Transactions of the Royal Society of South Australia (2014), 138(1): 113–123
EXPLOITATION BY COMMON CARP (CYPRINUS CARPIO) OF A FLOODPLAIN WETLAND OF THE LOWER RIVER MURRAY UNDER DROUGHT AND FLOODING CONDITIONS L. ViLizzi 1,4, L.A. ThwAiTes 2, & B.B. smiTh 3
1. murray-Darling Freshwater Research Centre P.O. Box 991, wodonga, Vic 3689, Australia 2. south Australian Research and Development institute Aquatic sciences, west Beach, sA 5024, Australia 3. Department for water, Level 2 25 Grenfell street, Adelaide, sA 5000, Australia 4. Corresponding Author. ichth-Oz environmental science Research, 972 irymple Ave., irymple Vic 3498, Australia. email:
[email protected]
Abstract
Common carp Cyprinus carpio are a declared pest fish throughout the murray-Darling Basin. successful management of this highly-invasive species aimed at mitigating its detrimental effects on aquatic ecosystems is therefore a priority issue for wetland restoration and rehabilitation programmes for the benefit of native fish communities. As part of a large-scale, field-based experiment on carp impacts, a population of carp was extensively sampled from 2009 to 2011 at Brenda Park wetland (lower River murray, south Australia) under both drought and flooding conditions. A consistent young-of-year (YoY) cohort was present in the 2009–10 breeding season, indicating successful spawning within the wetland. however, the same cohort appeared not to leave the wetland later in the season, possibly as a result of drought-induced low-flow conditions. Conversely, there was no evidence for within-wetland spawning in 2010–11 and 2011–12. Following a major flood event in summer/Autumn 2011, a consistent stock-size (210–440 mm TL) component was present in the carp population. Despite one successful spawning event out of the three breeding seasons of sampling, it cannot be ruled out that Brenda Park can act as a recruitment hot-spot for carp, especially in light of the large-scale population dynamics of this species. measures for wetland rehabilitation should account for this and rely on a combination of flow regulation practices, enhancement of native fish passage and integrated carp management/control measures involving use of selective traps or exclusion screens, removal programmes and, whenever feasible, water draw-downs. KeYwORDs: cohort, hot-spot, management, proportional stock density, young-of-year (YOY).
Introduction
Common carp (Cyprinus carpio; hereafter, ‘carp’) are a declared pest fish in south-eastern Australia (Koehn 2004), where they are implicated for a wide range of detrimental impacts on aquatic ecosystems (Koehn et al. 2000). successful management of this highly-invasive species has therefore become a priority issue as part of a broader strategy for river and wetland rehabilitation programmes aimed to benefit native fish communities across the murray-Darling Basin (mDB) (Barrett 2004). This is especially critical in those habitats already degraded by human disturbance (e.g. smith et al. 2009), which are readily exploited by carp in virtue of their opportunistic requirements (Balon 1974, 2004). The ultimate consequences of these invasions are an overall decrease in native biota diversity resulting from profound aquatic ecosystem changes caused by carp acting as an ‘ecosystem engineer’ (Jones et al. 1994; Byers et al. 2006), and often accompanied by high costs in measures for control and eradication (if at all possible/feasible) as well as economic losses due to deterioration of amenity values (e.g. Koehn et al. 2000).
in the mDB, carp comprise >80% of the fish biomass in some river reaches (harris & Gehrke 1997; Rolls 2005), and in many wetlands this figure is even higher (Nichols & Gilligan 2004). By way of example, carp 113
L. ViLizzi, L.A. ThwAiTes & B.B. smiTh
were found in 69 out of 74 wetlands of the lower River murray in south Australia surveyed between 2004 and 2007 (smith et al. 2009), and were the most abundant large-bodied fish species in five out of six wetlands of the middle River murray sampled between 2007 and 2010 (Beesley et al. 2012). indeed, carp are now the dominant fish across the mDB (Forsyth et al. 2013), and demands for management and control are persistent and widespread (Koehn et al. 2000).
