Effects of Flow on Lateral Interactions of Fish and Shrimps with Off-Channel

Hydrobiologia (2014) 729:161–174 DOI 10.1007/s10750-012-1352-1

WORLD’S LARGE RIVERS CONFERENCE

Effects of flow on lateral interactions of fish and shrimps with off-channel habitats in a large river-floodplain system

K. Go´rski • K. J. Collier • D. P. Hamilton • B. J. Hicks

Received: 31 January 2012 / Accepted: 3 October 2012 / Published online: 4 December 2012 Ó Springer Science+Business Media Dordrecht 2012

Abstract Off-channel habitats play a crucial role in into the wetland. High numbers of non-native larval the life-cycles of many riverine fish species, but lateral common carp (Cyprinus carpio) moved out of the movements of fish into these habitats are poorly wetland with retreating flood water. This study understood. We tested how flow dynamics affects the emphasises the importance of lateral connectivity movement of fish and shrimps between the main river and flooding in functioning of this river system where channel and different types of off-channel habitats: a numerous native fish, but also exotic fish, used off- riverine lake and a wetland. Our study site was the channel habitats. Floodplain management strategies lower Waikato River, North Island, New Zealand, should promote ‘controlled connectivity’ measures where there are numerous off-channel habitats. Fish that provide access for native species at key times were sampled using directional fyke nets. Shortfin eel while limiting opportunities for introduced species to (Anguilla australis) migrated mostly into the wetland utilise their favoured off-channel habitats. at night, particularly during high river flows. Common bullies (Gobiomorphus cotidianus) were most abun- Keywords Fish movement Habitat connectivity dant during the day and in low-discharge conditions, Floodplain Riverine lake Wetland Waikato River moving mostly into the riverine lake, whereas freshwater shrimp (Paratya curvirostris) moved mostly Introduction

