Hatchery‐reared endangered Colorado pikeminnow (Ptychocheilus lucius) undergo a gradual transition to piscivory after introduction to the wild

Abstract

Disruption of ecosystems by human activities has caused worldwide extinction threats, which has prompted conservationists to implement captive breeding programmes that aid the recovery of imperilled species. Understanding factors that limit the survival of hatchery‐spawned fishes after stocking is critical to future conservation efforts using captive populations. As the size at which juvenile piscivorous fishes shift to consuming other fish can influence their survival, the transition to piscivory by hatchery‐spawned Ptychocheilus lucius Girard, 1856 (Colorado pikeminnow) was investigated after stocking the San Juan River in New Mexico, Colorado, and Utah, USA. The stable isotope 15N was used to track ontogenetic changes in the trophic position of individuals stocked at age 0, with the expectation that they would become fully piscivorous by age 2. By sampling the isotopic signatures of Colorado pikeminnow across multiple years, the ability of river discharge and densities of fish prey to explain interannual variation in trophic position was also explored. Annual variation in river flow and the densities of fish prey had little predictive power in explaining variation in δ15N of age‐1 or age‐2 Colorado pikeminnow. After assessing the isotopic signatures of potential prey, a Bayesian isotopic mixing model suggested that invertebrates comprised nearly 25% of the diet of of both age‐1 and age‐2 individuals. The relationship between Colorado pikeminnow size and δ15N within stocked cohorts indicated that juveniles slowly transitioned to consuming fish prey as they grew, rather than abruptly switching, as indicated from a limited number of dietary studies using wild fish. Together, these results suggest that Colorado pikeminnow stocked into the San Juan River slowly transition to consuming fish prey. If this pattern leads to poor survival and recruitment, the effectiveness of this management action for conservation could be compromised.

Trophic niches of native and nonnative fishes along a river-reservoir continuum

“…Whereas trophic position of fish tends to increase with size 89 ; the random effect of fish length explained relatively little variation in our models. Nonetheless, our δ 15 N values for P. lucius (total length range: 222–526 mm), I. punctatus , and C. latipinnis are comparable to those reported previously from the San Juan River (range: 100–350 mm TL) 90 , although body sizes were not reported for the latter two species.…”

Changes to the trophic structure of a desert fish community following river regulation and species turnover: Implications for an endangered top predator

“…If invertebrate abundances are limited, competition for this prey could reduce overall fish abundances and may have caused the decline in the densities of invertivorous flannelmouth sucker, bluehead sucker, speckled dace, and red shiner observed in the San Juan River over the last two decades following ongoing deviations from the natural flow regime (Pennock et al, 2022; Van Poorten et al, 2018). The surprising and significant increase in niche overlap by Colorado pikeminnow into all other species may also be indicative of a reduction in fish prey given Colorado pikeminnow's presumed piscivorous nature (Franssen et al, 2007; Franssen et al, 2019; Vanicek & Kramer, 1969). Bioenergetics modeling assessing production and current fish prey abundance could be a useful path forward in quantifying the ability of the system to support Colorado pikeminnow, but would likely require substantial effort.…”

“…For these species, we sampled approximately 5 mm 2 of tissue from the caudal fins. In addition to contemporary fishes, we sampled invertebrates in riffles and backwaters (~2.5 mg dry mass collected per invertebrate order sample; see Franssen et al, 2019 for details). All contemporary tissue samples were preserved in table salt in the field and rinsed before processing (Arrington & Winemiller, 2002).…”

“…From these specimens, we obtained $0.5 g of white muscle. To obtain contemporary tissue samples, we leveraged riverwide late summer-fall fish community monitoring efforts conducted by San Juan River Basin Recovery Implementation Program researchers who used raft-mounted electrofishing and seines to capture fish (see Franssen et al, 2015Franssen et al, , 2016Franssen et al, , and 2019. Thus, the spatial extent of the historical collections was smaller than that of the recent samples.…”

Efficacy of fluorescent calcein mark retention and identification in endangered Colorado pikeminnow Ptychocheilus lucius

“…Calcein was assessed as a batch‐marking tool in the endangered Colorado pikeminnow Ptychocheilus lucius Girard, a species endemic to the Colorado River Basin and the largest cyprinid native to North America (Tyus, ). To aid recovery, over 400,000 age‐0 Colorado pikeminnow are stocked into the San Juan River, NM and UT, USA each year; nearly five million individuals have been stocked since 1998 (Franssen, Gilbert, Gido, & Propst, ). These stockings have increased the adult population size—maturity occurs at 8–10 years of age and >450 mm total length (TL)—resulting in the capture of wild spawned, age‐0 individuals in recent years (Schleicher, ; Zeigler & Ruhl, ).…”