A multidimensional typology of riverbank habitats explains the distribution of European grayling ( Thymallus thymallus L.) fry in a temperate river

A multidimensional typology of riverbank habitats explains the distribution of European grayling ( Thymallus thymallus L.) fry in a temperate river
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  A multidimensional typology of riverbankhabitats explains the distribution of Europeangrayling ( Thymallus thymallus  L.) fry in atemperate river  Franck Cattan  eo 1 , David Grimardias 1 , Marie Carayon 1,2 , Henri Persat 3 , Agn  es Bardonnet 2,4 1 Research Institute Earth, Nature, Environment, Hepia  –  University of Applied Sciences Western Switzerland, 1254, Jussy, Switzerland 2 INRA  –  UMR Ecologie comportementale et Biologie des populations de Poissons, 64310, Saint-P  ee sur Nivelle, France 3 LEHNA  –  Laboratoire d’Ecologie des Hydrosyst  emes Naturels et Anthropis  es, Universit  e Lyon 1  –  CNRS, UMR 5023, 69622, Villeurbanne Cedex, France 4 UMR Ecologie comportementale et Biologie des populations de Poissons, UFR Sciences c ^ ote Basque, Universit  e Pau & Adour, 64310, Anglet, FranceAccepted for publication September 16, 2013 Abstract   –  For species at risk, rehabilitation of riverbank habitats may be a promising way to sustain abundance of early-life stages, provided that fish  —  habitat relationships are significant and the scale at which they are described isrelevant to both fish ecology and management purposes. Here, habitat use of the threatened European grayling( Thymallus thymallus ) fry was analysed in an alluvial French-Swiss border river, the Allondon. Based on fivedescriptive variables, a practical typology of available bank habitats was first defined to categorise ecologicalconditions into five easily distinguishable habitat types (HTs). These represent marginal patches of multivariatehomogeneous habitat from a few to tens of square metres [median area  =  8 m  2 ; range (10%  –  90%)  = 1.9  –  32.6 m  2 ]. Comparing the habitat used by fry to those available revealed a clear selection for two of thefive HTs, that is, gravel bar shorelines (HT4) and depositional beaches along pool margins (HT3). Because of their multidimensional nature and larger spatial scale, HTs did not strictly reflect an up-scaling of univariate selectionpatterns at the microhabitat scale. Results emphasised that the preservation of the alluvial dynamics of the river andbank patchiness are basic to European grayling, and suggested that the patch scale may represent a goodcompromise between ecological relevancy and practical management needs. The herein developed typologicalapproach may be transferred to other rivers and help conserve European grayling populations by enhancing fryhabitat suitability. Key words: habitat use; conservation; alluvial river; patch scale; European grayling ( T. thymallus  ); young stages; habitatenhancement Introduction Ontogenetic habitat shifts are common in many river-ine species, enabling individuals to better exploit their environment while growing in body size,strengthening their swimming capacity, yet protectingagainst predators (Everest & Chapman 1972). Avail-ability of suitable habitat conditions at any season or life stage is then a key parameter, whose shortage isamong the main threats to population persistence(Bovee et al. 1998; Cramer & Ackerman 2009). Mul-tiple water uses (e.g., water abstraction for agricul-ture, damming for energy production or drinkablewater  … ) often result in habitat degradation and in adecrease in the connectivity among distinct habitats,hence threatening fish populations. Spatial and tem-poral patterns of required habitat conditions form acomplex mosaic, which has to be accounted for by Correspondence  : Franck Cattan  eo, Research Institute Earth, Nature, Environment, Hepia  –   University of Applied Sciences Western Switzerland, Route dePresinge 150, 1254 Jussy, Switzerland. E-mail: doi: 10.1111/eff.12106  1 Ecology of Freshwater Fish 2013   2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd  ECOLOGY OF FRESHWATER FISH  river managers to further implement restoration or conservation strategies (Rabeni & Sowa 1996; Lakeet al. 2007).Fish  —  habitat relationships have been explored at avariety of spatial scales, ranging from the microscale(about one metre), the mesoscale (tens of metres,e.g., riffle/pool successions) to the macroscale (hun-dreds of metres, e.g., comparison of river reachesacross sites or of distinct habitats across a flood-plain), each explaining part of the variability in fishabundance (Poizat & Pont 1996; Desch ^ enes & Rodri-guez 2007; Stewart-Koster et al. 2013). Microhabitat