Floodplain wetlands adjoining large lowland rivers are amongst the most biologically productive freshwater ecosystems (Tockner et al., 2009). Seasonal flow and Guest editors: H. Habersack, S. Muhar & H. Waidbacher / Impact of human activities on biodiversity of large rivers flood pulses increase connectivity between various components of riverine systems, expanding habitat K. Go´rski (&) K. J. Collier D. P. Hamilton availability for aquatic organisms (Junk et al., 1989; B. J. Hicks Tockner et al., 2000). Currently, large river–floodplain Environmental Research Institute—Te Pu¯tahi Rangahau Taiao, University of Waikato, P. O. Box 3105, Hamilton, systems that retain a high degree of natural function- New Zealand ality are rare in temperate regions, and most are highly e-mail: [email protected] modified by river regulation, e.g. construction of dams and dykes, changes in land use, and introductions K. J. Collier Waikato Regional Council, P. O. Box 4010, Hamilton, of non-native species (Bayley, 1995; Tockner & New Zealand Stanford, 2002; Nilsson et al., 2005). Off-channel 123 162 Hydrobiologia (2014) 729:161–174 habitats such as floodplain wetlands and lakes can be To make informed decisions about the rehabilita- crucial in supporting feeding, spawning and nursery tion of riverine fish production, it is essential to areas for many riverine fish species (Junk et al., 1989; understand the ecological role of off-channel habitats Baber et al., 2002; King et al., 2003; Jimeńez-Segura for both native and introduced species (Galat et al., et al., 2010;Go´rski et al., 2011a; Maganã, 2012). 1998; Buijse et al., 2002). Compared to large-scale Consequently, in many river systems, fish community longitudinal migrations within river corridors, how- structure and production are directly related to the ever, lateral fish movement and migrations in lowland quality and quantity of connections between main rivers remain one of the most poorly understood river channels and off-channel floodplain habitats dispersal mechanisms in temperate freshwater eco- (Welcomme, 1979; Moses, 1987; De Graaf, 2003; systems (Lucas & Baras, 2001; Nunn et al., 2010). Go´rski et al., 2011b). This is particularly so in New Zealand and other In European rivers, cyprinids have been shown to southern hemisphere temperate countries where the migrate from the main river channel into off-channel native fish fauna is dominated by diadromous species habitats, including tributaries (Nunn et al., 2010), (McDowall, 1990), and studies on lateral movement floodplain water bodies (Molls, 1999; Hohausova´ are scarce. et al., 2003) and temporarily inundated grasslands The Waikato River is New Zealand’s longest river. (Cucherousset et al., 2007), especially with increas- Its lower reaches were historically characterised by ing river discharge and during floods. Similar extensive river–floodplain interactions (Collier et al., phenomena have been observed in large tropical 2010a), and the river once supported the most rivers with highly predictable seasonal floods, where productive whitebait (Galaxias spp.) and eel (Anguilla the lateral migration of fish closely followed the spp.) fisheries in New Zealand (Stancliff et al., 1988; dynamic ‘pulsing’ of water levels (Junk et al., 1989), Chapman, 1996). Complex riverine habitats associ- enabling fish to access superior feeding and nursery ated with floodplain wetlands and riverine lakes habitats as water levels rose and advanced over provided important habitat for a range of species in terrestrial riparian habitats during seasonal floods the Waikato River, including many native fish that are (Wantzen et al., 2002; Castello, 2008). In the North now considered to be endangered (Collier et al., American temperate Kankakee River, for example, 2010b). Before European settlement, floodplain areas fish species adapted to seasonal flooding have been in the lower Waikato River extended to around shown to repeatedly seek out floodplain habitats 364 km2, but at present only about 53% of that area during high flows, whilst being forced to the main remains due to implementation of flood protection channel during low flows (Kwak, 1988). The schemes and changes in land-use (Collier et al., importance of off-channel habitats during low flow 2010a). Twelve introduced fish species now co-occur conditions has been stressed in recent studies from with native species, with common carp Cyprinus the Murray Darling Basin in Australia, where high carpio (Linnaeus, 1758) often dominating fish bio- numbers of both native and non-native fish moved to mass in the lower river (Hicks et al., 2010). accessible off-channel habitats during low water In this study, we investigated movement of larval levels (Lyon et al., 2010; Conallin et al., 2011). and juvenile fish between the main river channel and During winter, when low water temperatures limit two types of lateral habitats (riverine lake and metabolism and swimming performance, many tem- floodplain wetland) in the lower Waikato River. Our perate fish species prefer lentic backwater conditions specific objectives were to: (1) quantify the abundance to conserve energy (Lucas & Baras, 2001). Off- of fish moving into and out of a riverine-lake and channel habitats can also provide refuges for fish from wetland habitats in different seasons compared to high water velocities during high flows (Schwartz & movements within the main river channel, and (2) Herricks, 2005), and allow fish to restore energy investigate the role of selected environmental cues reserves after spawning (Fernandes, 1997;Go´rski associated with movement into and out of off-channel et al., 2010). However, organically enriched backwa- habitats for different fish species. We hypothesised ter and floodplain habitats may often suffer hypoxia, that seasonal variations in flow could play a crucial forcing fish to move back to flowing water of the main role in triggering lateral movement of both native and river channel (Knights et al., 1995). introduced fish from the Waikato River into off- 123 Hydrobiologia (2014) 729:161–174 163 channel habitats (Hohausova´ et al., 2003; Nunn et al., 2010). We further hypothesised that high flows could trigger movement of fish, so that shortfin eel, Anguilla australis (Richardson, 1841), might exploit off-channel habitats for feeding as well as exposed floodplain areas with retreating water, as shown for Anguill anguilla (Linnaeus, 1758) in Europe (Lasne et al., 2008). Small-bodied fish species such as¯nanga ı Galaxias maculatus (Jenyns, 1842) as well as freshwater shrimps Paratya curvirostris (Heller, 1862) might be expected to move into the off-channel habitats to avoid high water velocities in the main channel during floods (Schwartz & Herricks, 2005).

