Publication

Sustainability Management of Short-Lived Freshwater Fish in Human-Altered Ecosystems

URL: https://webapps.usgs.gov/mrgescp/documents/Hatch_2021_Sustainability-Management-of-Short-Lived-Freshwater-Fish-in-Human-Altered-Ecosystems.pdf

Date: 2021/05/01

Author(s): Hatch M.D.

Publication: A dissertation submitted to the Graduate School, New Mexico State University, 136 p.

Abstract:

CHAPTER 1 ABSTRACT - SUSTAINABILITY MANAGEMENT OF SHORT-LIVED FRESHWATER FISH IN HUMAN-ALTERED ECOSYSTEMS SHOULD FOCUS ON ADULT SURVIVAL

Evidence is presented that fish populations globally are susceptible to endangerment through exploitation and habitat loss. Theoretical simulations are presented to explore how reduced adult survival (age truncation) might affect short-lived freshwater fish species in human-altered contemporary environments. Simulations evaluate two hypothetical "average fish" and five example fish species of age 1 or age 2 maturity. From a population equilibrium baseline representing a natural, unaltered environment we impose systematic reductions in adult survival and quantify how age truncation affects the causes of variation in population growth rate. Estimates are presented of the relative contributions to population growth rate arising from simulated temporal variation in age-specific vital rates and population structure. At equilibrium and irrespective of example species, population structure (first adult age class) and survival probability of the first two adult age classes are the most important determinants of population growth. As adult survival decreases, the first reproductive age class becomes increasingly important to variation in population growth. All simulated examples show the same general pattern of change with age truncation as known for exploited, longer-lived fish species in marine and freshwater environments. This implies age truncation is a general potential concern for fish biodiversity across life history strategies and ecosystems. Managers of short-lived, freshwater fishes in contemporary environments often focus on supporting reproduction to ensure population persistence. However, a strong focus on water management to support reproduction may reduce adult survival. Sustainability management needs a focus on mitigating adult mortality in human-altered ecosystems. A watershed spatial extent embracing land and water uses may be necessary to identify and mitigate causes of age truncation in freshwater species. Achieving higher adult survival will require paradigm transformations in society and government about water management priorities.

CHAPTER 2 ABSTRACT – MITIGATION OF RECURRENT PERTURBATIONS IS AN IMPORTANT GOAL FOR CONSERVATION OF FRESHWATER FISHES

Human perturbations affect many aquatic ecosystems globally. I use matrix population models to explore the effects of environmental perturbations on population growth rates using short-lived freshwater fish species as examples. Estimates of annual flow intermittence frequency and average annual spatial extent of flow intermittence were used to approximate how contemporary hydrologic conditions may vary over time and space and affect population growth for the endangered Rio Grande silvery minnow (Hybognathus amarus). Deterministic calculations indicate that a single occurrence of flow intermittence may require 20 or more years for recovery to an initial population equilibrium baseline. Perturbation that reduces only juvenile survival has a shorter recovery time to initial population size and greater resilience of population growth than when adult survival is reduced. Consecutive occurrences of perturbation lengthen recovery time nonlinearly, more notably when adults experience perturbation mortality. I illustrate with an example how managers could identify multiple options to mitigate recurrent ecosystem perturbations by reducing perturbation frequency and/or mitigating perturbation mortality. When life history data are unavailable or uncertain for a specific species of concern, our simulations suggest parameter approximations for a hypothetical species of comparable size and lifespan would provide a useful general frame of reference for conservation assessments aimed at informing management measures needed to mitigate recurrent perturbations.

CHAPTER 3 ABSTRACT – MANAGEMENT OF WATER SUPPLY AND WATER SHORTAGES TO SUSTAIN AN ENDANGERED FISH SPECIES

Arid and semi-arid landscapes globally represent significant challenges to develop and manage regional water resources for human uses while simultaneously sustaining aquatic ecosystems. There is a pressing need for conservation initiatives that limit or mitigate the extent of human perturbations to aquatic ecosystems. However, little theory exists to effectively manage scarce water resources to support an environmental state that sustains aquatic species and water resources while limiting the cost of that protection to existing water uses. Results of demographic analyses along with statistical characterizations of flow intermittence, are used to identify how changing patterns of habitat size, flow continuity, and consecutive years of flow intermittence operate to shape population growth rates of the Rio Grande silvery minnow (Hybognathus amarus), a small-bodied endangered fish species endemic to the Rio Grande Basin of North America. The joint effects of demographic and hydrologic factors provide a logical basis to estimate limits of river drying to avoid levels of environmental mortality that threaten the capacity of H. amarus to sustain a positive capacity for population growth. I explore how demographic characteristics of H. amarus relates to water demand for species conservation, and how consideration of this need can be integrated into procedural conventions of water management. The analytic framework presented provides a structured context for integrated resource planning to identify promising combinations of management actions to restrict flow intermittence for conservation purposes, possibly including water supply augmentation and options for least-cost water shortage management under conditions of time and space variant flow intermittence.