investigations most often rely on point-sample mea-surements, where physical conditions are describednear the fish (Moyle & Baltz 1985; Bovee et al.1998). Data can then be utilised in preference curvesand be further implemented into a habitat-hydraulicsmodelling procedure (Heggenes 1996; Mallet et al.2000). However, point-sample microhabitat condi-tions are constrained by meso- and macroscale geo-morphic patterns (Frissell et al. 1986), implying that the physical characteristics at any point depend to acertain extent on the environmental conditions at alarger spatial scale. It is acknowledged that the appar-ent homogeneity of mesoscale geomorphic features(e.g., pool/riffle) actually masks a variety of micro-habitat conditions. The position of fish may then bet-ter reflect a global surrounding environment than thefocal habitat conditions at the strict microhabitat scale(Poizat & Pont 1996; Crook et al. 2001). Therefore,considering that habitat features can be stratified intolongitudinal (e.g., pool/riffle) and transversal (e.g.,mainstream/riverbank habitats) units, further definingdiscrete patches of several square metres within theseunits may add valuable information on how fish usethe available environmental conditions (Inoue &Nunokawa 2002).In many species, bank habitats are of primaryimportance for the early-life stages, which are partic-ularly critical to the population dynamics as the oper-ation of biotic and stochastic environmental processescan cause drastic mortality in the cohort (Harvey1991). Due to the narrow ecological requirementsand low mobility of fry, bank habitats should offer suitable hydraulic and trophic conditions while alsobeing a refuge from predators. Whether such habitatsare available in the vicinity of spawning grounds isfundamental to the fate of young fishes. The bankstructural diversity was shown to be a main driver of fry assemblage composition and is of high rele-vance to assess the ecological status of rivers (Schi-emer 2000; Jurajda et al. 2010; Leclere et al.2012). Thus, characterising the  ‘ riverbank land-scape ’  and its use by fish may help identify poten-tial shortage in habitat, and further guide restorationefforts.European grayling ( Thymallus thymallus , L.) is anative, threatened salmonid species in Central andWestern Europe. While abundant in the medium-sized, fast-flowing rivers (Northcote 1995), manypopulations of the southern and western margins of its distribution area have faced a dramatic declineduring the last 30 years (Persat 1996; Koskinen et al.2000; Uiblein et al. 2001). Causes of this decline aremultifaceted and include water quality and habitat degradation (Northcote 1995), flow regulation andsediment transport alterations (G € onczi & Adam1989), overfishing (N € aslund et al. 2005), increasingavian predation pressure (Staub et al. 1998) and cli-mate change (Wedekind & K € ung 2010; Wedekindet al. 2013). In Switzerland (Guthruf 1996; Fischnetz2004), as in other Alpine areas (Uiblein et al. 2001),population declines have been particularly severe.European grayling appears on the Swiss Red List (Kir-chhofer et al. 2007) and on the Federal Fishing Lawwith a  ‘ threatened  ’  status (according to IUCN criteria)therefore requiring substantial conservation efforts.So far, quantitative information about habitat useby grayling fry has relied on microhabitat scale sur-veys, where physical conditions were described bywater depth, velocity, substrate size and sometimescover (Scott 1985; Bardonnet et al. 1991; Sempeski& Gaudin 1995a). However, data are not comprehen-sive as (i) they only include a few distinct river types,with no alluvial rivers; (ii) they may not reflect trueresponses to environmental variability as they empha-sise on point-specific characteristics irrespective of the surrounding environment; and (iii) cross-river comparisons of fry habitat preference curves werepoorly transferable, suggesting that fish adapt to site-specific habitat conditions (Nyk € anen & Huusko2004). Except for water velocity which appeared tobe consistently selected whatever the river system,uses of depth, substrate and cover, and their combina-tions display a large flexibility, which implies that management of threatened populations should rely onhabitat preference criteria developed locally.Conversely to most other salmonids, the emer-gence pattern of grayling larvae is diurnal, with apeak just after sunrise (Gaudin & Persat 1985; Bar-donnet & Gaudin 1990). Fry then rest near spawninggrounds during daylight and move downstream dur-ing early night (Bardonnet et al. 1991; Grimardiaset al. 2012), to join slow-flowing marginal areas(Scott 1985; Sempeski & Gaudin 