Methods

Study area

The Waikato River flows in a northerly direction for around 442 km from its headwaters above Lake Taupo to the Tasman Sea at Port Waikato (Collier et al., 2010b). It drains a total catchment of 14,443 km2 and has a mean annual discharge at the mouth of approximately 450 m3 s-1 (Brown, 2010). The catchment has been significantly altered from its natural state, mostly for agriculture (62%) and exotic forestry Fig. 1 The study area indicating sampling sites (circled fish) (19%), as well as some urban development (Collier and its location in New Zealand et al., 2010b), whilst the upper river is punctuated by eight hydro dams. A dam 152 km upstream from the remaining which have relatively undisturbed connec- river mouth effectively acts as a barrier to the natural tivity with the river main channel in the lower Waikato. upstream movement of aquatic fauna. Soils in the Lake Whangape has a surface area of 14.5 km2 and Waikato River catchment are dominated by sediments muddy sediments, and is a turbid, hyper-eutrophic lake with low infiltration rates, and therefore the river with high productivity dominated by phytoplankton; system is highly responsive to rainfall, with large peak aquatic macrophytes are absent (Hamilton et al., 2010). flood flows after heavy rain (usually in winter and A stone weir constructed in the lake for water level spring) and low flows after periods of low rainfall control limits connectivity with the river main channel (summer and autumn) (Brown, 2010). in low flows, but during moderate to high flows the weir This study was conducted in the Waikato River’s is fully submerged, allowing unrestricted fish move- lower reaches, where it forms a low-gradient river ment. Opuatia wetland covers approximately 9.5 km2 accommodating extensive floodplains which are char- and is one of the few remaining wetlands retaining acterised by peat wetlands and several riverine lakes, dominance of native restiad rushes (Clarkson et al., although these interactions are now highly regulated 2004), supported by a peat bog surrounded by minera- by a flood protection scheme which was initiated in lised margins (Beard, 2010). Water originating from the 1950 and 1960s (Chapman, 1996). Lake Whangape the peat bog may periodically induce hypoxia in the (‘Lake’) and Opuatia wetland (‘Wetland’) were outflow of the wetland (Kuder & Kruge, 2001). Some selected as off-channel habitats to quantify fish move- flood protection works have been carried out in this ment to and from the river main channel at different wetland, including construction of bunds along the discharges (Fig. 1). These habitats are amongst the few boundaries of willow and pasture areas (Beard, 2010). 123 164 Hydrobiologia (2014) 729:161–174

Data collection derive length–frequency distributions. For the less abundant fish, all individuals were measured. Catch per Water level and temperature were recorded at 1-h unit effort (CPUE) was calculated for day and night intervals at the inlet to each off-channel habitat using samples, expressed as number of fish caught per hour. automatic data loggers (3001 LeveloggerÒ Junior; Solinst Canada, Ontario, Canada). Daily discharge data Data analysis of the Waikato River at Rangiriri (approximately 5 km upstream from the main channel sampling site (Fig. 1); We used permutational multivariate analysis of variance 37°25055.8800S, 175°7041.5000E) were obtained for the (PERMANOVA) (Anderson, 2001;McArdle&Ander- sampling period from Waikato Regional Council, son, 2001) to determine differences in CPUE between Hamilton, New Zealand. In addition, spot water the main channel and off-channel habitats for different temperature, dissolved oxygen and specific conduc- species. This test was chosen because we wished to tance were recorded approximately 1 m below the assess the differences between habitats and seasonal water surface on each sampling occasion using hand- dynamics as well as their interactions, and CPUE data held devices (YSI model 55 handheld DO meter and did not meet normality assumptions for parametric tests. YSI 30 meter; Yellow Springs Instruments, OH, USA) PERMANOVA provides analysis of the variance of for conductance and temperature, respectively. data for a set of explanatory factors on the basis of Fish were sampled bi-monthly between 29 September dissimilarity measures, thereby allowing a wide range of 2010 and 16 September 2011 to encompass seasonal empirical data distributions. Models were run based on a variations and different water levels and flows. Sam- Bray–Curtis dissimilarity matrix for the whole fish pling was conducted at the Lake Whangape inlet community, and separately for the four most abundant (37°25050.7900S, 175°4042.9000E), which was character- native fish species and freshwater shrimp based on ised by water depth 1–2 m, width c. 30 m in low flow similarity among dates and sites. Data were square root- conditions, muddy sediment and aquatic macrophytes transformed prior to analysis to reduce the effect of absent, and at Opuatia wetland inlet (37°24037.5000S, outliers. For the two most abundant species—the 175°3028.5500E), where the depth was 1–2 m, the width c. common bully Gobiomorphus cotidianus (McDowall, 20 m in low flow conditions, muddy sediment and about 1975) and shortfin eel—we compared size distributions 10% areal coverage with aquatic macrophytes (mostly of individuals between habitats and directions of hornwort Ceratophyllum demersum L.) (Fig. 1). movement using a Kolmogorov–Smirnov test. To compare movement patterns into and out Finally, to identify the relationship between move- of the lake and wetland with those in the main channel ment patterns and habitat type, flow magnitude, water during each sampling, fish were also collected from temperature and measured water quality variables the shore of a mid-channel island (37°2500.1600S, (dissolved oxygen concentration and conductance), 175°4015.1600E; Fig. 1), where the depth was 1–2 m we performed redundancy analyses (RDA) (based on and sediments were sandy, while aquatic macrophytes correlation matrices) (Jongman et al., 1995; ter Braak absent. On each sampling occasion, a set of two- &Sˇmilauer, 2002) on square root-transformed CPUE directional double-wing fyke nets (2.5 m wing span) data of the five most abundant species fish moving into was set in each location over the same 24-h period. As or out of off-channel habitats (river samples were this study concentrated on small and juvenile fish, fine- excluded). Global Monte Carlo permutation tests mesh (2 mm) nets were used. The nets were set at (1,000 permutations) were performed to determine similar depths in each location, facing both upstream the significance of the ordination at a = 0.05. (‘in’) and downstream (‘out’) to catch fish moving ‘in’ and ‘out’ of the off-channel habitats. Nets were checked close to dawn and dusk to give an indication Results of diurnal movement patterns. For each sample, fish were identified to species-level based on morpholog- Flow and water quality parameters ical features and pigmentation (Koblitskaya, 1981; McDowall, 1990). Fish were measured for total length The river flow during the sampling period was (±1 mm) for at least 100 individuals if available to characterised by high discharge in spring 2010 123 Hydrobiologia (2014) 729:161–174 165

