river health gudgeons and bony herring. The model also projected increases in abundance of popular angling species such as golden perch in the longer term. Modelled responses were more persistent over time than observed responses, indicating further refinement of the model is required to reproduce the short duration of observed responses. The model was not validated against biomass time series from this study because the default time step for temporal simulations in Ecosim is one year. Further refinement of the model is required for validation against time series data collected at intervals of less than one year. The model has potential for further development to estimate potential responses to carp reduction at other wetlands in the Murray-Darling Basin.
Discussion Uncertainty in the carp population estimates obtained means that the starting and final populations are not known accurately. However, the number and biomass of carp removed is known with certainty. Findings from this study are significant from several perspectives. Firstly, while ecological responses to carp removal have been widely demonstrated (Pinto et al., 2005), this study provides evidence of a succession of responses across multiple trophic levels following carp reduction. The specific succession pattern differed between lagoons, but the generalised succession following carp reduction was evidenced as (i) an increase in biomass of large zooplankton; (ii) an increase in taxa richness and species diversity of benthic macroinvertebrates; and (iii) increased biomass of gudgeons and bony herring. In Warra Lagoon, the specific sequence occurred as (i) increased biomass of large zooplankton; and (ii) a lagged increase in biomass of gudgeons and bony herring. Observed changes in nutrient availability and phytoplankton biovolume could not be attributed to carp removal. In Rainbow Lagoon, succession was observed as (i) increased biomass of large zooplankton; (ii) a lagged increase in taxa richness and diversity of benthic macroinvertebrates; and (iii) increased biomass of bony herring. An observed increase in nutrient availability was inconsistent with expected responses to carp removal, since carp typically contribute to the pool of bioavailable nutrients, especially NH4, so their removal should have reduced both N and P (King et al., 1997, Pinto et al., 2005). The increase may be explained
94 AUGUST 2011 water
Presentedatat presented
by factors such as drying and re-wetting of the lagoon’s sediments, which has been demonstrated to increase nutrient concentrations. It is possible that effects of carp removal on nutrient availability were masked by the drying-wetting cycle, combined with nutrient inputs with runoff. Carp manipulations typically affect zooplankton biomass and size structure. Addition of carp reduces large zooplankton such as Daphnia and total zooplankton biomass (Hrbáĉek et al., 1961). Shifts in the size composition of zooplankton have been associated with changes in the steady state of lakes and wetlands from turbid to clear water regimes (Folke et al., 2004). Responses of zooplankton taxa to carp removal tend to be size-specific (Weber and Brown, 2009). Small carp consume zooplankton as a major component of their diet (Vilizzi, 1998) and may reduce zooplankton biomass through predation pressure (Schrage and Downing, 2004; Weber and Brown, 2009), releasing phytoplankton from grazing effects. In the present study, large zooplankton taxa, particularly Boeckella, Daphnia carinata and Daphnia lumholtzii showed strong increases in biomass following carp reduction, and are likely to have exerted strong grazing pressure on phytoplankton. Once phytoplankton densities declined, zooplankton biomass stabilised near original biomass values on subsequent sampling occasions. Small zooplankton taxa such as Bosmina and Moina showed no response to carp reduction. These taxa tend to be ineffective grazers of phytoplankton, and have limited capacity for top-down regulation of phytoplankton by grazing. The short-term costs and benefits of carp reduction strongly favour carp removal. This study has shown that reducing carp biomass by 33%–43% can result in an increase in biomass of native fish species by between 240% and 1,130% for bony herring, and over 1,600% for gudgeons. In absolute terms, removal of 26–34kg ha-1 of carp biomass resulted in a three-fold increase in native fish biomass of more than 90 kg ha-1. This increase in native fish production is consistent with the increased availability of zooplankton following carp reduction, and known dietary preferences of bony herring and gudgeons (Meredith et al., 2003; Balcombe et al., 2005; Balcombe and Humphries, 2006; Sternberg et al., 2008; Medeiros and Arthington, 2008). Responses of bony herring are similar to observed dynamics of the closely-related gizzard shad (Dorosoma cepedianum) in North America. Gizzard
refereed paper
shad biomasses of less than 20–30kg ha-1 allow large zooplankton, especially Daphnia, to increase in abundance (Schaus et al., 2002). When zooplankton are not abundant, gizzard shad switch to feed on less-nutritious benthic detritus. In our treatment lagoons, bony herring biomass increased from less than 20kg ha-1 to 60–100kg ha-1 following the increase in zooplankton biomass. As bony herring biomass declined towards the end of the study to less than 20kg ha-1 zooplankton biomass increased again. These results suggest that of the full set of potential ecosystem responses to carp reduction, only a subset of responses may be demonstrated in individual locations because of the influence of local drivers and constraints. The transient nature of observed responses by zooplankton and native fish is in contrast with the longerterm response predicted by food web modelling. The observed short-term responses, therefore, provide an indication of the magnitude of potential environmental benefits of carp control, but further investigation is required to determine the level of responses that may be achieved over several years. Additional work is also required to optimise the subsequent carp removal efforts required to prevent recovery of carp populations. This study has demonstrated that modest reductions in carp biomass can provide significant benefits for native fish, which, if continued, may be expected to translate into longer-term increases in native fish populations. Carp in turbid wetlands interact strongly with native fish through pelagic food web pathways involving zooplankton, as well as benthic macroinvertebrate pathways. Carp reduction has the potential to contribute significantly to restoring populations of native fish by increasing food availability. Environmental outcomes of carp reduction include direct conservation benefits to native fish, potential increases in popular recreational species including golden perch and Murray cod, and improved aquatic ecosystem health. Because of the rapid increase in native fish abundance, reduction in the number of juvenile carp is not expected to reduce prey availability for large native fish, such as golden perch and Murray cod, that include small carp in their diets. Rather, piscivorous fish are likely to experience increased prey availability as a result of carp reduction. The magnitude of carp removal required to achieve these outcomes is realistic for local community groups to pursue with
technical features