Hydrobiologia (2017) 795:23–33 DOI 10.1007/s10750-016-3052-8

PRIMARY RESEARCH PAPER

Dietary traits of invasive bleak Alburnus alburnus (, ) between contrasting habitats in Iberian fresh waters

David Almeida . David H. Fletcher . Carlos Rangel . Emili Garcı´a-Berthou . Eduardo da Silva

Received: 16 February 2016 / Revised: 14 October 2016 / Accepted: 9 November 2016 / Published online: 17 November 2016 Ó Springer International Publishing Switzerland 2016

Abstract The bleak Alburnus alburnus (L., 1758) is detritus were more consumed in the river, especially a limnophilic fish native to most of Europe. This benthos and detritus during spring, whereas plant cyprinid species is a successful invader in the Iberian intake decreased during this season. Shannon index, as Peninsula, where it threatens the highly endemic fish a measure of trophic diversity, was higher in the fauna. However, studies on bleak autoecology are reservoir, except for spring. Overall results suggest scarce in this region, with few data on foraging that this wide trophic plasticity will facilitate the bleak strategies. The aim of this work was to compare the invasion process throughout Mediterranean Europe, dietary traits of bleak populations between contrasting which poses a serious risk to its highly valuable native habitats in Iberian fresh waters. For this purpose, bleak fish fauna. were seasonally collected from the small River Ge´vora and the Sierra Brava Reservoir (Guadiana River Basin, Keywords Gut content analysis Á Mediterranean southwestern Spain). As percentages of occurrence river Á Non-native fish Á Reservoir Á Trophic diversity and ingested mass, plant material and planktonic Crustacea were the most important food categories in the river and the reservoir, respectively. The intake of pelagic prey was higher in the reservoir, especially Introduction during summer. Benthic prey, plant material and Biological invasions are a major driver of biodiversity loss at the global scale (Mooney & Hobbs, 2000; Handling editor: Alison King Butchart et al., 2010). Multiple native fish species have been introduced globally, with adverse effects reported D. Almeida (&) Á E. Garcı´a-Berthou GRECO, Institute of Aquatic Ecology, University of on freshwater ecosystems (Cowx, 1998;Rahel,2002). Girona, 17003 Girona, Spain Ecological research has highlighted a variety of serious e-mail: [email protected] impacts on native communities by invasive fish via the mechanisms of hybridization, competition, predation D. H. Fletcher Department of Life and Environmental Sciences, Faculty and acting as disease vectors (e.g. Leunda, 2010; of Science and Technology, Bournemouth University, Ribeiro & Leunda, 2012).Thedisruptiveeffectoffish Poole BH12 5BB, UK invasions is of particular conservation concern in the Iberian Peninsula, where endemism is high in freshwa- C. Rangel Á E. da Silva Department of Anatomy, Cell Biology and Zoology, ter ecosystems. Indeed, [50% of the native fish species University of Extremadura, 06006 Badajoz, Spain are unique to this region, many of them being threatened 123 24 Hydrobiologia (2017) 795:23–33