(September–October) and winter 2011 (May–August), mostly during the day (Fig. 2). In contrast, the with a short (1-week) flood peak in summer (February freshwater shrimp showed less diurnal variation in 2011) (Fig. 2, top panels). Water temperature was directional movement, with similar numbers during highest in the lake inlet and the lowest in the wetland both day and night. High river discharge in spring inlet (Table 1) over the period of sampling. During triggered movement of shortfin eel into the off- spring flooding, water temperature gradually channel habitats at night, with numbers of eels moving increased in both the lake and the wetland, and into the wetland about fivefold higher than those reached 20–24°C during the summer months (Janu- moving into the lake (Fig. 2; Table 3). In contrast, ary–February), gradually decreasing during winter common bullies were most abundant during low high flows (Fig. 2, top panels). Daily fluctuations in discharge in summer and at the river inflow into the water temperature were about threefold larger at the lake, moving in both directions but with more than lake inlet compared to the wetland inlet (Fig. 2, top 70% moving into the lake (Fig. 2; Table 3). I¯nanga panels). On average, during the sampling period, moved into both the lake and wetland in similar oxygen saturation was high in the lake inlet (*102%) numbers, and their abundances at the inlets were about and main channel (*92%) and low in the wetland twofold higher than in the main channel. Common inlet (*38%), whereas specific conductance was smelt was most abundant in the main channel. We higher in both lake and wetland inlets (191 and observed about twofold higher abundances of fresh- 206 lScm-1, respectively) compared to the main water shrimp in the wetland compared to the main channel (148 lScm-1) (Table 1). channel, with significantly more individuals moving out of the wetland after high flows (Fig. 2; Table 3). Fish movement patterns Abundances of freshwater shrimp in the inlet to the lake were lower than those of the main channel. During the entire period of sampling, we caught a total We also observed significant numbers of intro- of 2,856 fish and 1,834 freshwater shrimp (Table 2). duced fish species at inlets to both off-channel Highest numbers of fish were captured at the inlet to habitats. Gambusia was the third most abundant the lake (1,506) with about half the number in the species in the lake inlet (after native common bully wetland (837) and fewer in the main channel (513). and shortfin eel), and moved in both directions. It was Freshwater shrimp were most abundant at the inlet to also present at the other locations, but in very low the wetland (1,029), followed by the main channel numbers (Table 2). We recorded numerous larvae of (539) and lake (266). The majority of fish species, as non-native common carp moving out of the wetland well as the freshwater shrimp, moved in both direc- (Table 2). One individual each of brown bullhead tions at all sampling locations, but the overall direction catfish and rudd was caught moving out of the wetland of movement varied between different habitats. (Table 2). There were no differences in size distribu- Overall, the majority of fish moved downstream in tion of shortfin eel between habitats (Fig. 3), but we the main channel (77%) or into the lake (69%), found some indication of slightly higher numbers of whereas for the wetland, similar numbers of fish smaller common bully moving into the lake compared moved in both directions. with moving out of the lake (Fig. 3; Table 2), although The frequency and direction of fish movements these differences were not statistically significant. varied both temporally (on both diel and seasonal Results of the redundancy analyses showed that scales) and between different off-channel habitats. habitat type, flow, water temperature, dissolved oxy- Comparisons were made of abundance over time gen and specific conductance explained much of the between the different habitats and directions of variation in the abundance of fish moving in both movement for the four most abundant native fish directions (51 and 59% for fish moving into and out species, as well as the abundant freshwater shrimp of the off-channel habitats, respectively; Table 4). We (Fig. 2). Similar sampling depths and method allowed observed consistent patterns in species—environment abundance comparisons between different sites and associations for some common species (Fig. 4). indicated species-specific behaviours. The movements Common bully and gambusia were positively associ- of shortfin eels were mostly nocturnal, whereas ated with lake habitat, higher dissolved oxygen ¯nanga,ı common bully and common smelt moved concentration and higher water temperature (summer 123 166 Hydrobiologia (2014) 729:161–174

