River pollution threatens preferred macroinvertebrate prey of endemic fishes in Aotearoa, New Zealand

Abstract

O_LIMacroinvertebrates are a critical source of food for riverine fishes. Anthropogenic stressors can transform riverine macroinvertebrate communities, yet the consequences of such transformations to the fishes that feed on them are poorly understood. Changes in any combination of food abundance, accessibility or quality may be referred to as changes in the foodscape of fishes. Stressor-driven increases of inaccessible prey that are difficult for fish to detect, capture or ingest, may render foodscapes less profitable to fishes. C_LIO_LIWe examined how nutrient enrichment, fine sediment, acid mine drainage (AMD), flooding and drying reshape fish foodscapes by altering the accessibility of macroinvertebrate prey for two New Zealand river fishes, Galaxias vulgaris and Gobiomorphus breviceps. We determined relative accessibility of macroinvertebrate taxa and traits to these fishes by comparing the abundance of prey taxa and traits in the environment with their corresponding abundances in fish diets. To make these comparisons, invertebrate and fish gut samples were collected from the same river reaches, at the same times, over three years. Environmental DNA (eDNA) metabarcoding of fish gut contents facilitated efficient diet characterisation of over 700 fish guts. Finally, to determine how multiple stressors might affect fish foodscapes, we related invertebrate trait-specific accessibilities to published data defining how the composition of those macroinvertebrate traits varies along stressor gradients. C_LIO_LIMacroinvertebrate traits that were most accessible to fishes were those promoting detection (e.g., active foraging behaviours exhibited by crawlers and scrapers) and ingestion (e.g., moderate size, morphologies lacking defensive structures), typical of mayfly and free-living caddisfly larvae. In contrast, prey with traits inhibiting detection (e.g., small size and burrowing behaviour) and ingestion by gape-limited fishes (e.g., large size), were less accessible to the fishes we studied. C_LIO_LITrait abundance-stressor relationships revealed that that increasing fine sediment, nutrients, AMD and drying, and decreased flooding frequency, favoured macroinvertebrates with traits that erode accessibility to fishes. These results show that multiple stressors may decrease the profitability of fish foodscapes by filtering out macroinvertebrate prey with traits promoting accessibility to fishes. C_LIO_LIFood web approaches to understanding multi-stressor impacts are rare, but are necessary to understand how the effects of multiple stressors on primary consumers then go on to affect predators of higher trophic position. A major barrier to fish foodscape research is the difficulty and cost of estimating trophic networks. We discuss how eDNA metabarcoding, coupled with trait-based approaches may help overcome these barriers, enabling generalisable assessments of foodscape changes caused by multiple stressors. C_LI