Abstract
The short larval ontogenetic phase has a large impact on success or failure of fish populations due to naturally high mortality rates. Yet, for the federally endangered fishes Colorado Pikeminnow Ptychocheilus lucius and Razorback Sucker Xyrauchen texanus, information about this sensitive life stage is limited. We determined species-specific age–length functions, spawning periodicity, and environmental factors related to growth and the onset of spawning by these species in the San Juan River of the Colorado River basin. Daily ages were determined from lapillar otoliths, and growth rates were calculated for subsamples of San Juan River larval Colorado Pikeminnows and Razorback Suckers collected from 2009 through 2017 and six candidate age–length functions were fit to evaluate the relationship between age and growth. Spawning periodicity estimates from the best-fit-model and published function were compared to those from observed otolith ages and differences were analyzed. The responses of spawning periodicity and growth to abiotic and temporal conditions were evaluated. For both species, spawning dates produced by otolith ages and best-fit functions were significantly different from those produced by the published functions, but not from each other. Age–length functions determined in this study enable more accurate back-calculation of ages and prediction of spawning periodicity than published functions. The spawning periodicity estimate of San Juan River Colorado Pikeminnow and Razorback Sucker were both impacted by multiple parameterizations of temperature and discharge. For both species, fish age had the biggest influence on growth. Accurate knowledge of spawning periodicity and factors affecting fish growth can improve timing of management activities to maximize benefit to Colorado Pikeminnows and Razorback Suckers in the San Juan River.
Highlights
L ARVAL fishes serve as valuable indicators of ecological integrity of rivers (Schiemer et al, 2003) and of population dynamics (Humphries and Lake, 2000)
Growth rates were significantly different across years (ANOVA: F 1⁄4 12.07, df 1⁄4 6, P, 0.0001) with highest growth in 2010 and lowest in 2011
Rapid growth enhances survival because of improved ability to feed and to avoid predators (Bestgen et al, 2006; Bestgen, 2008). Both species become less susceptible to gape-limited predatory cyprinids around 25 mm total length (TL) (Bestgen et al, 2006; Bestgen, 2008), so early-season larvae with slower growth rates (Bestgen et al, 2006) would be more susceptible to predation for longer than late-season larvae
Summary
L ARVAL fishes serve as valuable indicators of ecological integrity of rivers (Schiemer et al, 2003) and of population dynamics (Humphries and Lake, 2000). Colorado Pikeminnow Ptychocheilus lucius and Razorback Sucker Xyrauchen texanus are federally endangered species once prevalent in the Colorado River basin, including the San Juan River (USFWS, 2002a, 2002b). Populations of both species declined due to large-scale habitat modification, potential competition with and predation by nonnative fish species, and poor water quality (USFWS, 2002a, 2002b). Hypolimnetic releases from the dam largely alter the temperature regime of the San Juan River with cooler summer and warmer winter temperatures (SJRBRIP, 2018)
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