Main channel Lake Wetland ) 1 - 1200 5 s 30 5 3 30 4 4 24 800 24 3 3 18 18 2 2 400 12 1 12

Water level (m) level Water 1

6 Temperature (°C) 0 6 Daily discharge (m Daily Anguilla australis 12 12 12 In In In 6 6 6

0 0 0

6 6 6

12 Out 12 Out 12 Out

Galaxias maculatus In In In 2 2 2

0 0 0

22 2 Out Out Out

Gobiomorphus cotidianus 40 40 In 40

) In In -1 20 20 20 0 0 0 20 20 20 CPUE (no h CPUE 40 Out 40 Out 40 Out

Retropinna retropinna

4 In 4 In 4 In 2 2 2 0 0 0 2 2 2 4 4 4 Out Out Out

Paratya curvirostris 20 In 20 In 20 In 10 10 10 0 0 0 10 10 10 20 20 20 Out Out Out Oct Dec Feb Apr Jun Aug Oct Dec Feb Apr Jun Aug Oct Dec Feb Apr Jun Aug

Daily water temperature (mean ± range) Night Daily discharge / Relative water level Day

Fig. 2 Waikato River discharge at Rangiriri, and water level dynamics of ﬁsh and freshwater shrimp (bottom) in the main and temperature at Lake Whangape and Opuatia wetland channel as well as at inlets to Lake Whangape and Opuatia between September 2010 and September 2011 (top). Movement wetland between September 2010 and September 2011