Previous studies of carp in wetlands of the lower River murray have either focussed on specific life-history stages (e.g. Vilizzi 2008), provided ‘snapshots’ in time about their status (e.g. smith et al. 2009), or carried out intensively over a relatively short time-frame (Conallin et al. 2012). Also, persisting drought conditions as a result of the ‘millennium drought’ throughout the 2000s (Bond et al. 2008; murphy & Timbal 2008) have constrained monitoring studies on carp across the mDB within the widespread conditions of low and relatively stable flows (mDBA 2011). As part of a large-scale, field-based experiment aimed at quantifying the effects of carp on semi-arid floodplain ecosystems (Vilizzi et al. 2013c), a population of carp was extensively sampled over the course of two years in a wetland of the lower River murray under both drought and flooding conditions. This provided for a unique opportunity to study the size-structure and population dynamics of this carp population and its exploitation of the study area. The objectives of the present study were therefore to: (i) identify temporal changes in carp size-structure and recruitment and relate them to drought and flooding conditions occurring during the sampling period; (ii) investigate the possibility that the study wetland may act/have acted as a potential ‘hot-spot’ for carp recruitment; and (iii) discuss in light of the findings options for wetland rehabilitation, including flow regulation practices, enhancement of native fish passage as well as integrated carp management and control measures with potential for application to other wetlands of the lower River murray and elsewhere across the mDB. Study area
Methods
Brenda Park wetland is a terminal wetland of the lower River murray in south Australia (34° 40’ s, 139°41’ e), and is connected to the adjacent River murray channel via an inlet (scott’s Creek) (Fig. 1). in 1996, the Brenda Park scott’s Creek wetlands Rehabilitation Group (BPsC-wRG) was formed due to the wetland having a blue-green algae bloom that prevented use of the inlet creek. work began with eradication of weeds and removal of motorbikes, horses and livestock from the wetland area, and carp removal also occurred later in 1997. in 2001, Brenda Park station (on which the wetland is situated) was purchased as private property with the owners undertaking a works programme in 2002 (in partnership with BPsC-wRG, Riverland west Local Action Planning Group and Prince Alfred College), which again included carp removal and the installation of a flow control structure fitted with carp screens in the Brenda Park causeway road (pipe culverts) and in the causeway at the scott’s Creek end (box culverts). As a result, manipulation of flooding of the wetland has since become possible through scott’s Creek culvert. Also, some data on fish, vegetation, birds, frogs and groundwater exist from previous baseline surveys, as well as from student research on lignum (Muehlenbeckia florulenta) and river red gum (Eucalyptus camaldulensis) (Nickolai 2011).
Sampling
sampling took place over a two-year period for a total of four occasions: 03–06/11/09 (Round i: spring 2009), 13–16/04/10 (Round ii: Autumn 2010), 28–30/09/10 (Round iii: spring 2010), and 01–04/11/11 (Round iV: spring 2011). Notably, Rounds i and ii occurred during the 2009–10 fish breeding season (taken to start in August and end in July of each year: Vilizzi 2012), and Rounds iii and iV during the 2010–11 and 2011–12 seasons, respectively.
sampling occurred within forty-five experimental plots (20 m × 20 m each), consisting of 15 fully-fenced exclosures, 15 semi-fenced plots and 15 un-fenced plots set up across the range of habitats encountered in the wetland (Vilizzi et al. 2013c). Fenced plots consisted of double-galvanised waratah® chicken wire (hexagonal shape, 5 cm maximum horizontal aperture mesh, 1 mm thickness) and prevented access of carp above a threshold size of 181 mm TL (estimated after hillyard et al. 2010). 114
exPLOiTATiON BY COmmON CARP OF FLOODPLAiN weTLAND, LOweR RiVeR muRRAY, uNDeR DROuGhT AND FLOODiNG CONDiTiONs
Figure 1. map showing Brenda Park wetland (south Australia) with indication of the inlet channel scott’s Creek, the flow control structure and the adjacent River murray.
One small-mesh fyke net (10 mm stretched mesh, 5 m leader, 3 m funnel, seven support rings, three chambers) and one multi-panel gill net (15 m total length, comprising 3 × 5 m panels of 3″, 4″ and 5″ mesh each) were used within each of the 45 plots in Rounds i and ii, and within 36 plots in Rounds iii and iV (no sampling of nine plots was possible due to logistic constraints). All nets were set in the afternoon and retrieved the next morning, with nine to twelve plots sampled per night. Choice of the fishing gear was aimed to sample carp of all range of sizes, i.e. including their young-of-year (YoY), 1+ (and older) juveniles, and adults. All fish were processed immediately after capture, with the first 30 individuals sampled in larger numbers measured for total length (TL; nearest mm) and weighed (nearest g), and with the remaining fish only counted. Carp were returned to the water alive to avoid biassing future samples. Data preparation and statistical analysis
Because TL could not be measured on all carp sampled, it was estimated for the un-measured fish based on the mean TL of the other fish caught in each plot and round of sampling. This extrapolation method was justified in that the supernumerary fish not originally measured were in the same size class as those measured. To distinguish YoY from older individuals, a maximum size sL = 148 mm (Vilizzi et al. 2013b) was used and converted into TL = 186 mm (sL → TL conversion factor value of 1.256: Froese & Pauly, 2013). Also, a TL = 280 mm was taken as the mean value (males and females) for size at maturity (after Brown et al. 2005). For each sampling round, carp size structure was summarised using proportional stock density (PsD), which was calculated as the ratio of quality-sized to stock-sized fish (cumulative sums) multiplied by 100 (Ogle 2011). The following size classes (TL) were identified (after Abernethy 2006): ‘zero’ (840 mm). Proportional stock density was computed in the R language and software environment for statistical computing and graphics v2.14.2 (R Development Core Team 2010) using libraries FsA and NCstats, and following guidelines in Ogle (2011). 115
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Results
Following inundation of Brenda Park wetland on 14/04/09, flows in the adjacent River murray remained historically very low (
