Abstract
We developed a geochemical atlas of the Colorado River in Grand Canyon and in its tributary, the Little Colorado River, and used it to identify provenance and habitat use by Federally Endangered humpback chub, Gila cypha. Carbon stable isotope ratios (δ13C) discriminate best between the two rivers, but fine scale analysis in otoliths requires rare, expensive instrumentation. We therefore correlated other tracers (SrSr, Ba, and Se in ratio to Ca) to δ13C that are easier to quantify in otoliths with other microchemical techniques. Although the Little Colorado River’s water chemistry varies with major storm events, at base flow or near base flow (conditions occurring 84% of the time in our study) its chemistry differs sufficiently from the mainstem to discriminate one from the other. Additionally, when fish egress from the natal Little Colorado River to the mainstem, they encounter cold water which causes the otolith daily growth increments to decrease in size markedly. Combining otolith growth increment analysis and microchemistry permitted estimation of size and age at first egress; size at first birthday was also estimated. Emigrants < 1 year old averaged 51.2 ± 4.4 (SE) days and 35.5 ± 3.6 mm at egress; older fish that had recruited to the population averaged 100 ± 7.8 days old and 51.0 ± 2.2 mm at egress, suggesting that larger, older emigrants recruit better. Back-calculated size at age 1 was unimodal and large (78.2 ± 3.3 mm) in Little Colorado caught fish but was bimodally distributed in Colorado mainstem caught fish (49.9 ± 3.6 and 79 ± 4.9 mm) suggesting that humpback chub can also rear in the mainstem. The study demonstrates the coupled usage of the two rivers by this fish and highlights the need to consider both rivers when making management decisions for humpback chub recovery.
Highlights
It is a lo[a]thesome little stream, so filthy and muddy that it fairly stinks
The variability of the Little Colorado River chemical parameters was far greater than was the case for the mainstem Colorado
How does an understanding of these chemical tracers and otolith growth of humpback chub help in our understanding of the population ecology of this species? We suggest two key findings: (1) age and size at egress of juvenile humpback chub from the Little Colorado River into the mainstem differed between young-of-the-year and older individuals, and (2) size at age-1 of humpback chub differed between fish captured in the Little Colorado River vs. the mainstem
Summary
It is a lo[a]thesome little stream, so filthy and muddy that it fairly stinks. It is only 30 to 50 [yards] wide and in many places a man can cross it on the rocks without going on to his knees ... A miserably lonely place with no signs of life but lizards, bats and scorpions. [The Little Colorado was] as disgusting a stream as there is on the continent ... It seemed like the first gates of hell. The Colorado River provides water to more than 40 million people, substantial hydroelectric production, and billions of dollars in agricultural products [2]. President Theodore Roosevelt declared that this is “...the one great site every American should see” and more than 4 million people follow this advice annually to visit Grand Canyon National Park
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