The influence of natural flow and temperature and introduced brown trout on the temporal variation in native fish abundance in a “reference” stream

Abstract

Understanding what drives variation in fish abundance at reference sites provides perspective for assessing the effects of human alterations to river flow and land use. We examined temporal variation in fish community abundance in a headwater tributary of a large river in the upper South Island (New Zealand) over 10 years. We were interested in the influence of natural flow variation and temperature on native fish abundance within the context of potential competition/predation pressure from juvenile introduced trout. Results from biannual sampling highlighted the dominating influence of floods on fish population dynamics, overriding biotic effects. We found no evidence for adverse effects of trout on native fish, and flow-related habitat performed more poorly in explaining variation in fish abundance than low- and median-flow statistics. Differences in temporal variation in abundance between species were largely consistent with life histories that provide resistance and (or) resilience to flood flows and relative insensitivity to low flows. Long-term data (≥10 years) are needed for detecting meaningful trends and quantifying the effects of human activities on fish community abundance.

Remarkable response of native fishes to invasive trout suppression varies with trout density, temperature, and annual hydrology

“…The magnitude of the impact of invasive salmonids may diminish at warmer extremes of their thermal tolerance (Ward and Morton-Starner 2015;Shelton et al 2018;Yackulic et al 2018), and natural thermal and flow regimes may allow native species to persist in salmonid-invaded habitats (Propst et al 2008;Hayes et al 2019), but outcomes of invasions may vary by species. For instance, in laboratory studies, rainbow trout piscivory was greatest in colder waters, as the swimming ability of the obligate warmwater native prey species was hampered (Ward and Bonar 2003), whereas brown trout piscivory rates were always high over a range of water temperatures (Ward and Morton-Starner 2015).…”

“…Additionally, discharge regimes may dictate the invasion success and population dynamics of these invading trout species (Fausch et al 2001;Kawai et al 2013;Dibble et al 2015). For example, high flow variability in spring may limit brown trout invasions (Kawai et al 2013), and natural flow regimes may confer resistance to the effects of biotic interactions to native fish assemblages uniquely adapted to extreme conditions (Hayes et al 2019). Thus, environmental factors and invasive trout may interact to structure native fish communities, but the relationships among invasive trout, native fishes, and flow and thermal regimes are complex and not clearly understood.…”

Bootstrap methods can help evaluate monitoring program performance to inform restoration as part of an adaptive management program

“…To assess the performance of our monitoring data to inform our ongoing restoration efforts, we developed a framework of resampling data collected during the first 18 months of our monitoring effort post-construction to assess the ''informativeness'' (see Bolker, 2008;Johnson et al, 2015) of our oyster counts to detecting a response to the restoration. Using count data for many species of fish and invertebrate species as a response variable can be difficult because of high annual variation in survival, recruitment, and capture probability (see examples in George, Baldigo & Stich, 2019;Gillespie & Vincent, 2019;Gwinn et al, 2016;Hayes et al, 2019;Hilderbrand & Kazyak, 2017;Miner et al, 2018;Wagner et al, 2013;Wagner et al, 2014) which could mask restoration effects. This is an area of significant concern because failure to detect whether a restoration action has the desired effect in a timely way significantly limits opportunities actively to inform ongoing restoration actions.…”