Sustainability management of short-lived freshwater fish in human-altered ecosystems should focus on adult survival

Abstract

Fish populations globally are susceptible to endangerment through exploitation and habitat loss. We present theoretical simulations to explore how reduced adult survival (age truncation) might affect short-lived freshwater fish species in human-altered contemporary environments. Our 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. We estimate 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.

Mitigation of recurrent perturbation mortality is an important goal for river restoration and conservation of freshwater fish species

“…In this regard and as we demonstrate by example, a natural (pre‐perturbation) baseline is an informative perspective of a fish species' life history for establishing management goals for species conservation. Early in this diagnostic process, it is important to identify demographic factors that are the most important determinants of population growth (Koons et al 2016; Hatch et al 2020), and to subsequently direct management efforts at maximizing the effects of those determinants to positively affect population growth. For example, as adult survival decreases, resulting in age‐truncation, the first reproductive age class becomes increasingly important to variation in population growth (Hilborn & Walters 1992; Ottersen et al 2006; Wang et al 2017; Hatch et al 2020).…”

“…From the analysis of Hatch et al (2020), we focused on results for three congeners: wild Hybognathus placitus (plains minnow), wild Hybognathus argyritis (Western silvery minnow), and captive H. amarus (Caldwell et al 2019). For this paper, we used the regression equation for H. argyritis and predicted fecundity for H. amarus at its mean size at age (Hatch et al 2020).…”

“…For a natural baseline, we assume a species can be considered in a natural environment where adult survival probability determines its natural lifespan. Given an approximate lifespan and age‐specific fecundity, one can obtain an analytic estimate of juvenile survival probability necessary for the population growth rate to be stable ( λ very close to one), which implicitly assumes a species' fecundity is sufficient to offset juvenile mortality (Hatch et al 2020).…”

“…Matrix Population Model With Perturbations Hatch et al (2020) demonstrated that demographic simulation can yield insight into a population's potential fate relative to a "natural" population equilibrium baseline. A natural (pre-perturbation) baseline is a convenient and general way to consider a fish species' life history and it enables comparisons between different life histories.…”

Mathematical Model of Iteroparous and Semelparous Species Interaction

“…Research on semelparous species can be seen in [1]- [3]. Then, research on iteroparous species can be seen in [4]- [6]. Semelparous species are species that reproduce only once in their lifetime shortly before dying.…”