. However, little research has assessed the accuracy of ultrasound for determining maturation status of small-bodied fish, or reproductive potential early in a species’ reproductive cycle. To address these knowledge gaps, we tested whether ultrasound imaging could be used to identify maturing female Westslope Cutthroat Trout (Oncorhynchus clarkii lewisi).

We tested the accuracy of ultrasound methods to identify maturing cutthroat trout reared in a hatchery setting. We accurately identified maturing female fish up to eight months prior to spawning with error rates ≤4%. Hatchery reared fish tend to grow larger than wild fish. Therefore, we also tested imaged fish in a field setting to examine variation in the size of maturing females among six wild, resident populations of Westslope Cutthroat Trout in western Montana. The median size of maturing females varied significantly across populations. We observed egg development in females as small as 109 mm, which is smaller than previously documented for this species.

Key Findings

• In this study, we demonstrate that ultrasound imaging may be used to assess reproductive maturity in small-bodied salmonids up to eight months prior to spawning.
• The size of reproductive maturity varied significantly across wild populations of westslope cutthroat trout.
• We observed egg development in females as small as 109 mm, which is smaller than previously documented for this species.
• This information may be incorporated into viability models to better understand how this trait influences population growth rates and persistence.