Conflicting effects of woody debris on stream fish populations: implications for management

Abstract

Summary 1. Coarse woody debris (CWD) in stream channels causes changes in flow, sedimentation and ratios of pool to riffle areas. There is a consensus among fishery managers and scientists that CWD is beneficial to stream fish communities because of its enhancement of habitat diversity, invertebrate production and cover. Our hypothesis was that CWD accumulation or introduction would not increase in‐stream habitat capacity for all species and their ontogenic stages at reach and stream scales. 2. The study used a system of gravel‐bed streams with naturally dynamic CWD accumulations and a fish community consisting of Salmo trutta, Cotttus gobio, Phoxinus phoxinus, Lampetra cf planeri, Nemacheilus barbatulus and Anguilla anguilla. Cotttus gobio and L. cf planeri are protected by an EU Directive and S. trutta is exploited for angling. Riffles, pools and CWD matrices, considered as the basic habitat/spatial units of channel structure, were sampled separately and abundance of each fish species quantified seasonally at each spatial scale. 3. Multiple‐pass electric fishing techniques were used. Capture efficiencies were calculated for species, habitat and season. Areal densities (number m−2) were compared for habitat types and season using nonparametric anova. Canonical analysis and stepwise multiple regression were used to show the most influential physical variables on fish density. Densities were also compared by unit volume (numbers m−3) for pools and CWD matrices to investigate direct three‐dimensional use for cover. Reach‐scale densities for each fish species in relation to habitat composition were made using Spearman rank correlation of habitat‐scale densities with proportionate areas of the different habitat units in the reach. 4. Habitat‐scale densities of bullheads and age 0+ trout were negatively correlated with depth and CWD areas for some seasons. Densities of lampreys, older trout, eels and minnows were positively correlated with depth in some seasons. Water depth had the most consistent influence on fish abundance at the habitat unit scale. Three‐dimensional comparisons of pools and CWD matrices indicated that only trout older than 1+ may use CWD habitats as cover. 5. Reach‐scale densities of 0+ trout and bullheads were significantly correlated with proportion of riffle area and negatively with CWD and combined CWD‐pool habitat area in the reach. Densities of older trout, large eels and lampreys were positively correlated with CWD area and combined CWD‐pool area in some seasons. Inundation of riffles caused by impoundment upstream of CWD accumulations reduced spawning habitat for trout, bullheads, brook lampreys, minnows and stone loach. A trade‐off was an increase in refugia for older trout, minnows and eels. 6. Coarse woody debris accumulation in streams is not beneficial to all species or ontogenic stages in a mixed species population and could severely limit essential habitat areas for some species. Thus, physical manipulation of channels should be implemented only after a thorough study of the habitat ...

Disentangling the influences of habitat structure and limnological predictors on stream fish communities of a coastal basin, southeastern Brazil

“…We did not measure, for example bank slope, riparian vegetation cover, accumulation of wood debris or stream declivity, variables that may influence fish communities (Pulsey & Arthington, 2003;Cruz et al, 2013;Langford et al, 2012). Nonetheless, given the high magnitude of the unexplained variation, it is improbable that they would significantly improve the percent of explanatory variation.…”

Wood placement in river restoration: fact, fiction, and future direction

“…Differences in physical habitat and channel morphology produced from wood are strongly correlated with fish abundance, particularly for species that prefer pool habitat (e.g., Crook and Robertson 1999;Dolloff and Warren 2003;Harmon et al 1986;Zalewski et al 2003), though not all species and life stages benefit from wood placement (Langford et al 2012). In addition to creating physical habitats for fishes, wood has a number of biological functions in channels, including increasing nutrient retention (Bilby 1981), creation of surfaces that facilitate primary and secondary production (Benke et al 1985;Coe et al 2009;Lester et al 2009), and providing hiding cover or altering depth and substrate to provide spawning areas for fishes (e.g., Beechie et al 2005;Montgomery et al 1999).…”

“…), trout cod (Maccullochella macquariensis), and a number of other fish species (Dolloff and Warren 2003;Growns et al 2004;Warren et al 2000). However, many other non-salmonid fishes do not appear to benefit from the presence of wood in streams (Growns et al 2004;Langford et al 2012;Warren et al 2000).…”

Avances en el análisis del material leñoso en ríos: incorporación, transporte e influencia en el riesgo por inundaciones

“…Desde el punto de vista ecológico, el material leñoso proporciona un hábitat adecuado para la flora y fauna (Carlson et al, 1990;Jackson and Sturm, 2002;Langford et al, 2012), y además regula el flujo de agua y nutrientes (Welty et al, 2002). Por todo ello, actualmente la reintroducción de fracción leñosa en los cauces es cada vez más recomendada como método de restauración fluvial (Kail et al, 2007).…”