123 Hydrobiologia (2014) 729:161–174 167

Table 1 Mean (SE) for water quality parameters measured at hypothesis that seasonal variations in flow can play a the study sites during sampling period (n = 17) crucial role in triggering lateral movement of fish. Location Temperature Specific Dissolved Similar behaviour has been observed in Europe for (°C) conductance oxygen cyprinids, which move from the main river channel -1 -1 (lScm ) (mg l ) into tributary (Nunn et al., 2010) or floodplain (Grift Main channel 17 (1.3) 148 (4.3) 7.2 (0.4) et al., 2001; Hohausova´ et al., 2003) habitats at high Lake 18 (1.5) 191 (12.3) 8.0 (0.5) flows. Directional movement of shortfin eels is likely Wetland 16 (0.7) 206 (6.3) 3.8 (0.5) to be for opportunistic feeding (Chisnall, 1989; Chisnall & Hayes, 1991; Lasne et al., 2008), while ¯nangaı could be searching for refugia from high flow season) for both ‘in’ and ‘out’ movements. In contrast, velocities (Schwartz & Herricks, 2005) or alterna- shortfin eel and common smelt moved mostly into the tively seeking shelter in wetland habitats where wetland, especially with higher flows and lower water predation pressure by larger fish is likely to be lower temperatures (spring). Common carp moved mostly than in the main channel (McDowall, 1990). out of the wetland habitat, mostly with higher flows Aquatic insects and large-bodied zooplankton that (spring). do not normally grow in flowing waters (Baranyi et al., 2002; Kim et al., 2002; Collier & Lill, 2008) can flourish in wetlands and floodplain retention zones Discussion where they may serve as a suitable food source for ¯nangaı and other small-bodied fish species (McDo- We observed significant numbers of fish moving to wall, 1990; Rowe et al., 2002). In tropical river and from both the riverine lake and wetland habitats systems, with highly predictable flood pulses, feeding investigated in this southern hemisphere large river, migrations during floods are widely documented supporting the conclusion that lateral connectivity (Beńech & Penã´z, 1995; Fernandes, 1997; Wantzen between the main river channel and off-channel et al., 2002; Castello, 2008). These lateral movements habitats is a key feature of ecological integrity in between the main channel and riverine lakes and riverine ecosystems (Copp, 1989; Junk et al., 1989; wetlands have also been shown to play an essential Hohausova´ et al., 2003). Moreover, the present study role in exchange of organic carbon between off- highlights the importance of different off-channel channel and main river food webs (Burford et al., habitats (i.e. lakes, wetlands) for fish productivity as 2008; Hunt et al., 2012). Conversely, in the smaller different native species varied in their use of riverine, Manu River in Peru´, with more unpredictable, short- lake or wetland habitats. In this study, we refer to duration flood pulses, fish moving into the off-channel the exchange of biota between the main river channel lake had full gut contents more often than fish leaving and off-channel habitats as ‘movements’ rather than the lake, suggesting the main channel can also be a ‘migrations’, defined as synchronised movements that preferred feeding habitat (Osorio et al., 2011). are large relative to the average home range for a Common bully movements occurred predomi- species and occur at specific stages of the life-cycle nantly during low flow conditions and higher summer (Lucas & Baras, 2001). Although it is possible that temperatures, when mostly juvenile bullies were some species interact with off-channel habitats during moving into the lake. This indicates that the produc- migrations, most of the movements in the present tive Lake Whangape, and potentially other riverine study likely occurred over a small scale in response to lakes with intact connections to the main river, may be environmental factors associated with inundation of important for common bully which has an excellent lateral habitats. ability to maintain feeding in waters with low water clarity (Rowe, 1999). Lakes such as Whangape may Flow triggers the movement of native fish also serve as a juvenile nursery. Interestingly, in contrast to the main river channel and wetland, we did Movements of shortfin eels and¯nanga ı into the not observe larger ([7 cm) common bullies at the lake wetland were driven mostly by changes in river inlet, suggesting that the more structurally complex discharge, especially spring floods, supporting our main channel and wetland habitats are more suitable 123 168 123

Table 2 Numbers of ﬁsh species and freshwater shrimp caught moving into and out of the main river channel, and lake and wetland off-channel habitats over all sampling dates combined Scientiﬁc name Common name In Out

Main channel Lake Wetland Main channel Lake Wetland

No. L No. L No. L No. L No. L No. L

Ameiurus nebulosus (Lesueur, 1819)a Brown bullhead catfish – – 1 5.8 – – – – – – 1 6.6 Anguilla australis (Richardson, 1841) Shortfin eel 19 39.4 (4.5) 131 39.9 (0.9) 255 29.5 (0.8) 6 18.8 (8.5) 44 38.6 (1.3) 62 34.3 (1.8) Anguilla dieffenbachii (Gray, 1842) Longfin eel 8 45.5 (2.1) 4 41.4 (2.3) 3 43.3 (6) 1 51.5 – – 1 25.3 Carassius auratus (Linnaeus 1758)a Goldfish – – 9 5.7 (0.6) – – – – 4 5.5 (1.4) 4 5.6 (1.4) Cyprinus carpio (Linnaeus, 1758)a Common carp – – 1 7.8 – – – – 3 1.1 41 1.2 (0.03) Gambusia affinis (Baird & Girard, 1853)a Gambusia 2 2.9 (0.6) 36 3.1 (0.1) – – 1 2.1 77 2.9 (0.1) 2 2.4 (0.2) Galaxias maculatus (Jenyns, 1842)¯ Inanga 10 5.7 (0.2) 18 5.8 (0.2) 18 7.5 (0.3) 4 4.4 (1.4) 21 5.5 (0.3) 6 6.5 (0.5) Gobiomorphus cotidianus (McDowall, 1975) Common bully 35 5 (0.7) 834 2.4 (0.1) 123 4.3 (0.2) 371 3.1 (0.6) 296 3 (0.1) 305 4.7 (0.2) Retropinna retropinna (Richardson, 1848) Common smelt 45 6.2 (0.2) 1 5.7 15 8.1 (0.4) 11 6.1 (0.2) 26 6.5 (0.3) – – Scardinius erythrophthalmus (Linnaeus, 1758)a Rudd – – – – – – – – – – 1 11.2 Paratya curvirostris (Heller, 1862) Freshwater shrimp 247 92 391 292 174 638

