Medium-term environmental changes impact age-specific survival in a salmonid population near its southern range limit

Abstract

1. Human-induced environmental change is a major stressor on freshwater habitats that has resulted in the population declines of many freshwater species. Ontogenetic shifts in habitat use and associated (st)age-specific requirements mean that impacts of environmental stressors can influence (st)ages in a population differently, and yet relatively few studies of freshwater fish populations account for their detail.
2. We aimed to identify environmental and biotic factors affecting survival estimated for six age-classes of a European grayling population in the River Wylye, UK over a 17-year period. We used a Bayesian age-structured state space model to estimate survival of grayling cohorts between subsequent life stages (eggs to age 5 adults) for 16 annual transitions (2003–2004 to 2018–2019), whilst accounting for imperfect sampling of the population. We quantified the effects of seasonal water flow and temperature, in-stream habitat and prey resource, and potential competitors and predators on survival between subsequent life stages. We used Bayesian variable selection to gauge their relative importance on survival.
3. Grayling abundances declined during the study period (>75% in all age-classes), predominately driven by a loss of mature adults. Changes to seasonal flows negatively influenced their survival: increased days of summer low flow related to decreased survival of subadults and mature adults, and lower winter flows related to reduced recruitment of juveniles from eggs. Higher summer macrophyte cover negatively
influenced juvenile and subadult survival and increasing days of high temperature in summer appeared detrimental to juvenile survival. Abundance of brown trout (a potential competitor and predator) did not negatively influence grayling survival.
4. Our results reveal the implications of environmental change on a salmonid population, where recent low summer flows and high temperatures, and below average winter flows, have negatively influenced grayling survival. These conditions appear to be becoming more frequent and persistent in our study river, which is towards the species’ southern range limit, which could render the population vulnerable to climate change. Our study demonstrates how careful analysis of long-term population monitoring and environmental datasets can identify factors affecting (st)age-specific fish population dynamics, and when combined with local expertise, results in realistic mitigation proposals to promote wildlife population persistence.