(Reyjol et al., 2007; Almeida et al., 2013a). Yet, the its spread. In particular, dietary traits of non-native proportion of the ichthyofauna that non-native fishes fishes commonly show wide plasticity under contrast- represent ([30%) continues to increase (Leunda, 2010), ing environmental conditions (Almeida et al., 2009; and this is the reason why the Iberian region is Godard et al., 2013) and their assessment can be of considered a hotspot for fish invasions (Ribeiro et al., particular interest to reveal how non-native species 2008; Clavero, 2011). thrive in recipient ecosystems (Sakai et al., 2001). This The bleak Alburnus alburnus (L., 1758) is a has implications from a conservation perspective, as cyprinid species native to most of Europe, from the this information can be used by policy-makers and Pyrenees to the Ural Mountains. In their native range, environmental managers to assess the potential impact bleak inhabit lakes or still waters in medium-large of bleak on native ecosystems. Indeed, a better rivers, where they chiefly feed on invertebrates from understanding of trophic ecology can also assist the water-column or the surface (Freyhof & Kottelat, monitoring programmes in identifying which other 2008). In the Iberian Peninsula, reservoirs are key areas are most likely to be colonised by invasive locations for the introduction of non-native fish species (e.g. Almeida & Grossman, 2014). However, (Clavero & Hermoso, 2011; Clavero et al., 2013), few data on the diet of invasive bleak exist for the which is true of the bleak (Vinyoles et al., 2007). This Iberian Peninsula (e.g. Latorre et al., 2016). Therefore, limnophilic species was first introduced in Spanish information in the present paper would be highly reservoirs from France during the 1990s as a ‘forage relevant to better understand the invasive capacity of fish’ for non-native piscivorous fishes. Some examples this fish species in the Mediterranean region of Europe. of these species are northern pike Esox lucius L., 1758, In light ofthe dearth of research on the autoecology of largemouth bass Micropterus salmoides (Lace´pe`de, this invasive fish in the Iberian Peninsula, the aim of this 1802) or pikeperch Sander lucioperca (L., 1758), work was to compare bleak dietary traits between which are target fishes for recreational angling (Viny- contrasting habitats within this region. Specifically, we oles et al., 2007). Since its introduction, the bleak has analysed diet composition, seasonal variation of food displayed a strong invasive character throughout intake, trophic diversity and ontogenetic shift in bleak Iberian fresh waters, and is now a typical species in populations from a river and a reservoir located in the the fish assemblages of this region (Maceda-Veiga Guadiana River Basin (southwestern Spain). We et al., 2010). The bleak threatens a wide range of hypothesised that (i) bleak will display wide plasticity native fish fauna, mainly via trophic competition and in dietary traits according to the particular environmen- aggression, as Iberian fishes are poorly adapted to such tal conditions of each habitat type, with more intake of invaders, which usually displace them from ecological benthos in the river and pelagic prey in the reservoir resources (Leunda, 2010). For example, direct obser- (Almeida & Grossman, 2014); (ii) diet will be more vations have shown that bleak can disturb the foraging diverse in the river because of a higher habitat behaviour of the endemic Ebro nase Parachondros- complexity (Morley et al., 2008; Almeida & Grossman, toma miegii (Steindachner, 1866) (Almeida & Gross- 2014) and iii) ontogenetic shift will also reflect man, 2012). Moreover, the bleak also threatens the environmental conditions of each habitat type, showing genetic integrity of Iberian fishes through hybridiza- opposite patterns of variation between the river and the tion with the endemic calandino Squalius alburnoides reservoir (Almeida et al., 2009;Godardetal.,2013). (Steindachner, 1866) complex and Southern Iberian chub Squalius pyrenaicus (Gu¨nther, 1868) (Almodo´- var et al., 2012). Materials and methods Despite this ecological risk, the available informa- tion on bleak in the Iberian Peninsula mainly addresses Study areas spatial distribution (Vinyoles et al., 2007) and popu- lation traits (Almeida et al., 2014; Maso´ et al., 2016), The River Ge´vora (length: 74 km; catchment area: but little on ecological interactions or role. Conse- 983 km2; altitude: 170–1027 m.a.s.l.) is a small right- quently, there is a need for more research that yields margin tributary (wetted width: 3–10 m; water depth: accurate insights into bleak invasiveness throughout 0.5–2.0 m; annual discharge: 0.5–0.9 m3 s-1)ofthe Spain and Portugal, which would contribute to control River Guadiana (southwestern Spain). Bleak were 123 Hydrobiologia (2017) 795:23–33 25

Table 1 Diet composition of bleak Alburnus alburnus between habitat types. Percentages of occurrences (Oc., %) and ingested masses (Mass, %) are presented Habitat type River Reservoir Food category Oc. Mass Oc. Mass