1995a). Requiredhabitat conditions significantly change over the first weeks, with a shift towards higher flows. The frystage can then be split into functional groups basedon fish size (Sagnes et al. 1997). Fry of about 20 mm length use shallow (10  –  30 cm), low velocity( < 15 cm  s  1 ), fine substrate ( < 2 mm) microhabitats,with variable (10  –  70%) vegetation cover, located 2Cattan  eo et al.  0.2  –  1 m from the bank (Nyk € anen & Huusko 2003).At approximately 25 mm length, fry still coloniseslow-flowing ( < 15 cm  s  1 ), fine substrate, but deeper waters (30  –  90 cm). Larger fry (30 mm) become ben-thic and rheophilic and increasingly shift from marginaldead zones to the main channel, where they use30  –  90 cm height waters which are faster flowing(10  –  50 cm  s  1 ), with a coarser substrate (until 512 mm)and low vegetation cover ( < 20%). Bardonnet et al.(1991) found no preference for particular bank pro-files. A diel habitat use pattern has been revealed for all fry stages, with a common tendency to colonise at night very shallow, dead zones close to the banks andto rest just above the substrate (Sempeski & Gaudin1995b).In this study, we analysed use (i.e., selection) of bank habitats by grayling throughout the fry stage inan alluvial river, on the entire reach where the specieswas present, at a scale intermediate between themicrohabitat and the mesohabitat (referred to as the ‘ patch scale ’ ). A practical typology of available bankhabitats was defined to categorise ecological condi-tions into easily distinguishable habitat types (HTs).The distribution of grayling fry abundance acrossavailable HTs was analysed to highlight markedselection or avoidance. Results are discussed in light of methodological and ecological information that thisstudy could bring to managers and scientists involvedin the conservation of threatened fish populations. Material and methods Study reach The Allondon is a 22 km cross-border river, flowingin a piedmont area from the Jura Mountains (from anelevation of 649 m) in France to the Swiss Rh ^ oneRiver (alt. 350 m), approximately 10 km downstreamthe city of Geneva (Fig. 1). It drains a 148 km 2 area,mostly composed of deciduous forests and pastures,and has a mean annual flow of 3.44 m 3  s  1 at itsmouth. Its rain-fed hydrological regime is marked byan important karst component and a particularly highannual depth of runoff (730 mm per year), and exhib-its two periods of high flows in autumn and spring,and a low-flow period during summer. The lower-most 4.5 km of the river are alluvial, with widebraided zones alternating with single channel zones.The alluvial zone benefits from a national protectionstatus. Due to its mean gradient of 13.6 m  km  1 andits high annual runoff, the Allondon is a powerfulriver especially suited for Salmonids (brown trout, Salmo trutta , and European grayling,  Thymallus thy-mallus ). Grayling, however, only colonises the sixlowermost km of the river (mean gradi-ent   =  10.6 m  km  1 ). During the spawning period,most adult individuals observed in the Allondonprobably come from the Rh ^ one River to spawn onsuitable gravel areas.To define the study reach, we tracked the reproduc-tive activity and spawning grounds every 3 days byobservation from the banks, between the 15 Februaryand the 30 April 2011. After emergence, grayling lar-vae were assumed to be rapidly carried away down-stream, close to the bank, by water flow at early night (Bardonnet 1989). Therefore, an upstream limit to thestudy reach was set at the beginning of the poolimmediately upstream of the uppermost spawninggrounds observed, and a downstream limit one riffle/ pool sequence (about 100 m) after the lowermost grayling fry observed along the riverbank. This corres-ponded to a total reach length of about 2100 m. Characterisation of available bank habitats Five variables were described to qualitatively cha-racterise the riverbank habitat available for fry over the whole study reach. Variables and their modalities(Table 1) were selected so as to be ecologically rele-vant based on previous knowledge, easy and practicalto assess  in situ , discriminant enough to allow a clear distinction of various types of habitat, and in smallnumber to keep the method at an acceptable level of complexity. Bank slope, which could be viewed as aproxy for water depth (not measured), was describedby three modalities: level, gentle or steep. The domi-nant substrate size was assessed by means of theWentworth ’  modified scale (Malavoi & Souchon 2002),and coded in four categories: fine, gravel, pebble andboulder.  ‘ Flow type ’  characterised the hydraulics of the habitat, which is of main importance for fish withpoor swimming capacities. Areas where the water flow was mainly slowed by drag with substrate parti-cles (water velocity typically  < 15 cm  s  1 ), backwardflow areas and lentic areas where no velocity occurredwere defined. Shading was either low ( < 30%), med-ium (30%  –  70%) or high (70%), depending on theamount of light screened by the riparian or aquaticvegetation. Finally, to report shelters, the presence or absence of woody debris (immerged roots or branches), herbs or macrophytes was noted. All vari-ables were visually assessed by a team of two opera-tors so as to minimise subjectivity. Almost all fishobserved were shoaling within a 1 m strip width fromthe bank (see also Bardonnet et al. 1991); therefore,the bank habitat width was considered to be 1 m. At the beginning of emergence, available habitats wereobserved and described (on 15 April 2011) as linear units along both river banks on their entire length. Thebank length where all five descriptive variablesremained homogenous allowed categorising habitats.A significant change of modality in any of the five 3Habitat selection by European grayling fry  variables implied a change in habitat. Habitat lengthwas measured to further assess its area (in m 2 ). As flowconditions changed little over the whole fry stage(from 15 April to 26 May 2011: Mean dailyflow  =  0.59 m 3  s  1 , SD  =  0.12 m 3  s  1 , Min.  = 0.44 m 3  s  1 , Max.  =  0.93 m 3  s  1 ; Fig. 2), available Fig. 1.  Location of the Allondon River (right), with a focus on the study reach (left). The map shows the distribution of the five definedhabitat types (HTs) along both riverbanks, as well as the positioning of spawning grounds. Colours were attributed according to the overallselection value (mean Jacob ’ s D electivity index) for the HT, with the following ranks (from the more to the less selected):HT4  >  HT3  >  HT2  >  HT1  >  HT5. Note here that the riverbed and HT lengths are scaled, but not HT widths, which have been thickenedfor visibility. Also, spawning grounds may have been slightly displaced for clarity. 4Cattan  eo et al.  habitats were considered to be stable over the studyperiod. Except for area, data were binary coded (0 if themodality was absent, 1 if present) and reported in a(samples  9  16 variable modalities) matrix. Binary cod-ing was chosen both for its simplicity and to avoid ahigher subjectivity in more quantitative modalityestimates. Sampling of grayling fry and used habitat conditions Fry abundance was estimated at point samples byvisual counting from the banks (Heggenes et al. 1990;Mallet 1999). Two operators, one on each river side,walked downstream along the banks and carefullyobserved for the presence of fry. Observations con-cerned a 1.5  –  2 m strip width, but could be extendedto a 3  –  4 m width when the bank slope was level, andso the main channel habitat was farther from the bank.Each time fry were observed, the number of individu-als was counted, the GPS location recorded, and thehabitat conditions described for an area of about 1 m 󰂲 around the fish (i.e., microhabitat scale), using thesame variables (Table 1) as for the available habitat characterisation. Although counting errors inevitably Table 1. Variables used to describe the bank habitat conditions on the Allondon River.Variable Modality DescriptionBank slope Level No significant angle between the river bank and the water surface ( < 15 ° )Gentle 15 °  ≤  Angle  < 45 ° Steep Angle  ≥ 45 ° Dominant substrate Fine Particle diameter  < 2 mmIncludes clay, silt and sandGravel 2 mm  ≤  particle diameter  < 64 mmIncludes gravels and small pebblesPebble 64 mm  ≤  particle diameter  < 256 mmInclude large pebbles and cobblesBoulder Particle diameter  ≥ 256 mmHydraulics Drag Water is slowed down by friction with the substrate particlesBackward flow The main flow direction goes upstreamLentic No water velocityShading Low Small or no riparian vegetation that poorly screened sunlightMedium Riparian shrub strata that moderately screened sunlightHigh Riparian arboreous strata that highly screened sunlightShelter Woody debris Submerged stubs, roots or branchesHerbs/macrophytes Aquatic vegetationNo shelter Absence of vegetal material 00.511.522.5315 Feb01 Mar15 Mar29 Mar12 Apr26 Apr10 May24 May07 Jun    D  a   i   l  y   f   l  o  w   (  m    3  .  s   –   1    ) Date Spawning activityobservationFry countsJuvenile  AH Fig. 2.  Allondon River discharge (m 3  s  1 ) during the whole study period. Arrows indicate the sampling campaigns (fry counts), AH wasthe campaign for the available habitat description. 5Habitat selection by European grayling fry
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