To understand the demography of an evolutionarily naïve fish species that sometimes coexists with an invasive predator, we collected mark-recapture data and size-frequency data of two populations of southern leatherside chub (Lepidomeda aliciae), one of which coexists with nonnative brown trout (Salmo trutta). For each population, we estimated vital rates from mark-recapture data to inform a stage-structured matrix transition model. We also used size-frequency distributions from these populations in an integral projection model.

Study system

Southern leatherside chub are a rare fish species of the family Leucisidae found in small streams of the eastern Great Basin in central Utah in the western United States. They live up to 8 years and reach standard lengths (SL) of 140 mm. They have one breeding season annually, from mid to late summer.

Salina Creek (upstream from the town of Salina, Sevier County, Utah; 38.9 N 111.7 W, elev. 1807 m) and Lost Creek (just southwest of Salina, Sevier County, Utah; 38.8 N 111.9 W, elev. 1746 m) are two adjacent stream systems where populations of southern leatherside chub occur. Both river systems have similar habitat quality, such as range of depths (Salina Creek depth range was 0.25–1.5 m and Lost Creek depth range was 0.30–2.0 m during the study period) and the presence of backwaters and side channels. Habitats in these streams consist of heterogeneous instream structure, such as deep pools (> 1 m), riffles, including cover from riparian vegetation, and undercut banks. Brown trout —a nonnative, piscivorous salmonid— were introduced into Utah in the early 1900s and stocked in streams across Utah where native cutthroat trout (Oncorhynchus clarkii utah) were once present. Southern leatherside chub co-exist with nonnative brown trout in Lost Creek, which prey upon southern leatherside chub. Large aquatic predators capable of consuming adult southern leatherside chub are absent in Salina Creek. Other species present at both sites include speckled dace (Rhinichthys osculus), mottled sculpin (Cottus bairdii), and mountain sucker (Catostomus platyrhynchus), none of which prey upon adult southern leatherside chub. Here we refer to the population in Lost Creek with brown trout present as a predator environment and the population in Salina Creek, absent of brown trout as a predator-free environment, though it is likely that early life stages of our native species of interest experience predation from other native species, including adult southern leatherside chub.

Mark-recapture design

To obtain vital rates for southern leatherside chubs, we conducted a multi-year mark-recapture study in Lost Creek and Salina Creek from 2003 to 2006. In each stream, we established four contiguous 50-m segments and blocked each at the downstream end with nets. Fish were removed from each segment using three successive electrofishing passes with a Smith-Root LR-24 backpack electrofisher (Vancouver, Washington). Segments were sampled sequentially in an upstream direction. Captured fish were sorted into aerated tubs by species; non-target species were released immediately, while southern leatherside chub were retained for measurement and marking.

Southern leatherside chub were measured (standard length, SL) and assigned to one of three size classes based on size-at-maturity and known growth patterns. Individuals measuring 40–64 mm SL were marked with Visual Implant Elastomer (VIE; Northwest Marine Technology, Inc., Anacortes, Washington) at the dorsal insertion of the caudal fin. Fish between 65–84 mm SL were marked at the ventral insertion of the caudal fin, and those greater than 84 mm SL were marked at the base of the anal fin. Each year of capture was denoted by a unique elastomer color. Individuals less than 40 mm SL, presumed to be young-of-the-year, were excluded from the study due to the potential for increased marking-related mortality and limited detectability during electrofishing. Visual Implant Elastomer marks have been shown to remain visible in southern leatherside chub for multiple years. Because there is no evidence of sexual dimorphism in the species, individuals were not sexed.

In 2004, we expanded our sampling efforts beyond the original four 50-m segments. In this year, we sampled an additional 50-m segments downstream beginning at distances of 50, 100, 250, and 500 meters from the lower boundary of the original sampling area. Upstream, sampling included segments located at 0, 100, and 200 meters from the uppermost boundary of the original reach. We marked all southern leatherside chub recaptured from previous years captured in these extended segments using the same VIE protocol to indicate size class and year of capture. New captures in the extended segments were not marked.

In 2005, upstream and downstream sampling was standardized to 150 meters in each direction from the original four segments, sampled in 50-m increments. Unmarked individuals captured within the original segments were marked according to the same protocol used in previous years. In 2006, our sampling was restricted to Salina Creek, and no new marks were administered that year.