Algae and vegetation debris 73 59 39 19 Rotifera – – 1 \1 Planktonic Crustaceaa 11 4 80 64 Ephemeroptera and Plecoptera nymphs 17 \11\1 Odonata nymphs 6 \1–– Diptera larvae 23 4 7 \1 Trichoptera larvae 17 2 1 \1 Other benthic invertebratesb 51 1\1 Nektonic and neustonic insectsc 1 \1283 Flying insectsd 18 3 21 3 Terrestrial arthropodse 52 41 Detritus 42 24 27 8 a Cladocera, Copepoda and Ostracoda b e.g. Oligochaeta and Coleoptera larvae c e.g. Corixidae and Gerridae adults d e.g. Hymenoptera, Diptera and Lepidoptera e e.g. Araneae and Formicidae collected from a 10 km section of the River Ge´vora: the river section and the reservoir (around 100 km), all 3885902300N–0685600200Wand3885501400N– these above advantages justified the selection of the two 0685702900W for the upstream and downstream bound- study areas. Furthermore, the assessment of dietary aries of the section, respectively. The Sierra Brava traits between selected contrasting habitats has been Reservoir (area: 1650 ha; volume: 232 hm3; water previously shown as appropriate to reveal wide ranges depth: 10–20 m; altitude: 303–352 m.a.s.l.) is located of phenotypic plasticity in other invasive fishes, on the River Ruecas (3981303600N–0583800600W), a although the study areas were not spatially close (e.g. right-margin tributary of the Guadiana River. The Sierra Almeida et al., 2009, 2012). Both study habitats were Brava Dam (\15 m) was mainly built to create an located in the middle part of the same catchment impoundment for water storage and sport fishing. (Guadiana River Basin) and consequently, fish assem- Consequently, the water level shows small fluctuations blage composition was similar. In particular, bleak was in this reservoir. These two study areas were selected an important species among the fish community from because bleak abundances were high enough to achieve the river and the reservoir, with only (both invasive) representative sampling sizes, whilst avoiding the effect pumpkinseed Lepomis gibbosus (L., 1758) and mosqui- of fish removal on the population throughout the study tofish Gambusia holbrooki Girard, 1859 being more period. Also, the accessibility to the sampling sites was abundant. Calandino, Iberian long-snout barbel Lucio- easy with all the sampling equipment. Moreover, bleak barbus comizo (Steindachner, 1864) and Southern were virtually confined to each particular habitat type straight-mouth nase Pseudochondrostoma willkommii (i.e. no water impoundments nearby the study river (Steindachner, 1866) were the most abundant endemic section, and the dam and unsuitable tributaries for bleak species in the study areas (see Table 1 in Almeida et al., in the study reservoir). Thus, the effect of each habitat 2014 for a comprehensive list of fish assemblages, both type (i.e. either river or reservoir) on bleak dietary traits native and non-native species). was more integrally assessed by controlling the effect of Substratum in the study areas is siliceous, with fish movements/migrations from other different habitats Precambric and Palaeozoic slates and quartzites. The adjacent to the study areas. Despite the distance between climate in this region is continental Mediterranean, 123 26 Hydrobiologia (2017) 795:23–33 with rainfall concentrated in autumn–winter After each survey was concluded, bleak were (800–1000 mm) and intense summer drought immediately immersed in an overdose solution of (\500 mm). The average annual temperature is typ- anaesthetic (MS-222) for 15 min followed by sever- ically between 10 and 15°C. The lowest temperatures ance of the spinal cord. Bleak individuals were stored occur in winter (down to –5°C) and the highest in in ice during transport to the laboratory: n = 502 from summer ([35°C). Riparian vegetation mainly consists the river (168 in spring, 121 in summer, 110 in of ash Fraxinus spp., willow Salix spp., alder Alnus autumn, 103 in winter) and n = 1015 from the spp., poplar Populus spp. and bramble Rubus spp. reservoir (353 in spring, 261 in summer, 215 in Aquatic vegetation consists of macrophytes such as autumn, 186 in winter). Individuals of the remaining water crowfoot Ranunculus spp., rushes Juncus spp. fish species were identified, counted and kept in a tank and Scirpus spp., as well as freshwater weeds of the with supplied oxygen (two battery operated aerators genera Chara and Spirogyra. The land use is mainly with portable pump) until fully recovered before being for agriculture and cattle rearing. released (only native species). All field procedures complied with use and care regulations of Field sampling and laboratory procedures Europe and Spain (specific licences were granted for Scientific Field Research in Extremadura, Spain). Fish Fish were collected from April 2007 to February 2008. were