Average (SE) total length (L) of species caught is also shown yrbooi 21)729:161–174 (2014) Hydrobiologia a Introduced species Hydrobiologia (2014) 729:161–174 169

Table 3 PERMANOVA results conducted on CPUE for the ﬁsh community as well as four most abundant native ﬁsh and native freshwater shrimp individually Fish considered Source df SS Pseudo-FP

Community (all ﬁsh species) Location 2 7,556.4 5.5792 0.001 Date 5 8,860.6 2.6168 0.001 Location 9 date 10 10,482.0 1.5479 0.023 Residual 36 24,379.0 Anguilla australis Location 2 1,662.9 6.3287 0.004 Residual 36 4,729.7 Galaxias maculatus NS Gobiomorphus cotidianus Location 2 4,649.0 13.914 0.001 Date 5 4,547.4 5.4441 0.001 Location 9 date 10 5,427.6 3.2489 0.002 Residual 36 6,014.1 Retropinna retropinna NS Paratya curvirostris Location 2 958.0 4.0972 0.006 Date 5 2,687.6 4.5978 0.001 Location 9 date 10 8,588.8 7.3467 0.001 Residual 36 4,208.7 df Degrees of freedom, SS sums of squares, Pseudo-F distance-based pseudo F statistic, P probability values (obtained using 999 permutations of residuals under a reduced model)

Main channel Lake Wetland Anguilla australis In In In 60 40 20 0 Out Out Out 20 40 60

<10 -69 <10 <10 10-19 20-29 30-39 40-49 50-59 60 70-79 10-19 20-29 30-39 40-49 50-59 60-69 70-79 10-19 20-29 30-39 40-49 50-59 60-69 70-79

Gobiomorphus cotidianus

Frequency (%) In In In 60 40 20 0 Out Out Out 20 40 60

<1 .9 .9 <1 .9 .9 <1 .9 -8 -9 -2 9 1-1.92-2.93-3.94-4.95-5.96-6.97-7.98 9 -10.9 1-1.92 3-3.94-4.95-5.96-6.97-7.98-8.99-9.9 1-1.92-2.93-3.94-4.95-5.96-6.97-7.98-8.99- 2-12.9 0-10.9 2-12.9 0-10.9 10 11-11.91 1 11-111 1 11-11.912-12.9 Length class (cm)

Fig. 3 Length–frequency relationships of the two most abundant native species, shortﬁn eel (Anguilla australis) and common bully (Gobiomorphus cotidianus), for each location and movement direction 123 170 Hydrobiologia (2014) 729:161–174

Table 4 Eigenvalues and cumulative percent variance explained for wetland habitats with abundant vegetation (in parentheses) from redundancy analyses (axes I–III) of habitat (Carpenter, 1982). Vegetated wetland habitats can type, flow magnitude and temperature explaining the abundance of provide shelter from predation (Jordan et al., 1996; moving fish Banha & Anastaćio, 2011), as well as detritus and a Direction P Axes periphyton as a source of food (Azim, 2005). Indeed, I II III shrimps moved in large numbers into and out of both of the off-channel habitats, mainly during low flows In \0.01 0.450 0.036 0.015 and high summer temperatures, suggesting dynamic (45) (48.6) (50.1) interactions between river and off-channel habitats for Out \0.01 0.375 0.119 0.048 this species, depending on environmental cues. (37.5) (49.4) (54.3) a Based on 999 permutations (test of significance of the first The role of temperature and water quality canonical axis vs. all canonical axes gave the same results) As ectotherms, fish are generally more active at the for adults of this species which can use aquatic higher end of their preferred thermal optimum (Lucas vegetation as a spawning substrate (McDowall, 1990). & Baras, 2001). This would explain the frequent Habitat complexity may also account to some movements to off-channel habitats that we observed degree for the strong preference of freshwater shrimp during high discharge in spring, but not during winter

Fig. 4 Redundancy In analyses of ﬁsh CPUE of 1 different species explained by habitat type, ﬂow magnitude, temperature and Flow A. australis selected water quality G. affinis variables. The greater the Temperature similarity in length and G. maculatus Dissolved oxygen Lake direction of the vectors, 0 the stronger the association AX2 G. cotidianus between the abundance Wetland Specific of particular species and R. retropinna conductance associated environmental characteristics

