Species Status Assessment Report for the Headwater Chub and Lower Colorado River Distinct Population Segment of Roundtail Chub

Species Status Assessment Report for the Headwater Chub and Lower Colorado River Distinct Population Segment of Roundtail Chub Version 1.0 September 2015 U.S. Fish and Wildlife Service Region 2 Albuquerque, NM Arizona Ecological Services Field Office Tucson, AZ Headwater and Rountail Chub SSA Report September 2015 Suggested reference: U.S. Fish and Wildlife Service. 2015. Species status assessment report for the headwater chub and the lower Colorado River distinct population segment of roundtail chub. Version 1.0, September 2015. U.S. Fish and Wildlife Service, Southwest Region, Albuquerque, NM. 2 Headwater and Rountail Chub SSA Report September 2015 Chapter 1. Introduction The headwater chub (Gila nigra) and the Lower Colorado River distinct population segment (DPS) of the roundtail chub (Gila robusta)1 (roundtail chub DPS)2 occur in streams in New Mexico and Arizona (Service 2006, 2009). These entities are being reviewed for listing as threatened or endangered species under the Endangered Species Act of 1973, as amended (Act). This Species Status Assessment Report for the Headwater Chub and Lower Colorado River Distinct Population Segment of Roundtail Chub (SSA Report) is a summary of the information assembled and reviewed by the U.S. Fish and Wildlife Service (Service) and incorporates the best scientific and commercial data available. This SSA Report documents the results of the comprehensive status review for the headwater chub and roundtail chub DPS to inform the listing decision under the Act and to inform future conservation efforts. The Service is engaged in a number of efforts to improve the implementation of the Act (see www.fws.gov/endangered/improving_ESA). As part of this effort, our Endangered Species Program has begun to develop a new framework to guide how we assess the biological status of species. Because biological status assessments are frequently used in all of our Endangered Species Program areas, developing a single, scientifically sound document is more efficient than compiling separate documents for use in our listing, recovery, and consultation programs. Therefore, we have developed the following SSA Report that contains summary information regarding life history, biology, and current and future risk categories facing the headwater chub and roundtail chub DPS. The objective of the species status assessment (SSA) is to thoroughly describe the viability of the headwater chub and roundtail chub DPS based on the best scientific and commercial data available. Through this description, we will determine what the species need to remain viable, their current condition in terms of those needs, and their forecasted future condition. In conducting this analysis, we take into consideration the changes that are happening in the environment – past, current, and future – to help us understand what conditions drive the viability of the species. We developed a model to analyze the current and future condition using Microsoft Excel. The purpose of the model analysis is to reflect our understanding of the viability of these chub species by applying the best available information to assess the status of these species in a standardized and transparent method. 1 The Lower Colorado River DPS of roundtail chub is the candidate entity under evaluation in this SSA. The DPS includes three watersheds below Lees Ferry on the Colorado River; the Bill Williams, Gila, and Little Colorado rivers. In this SSA the entity under consideration is the DPS and not the entire species. Where appropriate, information on life history and habitat in this assessment relies on information generated from the entire range of roundtail chub. 2 Under the Act, a distinct population segment meets the definition of a species and is a listable entity. 3 Headwater and Rountail Chub SSA Report September 2015 For the purpose of this assessment, we define viability3 as a description of the ability of the species to sustain populations in the wild beyond a biologically meaningful timeframe. Using the SSA framework, we consider what the species needs to maintain viability by characterizing the status of the species in terms of its resiliency, redundancy, and representation. • Resiliency is having sufficiently large populations for the species to withstand stochastic events. Stochastic events are those arising from random factors such as weather, flooding, or fire. We can measure resiliency based on metrics of population health; in the case of the headwater chub and roundtail chub DPS, abundance and persistence of chubs in currently occupied streams over time, the amount of habitat available in the stream, and presence of nonnative species are primary indicators of resiliency. Resilient populations are better