collected by trained personnel (i.e. the holder of This was a hydrologically average period for the study the licences, E. da Silva). Thus, no adverse effects areas (Ministry of Environment Spain, 2016), which were caused on the wildlife in the study habitats and avoids the effects of particular dry or wet years. Also, all native fish fully recovered from the collection this ‘average character’ of the sampling period allows methods. data to be considered as representative for this species in On arrival at the laboratory, bleak were measured the Mediterranean region of the Iberian Peninsula, for standard length (SL, ± 1 mm). This particular fish increasing the potential generality of the present find- length was the only measure of size used for data ings. To achieve an accurate assessment of the seasonal analyses, as it avoids ‘noise’ given by variation of effect on dietary traits, surveys were carried out at each caudal fin length not related to body size (e.g. wounds habitat type in April–May 2007 for spring, July–August and cuts in the fin skin and rays). Specifically, bleak 2007 for summer, October–November 2007 for autumn size ranged between 43–166 and 46–159 mm SL for and January–February 2008 for winter. To encompass the river and reservoir populations, respectively. After the existing environmental variability at each habitat fish dissections, only the anterior one-third of the type, surveys were evenly distributed across the study intestinal tract was preserved in 4% formalin for areas and fish were collected from all mesohabitats subsequent examination to avoid food remains present in both river (e.g. runs, riffles and pools) and severely digested (e.g. Duffy, 1998; Godard et al., reservoir (e.g. littoral and open-water zones). This 2013). Food types were identified to the lowest allowed collecting a representative sample of bleak possible taxonomic level (e.g. Tachet et al., 2003) populations along a wide size range from the two habitat using a dissecting microscope (409) and their types. The most appropriate catch methods were used to ingested masses were weighed using an electronic thoroughly sample each habitat type. Specifically, balance (±0.1 mg). electrofishing (2000 W DC generator at 200–250 V, 2–3 A) was used along with dip nets (1.5-m-long pole, Data analyses 30-cm-diameter net, 10-mm mesh size) in the river, following a zigzagging and upstream direction. Tram- Log-linear analysis was used to test for differences in mel nets (20 9 2m,50mmouterand10mminner the frequency of ‘gut content’ (either empty or full) mesh sizes) were also used in this lotic habitat. The across the categorical factors ‘habitat type’ and ‘season’ reservoir was sampled by electrofishing in the littoral (e.g. Godard et al., 2013). To explore seasonal patterns zone (the same procedure and equipment as described in dietary traits, indirect gradient techniques were above for the river sampling) and using two boats with performed (e.g. Garcı´a-Berthou & Moreno-Amich, trammel (see details above) and seine (40 9 5m, 2000a, b). However, non-metric multidimensional 10 mm mesh size) nets in the open-water zone. scaling (NMDS), principal component analysis (PCA) 123 Hydrobiologia (2017) 795:23–33 27 and detrended correspondence analysis (DCA) did not shape for the relationship between variables. The reveal any clear pattern, which probably indicates that complexity of GAMs was selected by a stepwise multivariate ordination methods were not suitable for procedure according to Akaike information criterion this dataset. Thus, two overall dietary indices were (AIC). Thus, only models showing the highest good- calculated (omitting empty guts) and expressed as a ness of fit and parsimony were extracted. percentage for each food category (e.g. Almeida & Gender (i.e. male/female) was not used as a Grossman, 2014): (1) occurrence (frequency of fish guts categorical factor within the statistical analyses, as in which a particular food category occurred relative to no significant effect was detected by using initial the total number of fish individuals); and (2) ingested general linear models (GLMs, overall P [ 0.05). This mass (frequency of the mass of a particular food allowed the simplification of data analyses and thus category relative to the total mass of all food cate- increased the statistical power of the remaining gories). Examination of both of these percentages sources of variation, which would otherwise be indicates whether a given food category is commonly seriously compromised. Data were transformed by eaten within the population and whether this food using ln (x ? 