Environmental drivers of benthic fish assemblages and fish‐habitat associations in offshore areas of a very large river

Zoltan Szalóky,Vivien Füstös,Tibor Erős,Balazs Tóth

doi:10.1002/rra.3793

Abstract

AbstractFish‐habitat associations are poorly known in offshore areas of very large rivers. We examined physical habitat structure and its effect on habitat use and assemblage formation of benthic fishes in the main channel of the Danube River, Hungary. Principal component analysis of physical variables showed that sample unit (i.e., 500 m long reaches) and cross‐channel transect‐level data of corresponding reaches were highly correlated. We found clear gradients in physical variables from areas with high velocity and higher proportion of hard substratum (pebble and cobble) to areas with low velocity, high mean depth and finer substratum (mainly sand) composition. Variation in velocity was coupled with variation in both mean depth and substratum composition (i.e., Shannon diversity of sediment composition) and higher proportion of silt material. Differences in physical habitat structure (flow, substrate) also manifested among river segments. Classification and regression tree analyses (CART) and fish abundance – occupancy patterns in the PCA template revealed that many species showed clear responses to environmental heterogeneity (barbel, Barbus barbus; schraetser, Gymnocephalus schraetser; Danube streber, Zingel streber; whitefinned gudgeon, Romanogobio vladikovy; round goby, Neogobius melanostomus) while others (white bream, Blicca bjoerkna) showed very elusive habitat use patterns. Multivariate regression tree analysis confirmed the results of CART and indicated that transect‐level substratum composition was the most important determinant in the formation of benthic assemblages. These results on habitat use can contribute to the more effective conservation management of offshore fish assemblages, which is important due to increasing inland navigation in the Danube River.

Highlights

Determination of how physical structure of the habitat influences the distribution, abundance and assembly of species is a prerequisite for effective nature conservation and environmental management (Allan & Flecker, 1993)
This is the first study that directly models the relationships between physical habitat variables and the abundance of several offshore benthic fish species in the very large Danube River
We found strong associations in the values of sample unit and transect-level physical variables (Figure 2), which indicates relatively small mesoscale level (101–103 m) environmental heterogeneity offshore in this very large river, as expected

Summary

| INTRODUCTION

Determination of how physical structure of the habitat influences the distribution, abundance and assembly of species is a prerequisite for effective nature conservation and environmental management (Allan & Flecker, 1993). Environmental gradients in substrate composition and water velocity are relatively short in the potamon, at least compared with the littoral zone or with wadeable streams of the rhithron, where contrasting changes in physical habitat quality and fish assemblages are more common (Allan & Castillo, 2007; Ero}s, 2017; Ero}s et al, 2017) This may result very elusive fish-habitat relationships for offshore areas. Advanced statistical and machine learning techniques, which can handle nonlinear and complex interaction effects, may be better applicable to explore fish-habitat relationships in these cases than traditional methods such as linear or multiple regressions (Knudby, Brenning, & LeDrew, 2010; Olden, Lawler, & Poff, 2008) In this regard, regression trees proved to be especially useful for modelling fish-habitat relationships (Knudby et al, 2010; Vezza, Parasiewicz, Calles, Spairani, & Comoglio, 2014). We predicted that the joint analysis of focal scale (i.e., sample unit level) physical attributes with physical attributes of the surrounding environment

| MATERIAL AND METHODS

| RESULTS

Findings

| DISCUSSION