Demographics of Bonytail and Razorback Sucker in isolated ponds

Authors: Brian Kesner, Kurt Shollenberger, Paul Marsh

Year: 2019

Abstract

Bonytail, Gila elegans, and Razorback Sucker, Xyrauchen texanus, are two critically endangered fishes that have no known self-sustaining populations in the Colorado River. Both species have been stocked extensively throughout the basin and for Razorback Sucker this has resulted in thousands of adults persisting near several stocking centers. However, their presence is reliant on the release of tens of thousands of fish per year. For Bonytail, persistence beyond a year post-release has rarely been recorded despite large scale stocking similar in scope and numbers to Razorback Sucker. A major factor of both species failure to achieve recruitment levels that match mortality is a suite of predatory non-native fishes that have been introduced and established throughout the basin. These introduced species consume nearly all larval production by Bonytail and Razorback Sucker. They also increase post-stocking and adult mortality of both species by consuming stocked fish. Many of the introduced species are sport fish, and their wide-scale removal is not considered as a management option because states rely on revenue from anglers to support their programs. Bonytail and Razorback Sucker recovery in the Colorado River therefore depends on dramatic changes in wildlife management strategies or the discovery of novel approaches that allow large scale recruitment within the system in the presence of non-native sport fish. In the meantime, the only viable solution may be creation of aquatic environments that exclude piscivorous non-native fishes. For the last 15 years proponents of the “backwater concept” in the lower Colorado River basin have worked to develop and test the idea that naturally recruiting populations of Bonytail and Razorback Sucker can be maintained in isolated, predator free off-channel habitat. Initial estimates of important demographic parameters have been calculated from monitoring data available through the online component of the lower Colorado River Native Fishes Database (ncreased.net). Monitoring in these backwaters includes continuous PIT scanning to track tagged individuals and annual netting efforts to capture and tag new recruits as well as recapture adults for health and growth studies. Annual adult survival of Razorback Sucker within monitored backwaters is consistently over 90%. Natural recruitment of Razorback Sucker in one backwater (Yuma Cove backwater) has resulted in a sustained population of more than 300 fish since 2013. Bonytail annual survival in backwaters typically has been less than 50%, but large recruitment events appear to maintain a relatively young population in at least one backwater (Imperial Ponds, Imperial National Wildlife Refuge). PIT scanning thus far has been less effective at tracking Bonytail populations due to the high turnover rate of these populations; most of the population is made up of untagged young fish. Results thus far are encouraging and support continued investigation toward eventual large-scale implementation of the backwater program.