1). Particularly for percentage data, category is energetically important to the population. logit-transformation was used, since Warton & Hui For specific assessment on seasonal variation, ingested (2011) recommend this method to analyse proportions mass of four main food items was allocated according to in ecology research, as an alternative to arcsine square the particular ecological features of each trophic root transformation. Assumptions of normality of resource (e.g. Almeida et al., 2012): (1) ‘pelagic prey’ distributions and homogeneity of variances were sensu lato for taken from the water-column (i.e. verified through Shapiro–Wilks and Levene’s tests, plankton and nekton), water-surface (i.e. neuston and respectively. Statistical analyses were performed with terrestrial arthropods fallen on the water) or the aerial SPSS v.19 (SYSTAT Software Inc., Chicago, USA). zone (i.e. flying insects); (2) ‘benthic prey’ for animals The significance level was set at P value = 0.05. taken at the bottom (i.e. benthos); (3) ‘plant material’ Sequential Bonferroni corrections were performed for for Algae and vegetation debris and (4) ‘detritus’ for every set of multiple tests. decaying organic matter. Trophic diversity (Shannon index, H0) was also calculated for each bleak individual. Two-way analysis of covariance (ANCOVA) was Results performed to compare the dietary traits of bleak across habitat types and seasons, as well as assessing the For the frequency of gut content (i.e. empty/full), the interaction of these two factors (Godard et al., 2013). final model obtained from the log-linear analysis The effect of fish size was controlled by using SL as included the interactions ‘content’ 9 ‘habitat’ (partial the covariate. The response variables for dietary traits association: G1 = 25.24, P \ 0.001) and ‘con- were the ingested mass of four main food items (see tent’ 9 ‘season’ (partial association: G3 = 57.12, above) and the trophic diversity. ANCOVAs were P \ 0.001). Specifically, the proportion of empty guts followed by a post hoc Tukey–Kramer honestly increased from spring (7%) to winter (46%) in the significant difference (HSD) test to compare the river (also 17% for summer and 29% in autumn). The means among groups per habitat or season. Ontoge- proportion of empty guts in the river during winter was netic shift in dietary traits was analysed for each the highest percentage found for all habitat types and habitat type by estimating the relationships with fish seasons. In the reservoir, the percentages increased size (i.e. independent variable). The five response from summer (6%) to winter (30%), although they variables for dietary traits were the intake (percentage were clearly lower than those found in the river (also of ingested mass) of four main food items (see above) 18% in autumn). The proportion of empty guts was and trophic diversity (Godard et al., 2013). Relation- similar between habitat types during spring (10% in ships between independent and response variables the reservoir). were fitted by generalised additive models (GAMs) Plant material was the most common food category (e.g. Carol et al., 2006). This technique was performed in occurrence and ingested mass for the river popu- because, unlike more conventional regression meth- lation, followed by detritus. In the reservoir, plank- ods, it does not require the assumption of a particular tonic Crustacea was the most important food resource, 123 28 Hydrobiologia (2017) 795:23–33 followed by plant material (Table 1). Other food showing a seasonal ‘peak’ in this lentic habitat categories were also frequent in the river, such as (Fig. 1a). The ingested mass of benthic prey was nymphs, larvae and flying insects (17–23% in occur- higher in the river habitat throughout the seasons, but rence), although they all were only &10% of ingested particularly so in spring (Fig. 1b). Similarly, plant mass. In the reservoir, nektonic, neustonic and flying material was always consumed more in the river, insects reached 21–28% of occurrence, although they although this ingested mass decreased during spring only represented a 6% of ingested mass. Detritus also (Fig. 1c). Consumption of detritus was also higher in showed a high occurrence (27%) in the reservoir, with the river, except for autumn (i.e. similar ingested mass this food category being less important (8%) in terms between habitat types), with this season being signif- of ingested mass (Table 1). icantly lower than spring (Fig. 1d). A significant A significant interaction was found between the interaction was found between habitat types and factors ‘habitat’ and ‘season’ for the ingested mass of seasons for trophic diversity (F3,1253 = 7.76, pelagic prey (F3,1253 = 6.10, P = 0.0004). The inter- P \ 0.0001). Shannon index was higher in the reser- action was marginally significant for benthic prey voir, except for spring, when no difference was found

