Assessment of age-0 Colorado Pikeminnow survival in backwater nursery habitats of the middle Green River, Utah

Authors: Matthew Breen, Tildon Jones

Year: 2019

Abstract

Despite consistent Colorado Pikeminnow, Ptychocheilus lucius, reproductive success, first year survival of larval fish remains poor in the middle Green River, Utah. In 2012, we examined potential factors impeding age-0 Colorado Pikeminnow survival in nursery backwater habitats. Presence and arrival of larval Colorado Pikeminnow was verified by drift net sampling conducted near the Split Mountain boat ramp (river mile [RM] 310) in Dinosaur National Monument and larval seining that took place in downstream backwaters. Twelve randomly selected backwaters (RM 305–273.5) were blocked off and depleted of nonnative fishes with a beach seine (≥ 90% depletion), then assigned one of three possible treatments: (1) four backwaters that were not blocked after initial depletions, (2) four backwaters blocked by 1/4“ mesh block nets, and (3) four backwaters blocked by 1/2“ mesh block nets. Experimental backwaters were revisited every other week on six separate occasions (10 July–25 September 2012) to monitor fish community response over time; changes in fish community composition were analyzed using a Repeated Measures ANOVA. Given that environmental conditions (i.e., dry hydrology) and deteriorating habitat quality were not conducive to Colorado Pikeminnow larval drift, arrival in nursery areas, or age-0 recruitment in 2012, other age-0 native fishes (mainly Flannelmouth Sucker, Catostomus latipinnis) provided helpful insights on the effects of our experimental design. Abundance of both native and nonnative fish (mainly small-bodied cyprinids) was greatest in 1/2“ mesh blocking treatments. Moreover, native fish were completely absent from unblocked backwaters before the fourth sample period (20–22 August 2012) and nearly eliminated from 1/4“ mesh blocking treatments before the fifth sample period (4–5 September 2012). Additionally, unblocked backwaters contained the lowest abundance of all species, suggesting that predation by nonnative piscivores (e.g., Smallmouth Bass, Micropterus dolomieu) coming from riverine habitats may be a significant threat to small-bodied fishes and blocking treatments can positively influence survival in nursery areas. By blocking backwater nursery areas, we created predator free habitats that benefited most species of small-bodied fishes. Despite higher abundance of small-bodied nonnatives in 1/4” and 1/2” mesh blocking treatments throughout our experiment, age-0 native fishes were also more abundant (1/2” mesh treatments containing the highest abundance). We do not discount that competition between age-0 native fishes and nonnative cyprinids can be intense in backwaters, but if we can control for predation by blocking backwaters, survival can be positively influenced despite ongoing competition for resources in nursery habitats.