able to withstand disturbances such as random fluctuations in birth rates (demographic stochasticity), variations in rainfall (environmental stochasticity), or the effects of climate change and increasing human uses of water. • Redundancy is having a sufficient number of populations for the species to withstand catastrophic events. A catastrophic event is defined here as a rare destructive event or episode involving many populations and occurring suddenly. Redundancy is about spreading risk and can be measured through the duplication and broad distribution of resilient populations across the range of the species. The more resilient populations the species has, distributed over a larger landscape area, the better chances that the species can withstand catastrophic events. For the headwater chub and roundtail chub DPS, we measure redundancy based on the geographic distribution of the occupied streams (evaluated as analysis units [AUs]) across the range of the entities. • Representation is having the breadth of genetic makeup of the species to adapt to changing environmental conditions. Representation can be measured through the genetic diversity within and among populations, and the ecological diversity (variety of ways species interact with each other and the environment) of populations across the species’ range. The more representation, or diversity, the species has, the more it is capable of adapting to changes (natural or human caused) in its environment. In the case of the headwater chub and roundtail chub DPS, we evaluate representation based on the maintenance of the identified genetic diversity in populations across the geographic range of the two species. Our objectives for this SSA were: 1) to assess the current condition of each extant headwater chub and roundtail chub DPS analysis unit (AU) relative to a series of condition categories; and 2) to describe the future condition of the headwater chub and roundtail chub DPS AUs by examining a set of future scenarios related to those condition categories; and 3) to use the assessments of current and future conditions to describe of viability in terms of the species’ resiliency, redundancy, and representation over our measured time frame to 2046. In this SSA 3 Viability is not a specific state, but rather a continuous measure of the likelihood that the species will sustain populations over time. From, USFWS. 2015. Draft Species Status Assessment Framework. Version 3.1 for FWS ES HQ and Science Applications Review. January 2015. 4 Headwater and Rountail Chub SSA Report September 2015 we refer to populations of headwater chub and roundtail chub DPS as analysis units (AUs). The AUs are based on the hydrological connectivity of the occupied streams. This is further explained in section 1.1 below. This SSA Report presents the following information: (1) Introduction; (2) Taxonomy and the historical and current distribution of the two species (Chapter 2); (3) Life history, biology, and resource needs of individuals (Chapter 3); (4) Taxonomy (Chapter 4); (5) Population and species needs and current condition of populations (Chapter 5); (6) Discussion of risk categories and conservation actions (Chapter 6); (7) Current Condition (Chapter 7). (8) Future Condition and Viability (Chapter 8) (9) Appendix A-Glossary of Selected Terms (10) Appendix B-Evaluating Causes and Effects for Headwater and Roundtail Chub (11) Appendix C-Status Assessment Model Headwater and Roundtail Chub (12) Appendix D-Literature Cited (13) Appendix E – Supplemental Information for Assessment Model (14) Appendix F- Effects Pathway Conceptual Model For a glossary of selected terms used in this SSA Report, reference Appendix A. The detailed analysis of potential threats to the chubs as summarized in Chapter 6 is found in the Evaluating Causes and Effects tables in Appendix B. We conducted an analysis to qualitatively characterize the viability of the headwater and roundtail chubs as described in Draft Status Assessment Model Headwater and Roundtail Chub in Appendix C. The literature cited in this SSA Report is in Appendix D4. Appendix E contains additional information on our assessment methods, uncertainties, and assumptions. The Effects Pathway Conceptual Model is in Appendix F. There is a considerable lack of specific information on population size or density for either chub species. Consequently, determining an effective population size is difficult. Further, data is lacking as to the amount and quality of habitats in a stream needed to support a population size or effective population sizes to avoid loss of genetic or ecological diversity. In addition, the relationship between chubs and various nonnative aquatic species that now occupy chub habitat under different physical conditions is unclear. However, we used the best available

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