(F3,1253 = 3.63, P = 0.0126), after Bonferroni cor- between habitat types. Trophic diversity was stable in rection (re-calculated P = 0.0167). Seasonal variation the reservoir throughout the year, whereas Shannon was similar between habitat types for plant material index strongly increased in the river during spring with

(F3,1253 = 1.10, P = 0.3481) and detritus respect to the rest of seasons (Fig. 2). (F3,1253 = 2.01, P = 0.1107). Specifically, the Concerning ontogenetic shift in dietary traits, the ingested mass of pelagic prey was higher in the GAM technique provided only one significant and reservoir, irrespective of the season, with summer positive relationship, after Bonferroni corrections.

60 a Pelagic prey 100 c Plant material a 50 80 a 1 a 40 a a a a 60 30 a 40 20 1

20 10

0 0 Spring Summer Autumn Winter Spring Summer Autumn Winter

20 b Benthic prey 30 d a Detritus Ingested mass (mg) a Ingested mass (mg) a 25 a 15 1 1 a 20 a a 10 15 2 10 5 5

0 0 Spring Summer Autumn Winter Spring Summer Autumn Winter

Fig. 1 Ingested mass (mg) of the four main food items for bleak ANCOVAs (covariate: SL). Significant differences (Tukey– Alburnus alburnus between habitat types and seasons. River: Kramer HSD tests) between habitat types per season are open circles and dashed lines; Reservoir: full circles and solid indicated by letters (above mean) and between seasons per lines. Results are adjusted mean ± SE, after two-way habitat type by numbers (below mean) 123 Hydrobiologia (2017) 795:23–33 29

