Humpback Chub range expansion in the lower Grand Canyon or better sampling?

Authors: David L. Rogowski

Year: 2016 (xlviii)

Abstract

The Humpback Chub, Gila cypha is an endangered fish endemic to the Colorado River Basin. Standardized long term monitoring of the Colorado River between Glen Canyon Dam and Lake Mead has been occurring since about 2000 by various entities. Nighttime boat electrofishing has been the primary method of sampling native and nonnative fish during this monitoring. Most of this monitoring occurred between river mile zero at Lees Ferry and Diamond Creek, river mile 225. Below Diamond Creek the Colorado River was essentially part of Lake Mead for many years. In more recent years with declining water levels the Colorad o River has extended well past Diamond Creek to Pearce Ferry (RM 281.4) and beyond. Subsequently Arizona Game and Fish Department monitoring has extended to Pearce Ferry in recent years (2011-current). While electrofishing has not been very successful at capturing adult Humpback Chub, we do capture a small number of chub and with the recent (2016) addition of riverwide hoop net sampling we have documented Humpback Chub in the Western Grand Canyon. Additionally these Humpback Chub captured in the Western Grand (> RM 220) Canyon were not recaptures, whereas around 80% of Humpback Chub captured around the Little Colorado River inflow area were recaptures. This suggests that perhaps Humpback Chub in Western Grand Canyon are not utilizing the Little Colorado River for reproduction and might constitute a new ‘aggregation’.