-1 -1 0 1 AX1

Out 1

G. affinis Specific G. maculatus C. carpio conductance Temperature Lake A. australis Dissolved oxygen 0

AX2 G. cotidianus Flow Wetland

-1 -1 0 1 AX1 123 Hydrobiologia (2014) 729:161–174 171 when water temperatures remained low. Off-channel maximise habitat availability and feeding opportunities habitats inundated during spring floods are often for key life stages of native fish as well as providing warmer than the main river channel and thermally refugia at important times of year. Similar findings have heterogeneous (Go´rski et al., 2010), suggesting inun- been shown for large European rivers (Lasne et al., dation may trigger movements in search of optimal 2007), where numbers of native species increased with temperature regimes (Lucas & Baras, 2001). In increasing connectivity. Furthermore, body condition support of the role of thermal cues, adult barbel of European eels Anguilla anguilla (Linnaeus, 1758) Barbus barbus (Linnaeus, 1758) were shown to has been shown to decrease with decreasing lateral progressively shift their diurnal pattern of feeding in connectivity (Lasne et al., 2008), potentially because order to move to the foraging places at the time of the inundated terrestrial habitats provided food of better day when water temperature is the closest to their nutritional quality (Van Liefferinge et al., 2012). A thermal optimum (Baras, 1995). In contrast to similar reliance on lateral connectivity with wetlands is Hohausova´ et al. (2003), who showed that water supported for shortfin eels in the lower Waikato River quality parameters had little influence on fish move- (Chisnall & Hayes, 1991). ment into off-channel habitats in the River Morava However, high numbers of non-native larval com- (Czech Republic), we observed that variations in mon carp moved out of the wetland with retreating specific conductance and dissolved oxygen between flood water, and the introduced gambusia was different off-channel habitats could partly explain fish recorded in both off-channel habitats through the movement into and out of these habitats. This finding year. Maintenance of high flows and artificial floods indicates that, after the governing roles of flow and may potentially be important for preservation of native temperature are accounted for, other environmental fish communities in river systems in which most of the characteristics that define off-channel habitats (i.e. introduced fish species are adapted to lentic conditions water quality) may also play a role in initiating fish (Bernardo et al., 2003). In the lower Waikato River, movement. In support of this, North American however, flooded wetland habitats appear to serve as centrarchids overwintering in backwater lakes on the superior spawning habitat for phytophilic common upper Mississippi River were shown to move out of carp whose larvae were highly abundant moving out lentic areas when oxygen levels dropped below of the wetland with receding spring floods, as has 2mgl-1 (Knights et al., 1995). Similarly, higher also been observed for this species in the Australian numbers of common bullies moving into the lake Murray River (Stuart & Jones, 2006). Indeed, adult habitat in our study could, to some extent, reflect a common carp were recorded to move frequently to off- preference for turbid waters (Rowe, 1999). Factors channel habitats during spring spawning time in both stimulating fish migration and movement are complex the Australian Murray River (Jones & Stuart, 2009) (Lucas & Baras, 2001), with multiple cues working in and the lower Waikato River (Daniel et al., 2011). concert, and movement occurring when factors pro- Therefore, to be successful, management aimed at moting residency are outweighed by those stimulating enhancing ecological integrity must consider the movement, irrespective of the physiological or envi- potential negative and ongoing implications of non- ronmental nature of those cues (Ovidio et al., 1998). native fish movement and reproduction in off-channel habitats, which our study has shown can provide Management implications important nurseries for invasive common carp. Para- doxically, main channel–off-channel connections can This study emphasises the importance of lateral serve as natural movement bottlenecks for such hydrologic connectivity and seasonal flooding of off- species and therefore may provide opportunities for channel habitats in the functioning of this southern targeted control of non-native invasive fish species hemisphere temperate large river system, as demon- during movement phases. Appropriate management strated by the numerous native fish using the lake and of floodplain ecosystems may therefore involve wetland during the flood and at low flows. Therefore, implementing ‘controlled connectivity’ measures that management strategies that promote connectivity provide access for native species at key times while within lowland river–floodplain ecosystems and reha- limiting opportunities for introduced species to utilise bilitate (semi-)natural flow dynamics are likely to favoured off-channel habitats. 123 172 Hydrobiologia (2014) 729:161–174

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