1.8 Trophic diversity current during autumn–winter, along with higher

1.6 temperatures and oxygen depletion during summer

) (Magalha˜es et al., 2007), may be behind the higher 1.4 H’ a a incidence of empty guts in the river. In the case of the 1 a 1.2 Sierra Brava Reservoir, flow regulation allows this 1.0 lentic habitat to dampen the natural hydrological 0.8 oscillations, which will better match the ecological Shannon index ( Shannon index 0.6 requirements of a limnophilic species such as the bleak. Additionally, reservoirs are warmer in winter 0.4 Spring Summer Autumn Winter and cooler in summer than typical Mediterranean rivers because of its high water volume (Wetzel, 0 Fig. 2 Trophic diversity (Shannon index, H ) of bleak Alburnus 2001). These favourable conditions of the Sierra Brava alburnus between habitat types and seasons. River: open circles and dashed lines; Reservoir: full circles and solid lines. Results Reservoir have also been evident when assessing other are adjusted mean ± SE, after two-way ANCOVAs (covariate: ecological traits of invasive bleak (e.g. see variations SL). Significant differences (Tukey–Kramer HSD tests) of size structure in Almeida et al., 2014). between habitat types per season are indicated by letters (above Spring and summer were the most ‘plentiful’ mean) and between seasons per habitat type by numbers (below mean) periods (i.e. the observed lowest frequencies of empty guts) for bleak foraging in the river and the reservoir, respectively. This finding appears to be associated This model consisted of a linear regression between with the habitat-dependent seasonal variation in trophic diversity and (log-transformed) fish size for the dietary traits, and it demonstrates the opportunistic reservoir population (H0 = 1.23 9 SL - 4.18, foraging strategy of the bleak, i.e. proportional use r = 0.49, F1,869 = 13.81, P = 0.0002). according to the environmental availability. Thus, the intake of benthic prey was higher in the river, especially during spring, when benthos activity is at Discussion its maximum in Mediterranean water courses, just prior to the summer drought (Almeida et al., 2013b). The frequency of empty guts showed that autumn and In the reservoir, the higher availability of the open- (mainly) winter months were the most restrictive water mesohabitat, along with the year-round hydro- periods for bleak foraging in both habitat types (i.e. logical stability, allowed bleak to feed largely on river and reservoir). This seasonal effect can be pelagic prey (chiefly zooplankton), especially during understood in terms of food supply and fish metabo- the hot summer. This foraging strategy (i.e. higher lism, as highlighted by Hurst (2007) in relation to the zooplanktivory in summer) is more similar to that main sources of winter mortality. At the ecosystem displayed in the native range, given the morphological level, productivity (i.e. food) is reduced during ‘cold adaptations (e.g. superior mouth) of this fish species months’. At the individual level, low temperatures (Vinni et al., 2000; Vasˇek & Kubecˇka, 2004). There- typically decrease fish activity (i.e. metabolism) and fore, bleak are able to deeply shift the natural patterns consequently, the foraging capacity (e.g. Persson, of food intake under variable conditions of flowing 1986). Furthermore, an effect of the habitat type was waters, where pelagic prey are a more limited trophic clearly observed, with the percentage of empty guts resource. This particular dietary shift (i.e. from a being globally higher in the river, especially during the pelagic to a benthic feeder) has been shown as summer season (i.e. &3 9). Small Mediterranean- advantageous elsewhere in Iberian rivers for the type rivers commonly show a high hydrological establishment of other invasive fishes, which are variability in the Iberian Peninsula (Gasith & Resh, adapted to more stable hydrological regimes in their 1999; Boix et al., 2010), with autumn–winter floods native ranges (e.g. Almeida et al., 2009, 2012; and summer droughts, which may promote harsher Almeida & Grossman, 2014). environmental conditions in the River Ge´vora (e.g. Despite these results, we found partial support for seasonal scarcity of food and habitat suitability). Thus, our first hypothesis, as bleak displayed wide plasticity a combination of lower temperatures and strong in dietary traits between habitats, with benthic prey 123 30 Hydrobiologia (2017) 795:23–33 being more consumed in the river and pelagic prey in of 1.1 to a maximum of 1.2. These values are relatively the reservoir. However, benthos was not a major food low, but they are consistent with the foraging strategy resource for bleak within the global diet composition of bleak in the native range, where the main ingested of the river. Plant material and detritus were much mass of zooplankton corresponds to only a few species more important in this lotic habitat instead. These food of Cladocera and Copepoda (Vasˇek & Kubecˇka, items are less ‘profitable’ in terms of net energy 2004). In relation to these values of trophic diversity, content, but bleak may ingest great amounts of Encina et al. (2004) considered that an H0 index equal vegetation and detritus because of strong environ- or lower than 1 would be indicating a specialist feeding mental constraints (e.g. food scarcity), as suggested by strategy. As an example, these authors found that Chappaz et al. (1987) and Vinni et al. (2000). Indeed, zooplankton foraging was highly displayed by the non- these authors reported a relationship between the high native roach Rutilus rutilus (L., 1758) in an Iberian presence of Algae and detritus in the diet of some reservoir. Their low values of trophic diversity bleak and their low growth/fecundity levels. Thus, (H0 & 1, i.e. specialist strategy) were consequently more severe conditions may be occurring in the river similar to the bleak population from the Sierra Brava throughout the year (see comments above and Reservoir, which was much more zooplanktivorous. Almeida et al., 2014), which allows plant debris and Trophic diversity was even lower in the river, although detritus to increase their benefit–cost ratios for bleak, structural heterogeneity of this habitat is overall according to the predictions of ‘optimal foraging higher. This mismatch of results with our expectations theory’ (Schoener, 1971; Griffiths, 1980). In the was likely because of the great intake of plant material reservoir, plant material and detritus were of relative and detritus in the river. Remarkably, Shannon index importance as ingested mass (&20 and &10%, showed a strong increase under lotic conditions during respectively) within the global diet composition, spring. This finding is probably related to a higher which may be explained as a mechanism for bleak to activity of different taxonomic groups of benthos (see widen their trophic niche, enabling them to more comments above and Almeida et al., 2013b), which successfully invade novel areas. This finding was suggests that benthic invertebrates are more available similarly observed