AUTHOR=Meijer Christopher G. , Hickford Michael J. H. , Gray Duncan P. , Schiel David R. TITLE=Disentangling the simultaneous effects of habitat degradation and introduced species on naturally depauperate riverine fish communities JOURNAL=Frontiers in Freshwater Science VOLUME=2 YEAR=2024 URL=https://www.frontiersin.org/journals/freshwater-science/articles/10.3389/ffwsc.2024.1398975 DOI=10.3389/ffwsc.2024.1398975 ISSN=2813-7124 ABSTRACT=Introduction

Global change is a multi-faceted issue putting many species at risk. The broad range of potentially interacting environmental stressors is problematic for effective and efficient conservation and management. In freshwater systems, habitat degradation and introduced species have been repeatedly recognized for their extensive impacts on native ecosystems. However, the simultaneous impacts of these environmental stressors on naturally depauperate and inherently vulnerable communities are poorly understood.

Methods

In southern New Zealand, the fish communities in 14 tributaries of three lowland lakes were surveyed to quantify the within- and between-community changes along gradients of habitat complexity and abundance of introduced species, specifically brown trout (Salmo trutta Linnaeus) and redfin perch (Perca fluviatilis Linnaeus).

Results

Stable isotope analyses identified that trophic diversity increased with habitat complexity and an abundance of native eels (Anguilla spp.) but was unaffected by introduced species. Within each community, only perch exhibited distinct dietary shifts along all environmental gradients, whereas trout and the native fish had consistent, generalist diets. When supported with length–weight regressions, these impacts became increasingly size-dependent. For example, among the native fish, only the larger eels were unaffected by habitat and achieved greater body conditions with increased numbers of eels and perch; however, more trout were detrimental to eel body condition. In contrast, the smaller bodied natives, including elvers, all had improved body conditions from increased habitat complexity and reduced numbers of trout and perch. For the introduced species, perch weights were consistent regardless of the local environment due to their variable diet, but larger trout generally increased in weight with reduced habitat complexity and greater numbers of introduced fish, although high eel densities were detrimental.

Discussion

Overall, our results highlight how the responses to environmental stressors, even in depauperate communities, are complicated and generally species-specific. Nonetheless, habitat degradation had the most wide-ranging negative impacts on native fish, with perch numbers only affecting the smaller bodied natives and trout only affecting one native species. We conclude that focusing on habitat restoration in conservation strategies will provide the most efficient and effective use of resources, although the realized benefits for native species will be limited if introduced species are overly abundant.