for the non-native zooplanktivo- to be consumed by bleak. Detritus intake also rous rudd Scardinius erythrophthalmus (L. 1758) increased in the river during spring, although this introduced into Iberian lakes, where this cyprinid seasonal difference was less pronounced. The enrich- displays higher levels of herbivory and detritivory ment of detritus during spring could provide a than con-specifics from the native range (Garcı´a- potential explanation to understand this result (e.g. Berthou & Moreno-Amich, 2000a). Ahlgren et al., 1997). The seasonal production of River ecosystems are structurally more complex organic matter from vegetation and benthos may turn than nearby reservoirs. Therefore, trophic resources detritus into a food resource of higher nutritional that rivers can supply are usually more diverse (Morley ‘appeal’ for bleak during spring (see another example et al., 2008; Terra & Arau´jo, 2011). Accordingly, we for the cyprinid fathead Pimephales promelas expected a higher level of trophic diversity in the river Rafinesque, 1820 in Herwig & Zimmer, 2007). Thus, population, but results did not support this second the steep decrease of vegetation intake during spring, hypothesis. On the contrary, diet was more diverse in from &60 to &40 mg, may be a response to a higher the reservoir for the majority of the year, with Shannon consumption of detritus and mainly a variety of index being only similar between habitat types during benthic prey. This was the most likely cause for the spring. In the reservoir, no seasonal difference was observed increase of trophic diversity in the river found for trophic diversity, as per the intake of main during this season. food items. The only exception was the ingested mass All size classes of bleak ingested similar propor- of pelagic prey in this lentic habitat during summer, tional amounts of the main food items. This was reaching the highest seasonal mean. Despite this slight puzzling, as larger fish individuals usually have a difference and given that diet composition was virtu- greater efficiency to process different food resources ally the same in the reservoir throughout the year, because of a more developed digestive system (Mit- trophic diversity also remained stable across the telbach, 1981; Kolkovski, 2001). This generally pro- seasons, with Shannon index ranging from a minimum motes clear patterns of ontogenetic shift in dietary 123 Hydrobiologia (2017) 795:23–33 31 traits, including those of non-native fishes introduced the endemic cyprinids mentioned above. Data on diet into the Iberian Peninsula (Garcı´a-Berthou, 1999; composition for calandino and Iberian arched-mouth Garcı´a-Berthou & Moreno-Amich, 2000a, b; Garcı´a- nase appear to overlap with those from bleak, especially Berthou, 2001; Almeida et al., 2009). Only one food on benthic invertebrates (Ferna´ndez-Delgado & Her- category accounted for the main bulk of ingested mass rera, 1995; Gomes-Ferreira et al., 2005). Another (&60%) at each habitat type, which was plant material suggested impact by bleak is the ‘predation’ on Iberian in the river and planktonic Crustacea in the reservoir. fisheggs(Leunda,2010). However, this food category These were probably the most available food resources (i.e. eggs) has not been found in the present work or in for all size classes of the two study populations. This other recent publications in the Iberian Peninsula (e.g. may explain the lack of relationships between the Latorre et al., 2016). Nevertheless, environmental remaining food items and fish size. In fact, detritus managers should be aware of this possible ecological intake usually shows clear patterns of ontogenetic shift risk, given that other similar invasive cyprinids can feed for other non-native cyprinid species such as fathead on fish eggs in Iberian fresh waters (Encina et al., 2004). minnow (Godard et al., 2013). Trophic diversity was In conclusion, the present study highlights the wide the only dietary trait found effective to ‘detect’ plasticity in dietary traits displayed by bleak between ontogenetic shift, particularly in the reservoir, where contrasting environmental conditions (i.e. river vs. larger individuals showed a higher value of Shannon reservoir), when colonising new areas. Such flexibility index. Similar to lakes in the native range, juvenile/ is a common trait of invasive species (Agrawal, 2001), smaller bleak were more restricted to a mesohabitat with this being particularly evident for freshwater type in the reservoir, specifically the littoral zone (D. fishes (e.g. Marchetti et al., 2004; Ribeiro et al., 2008). Almeida, pers. observ.). However, adult/larger indi- In the Iberian Peninsula, overall results suggest that viduals display a higher ambulation capacity, which bleak invasiveness is potentially high, as this cyprinid allows them to exploit food resources at both littoral can also thrive far from reservoirs (e.g. by exploiting and open-water zones (Bı´ro´ & Musko´, 1995; Bogacka- the available food resources in flowing waters), more Kapusta & Kapusta, 2007), making the diet more than expected according to previous data (Vinyoles diverse. This positive relationship between trophic et al., 2007). As shown for other biological attributes diversity and fish size provided a degree of support for (Almeida et al., 2014; Maso´ et al., 2016), trophic the third hypothesis, which even predicted opposite plasticity will surely facilitate the invasion process of patterns of variation between habitat types, but this this fish species throughout a variety of Iberian fresh result was not observed. In small Mediterranean-type waters, including lentic and lotic habitats. Therefore, rivers, the ‘transition’ among mesohabitat types (i.e. the future spread, along with the range of impacts ecotone-like at a fine-scale) is more variable across the exerted by this bioinvasion, poses a serious risk to the seasons than in reservoirs, which could disguise the highly valuable native fish fauna of the Mediterranean partitioning/segregation of bleak population by size Europe (Ferreira et al., 2007; Reyjol et al., 2007). (e.g. Mora´n-Lo´pez et al., 2006). Consequently, the effect of mesohabitat on trophic diversity’s relation- Acknowledgements This project was funded by the FEDER ship with fish size may be also ‘diluted’. programme through the Junta de Extremadura (Project Code: 045/06, 2006–2007). David Almeida held a postdoctoral With respect to the interactions with native fish, fellowship from the ‘Beatriu de Pino´s’ programme, funded by Almeida et al. (2014) highlighted the potential compe- the Generalitat of Catalonia and the Marie Curie COFUND tition for food resources with calandino and Iberian through the University of Girona. Authors also thank the arched-mouth nase Iberochondrostoma lemmingii Associate Editor and two anonymous reviewers for helpful comments to improve the manuscript. (Steindachner, 1866), particularly in the River Ge´vora, as these two species show similar eco-morphological traits as per the bleak. 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