Predator-induced control of pests depends on the predator's preference for the target pest over naturally co-occurring prey species. We compared the larvivorous efficiency of three common freshwater predators: mosquitofish (Gambusia affinis; Baird and Girard, 1854), dragonfly naiads (Zyxomma petiolatum; Rambur, 1842) and copepods (Mesocyclops aspericornis; Daday, 1906) on different instars and the relative abundances of the mosquito Anopheles stephensi (Liston, 1901) in the presence of alternative cladoceran prey, either Moina macrocopa (Straus, 1820) or Daphnia similoides (Hudec, 1991). Larval removal rate decreased with increasing larval size and instar stage. The maximum consumption rate was by mosquitofish, followed by dragonfly naiads and copepods. The presence of either of the alternative prey significantly reduced larval consumption by all three predators, except in the D. similoides-mosquito larvae combination for naiads. Mosquitofish and copepods preferred early instars of the mosquito. Prey selectivity indices for early mosquito instars against D. similoides did not differ between mosquitofish and copepods, whereas naiads had significantly lower index values than the other two predators. Considering the negative impacts of mosquitofish on native assemblages, that is, its invasiveness and its lower selectivity for mosquito larvae, our results suggest that the feasibility of using copepods in large-scale control programs needs to be evaluated.

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There is debate about the relative importance of controlling anthropogenic nitrogen (N) versus phosphorus (P) inputs to limit algal growth in lakes and reservoirs. Our study examined nutrient responses in a subtropical reservoir using short-term algal bioassays on 3 occasions, once during the austral winter and twice during the austral summer. Measurement of photosynthetic yield (Fv/Fm) was used to determine the response to nutrient addition. For the 2 summer sampling occasions, the photosynthetic yields of the N+P treatments were significantly higher than the control. At some sites and on some occasions there was a response to P or N alone, but there was no consistent pattern. The one winter sampling occasion had no response to nutrient addition. Overall, the magnitude of the photosynthetic yield of algal samples correlated with nitrate/nitrite (NO 2 − /NO 3 − ) and soluble reactive phosphorus (SRP) concentrations, but not with ammonium (NH 4 + ), or dissolved organic N (DON) or P (DOP), despite relatively high concentrations of DON. Therefore we concluded that both N and P co-limited the growth of phytoplankton in the 2 austral summer sampling occasions. This contrasted with high N:P ratios and low P concentrations observed, which suggested that the reservoir was most likely to be P limited. This study highlights the importance of determining algal responses to nutrients and measures nutrient concentrations and ratios to determine whether N or P should be controlled to prevent algal blooms.

Background: Mosquitofish were introduced to several countries of the tropics and subtropics as biological agents for the control of mosquito larvae. Meanwhile, they became a threat to native communities and fish worldwide, similar to other invasive species through resource competition, overexploitation, or habitat alteration. We investigated prey selectivity patterns of Gambusia affinis (mosquitofish) preying on larvae of the two Indian major carps (Catla catla and Labeo rohita) in the presence of varied proportions of alternative prey (rotifers, cladocerans, chironomid and mosquito larvae) under laboratory conditions. Results: The patterns of prey selectivity in mosquitofish were influenced by the presence of alternative prey and their relative abundance in the environment. Carp larvae, when present in equal proportions, were randomly selected by mosquitofish, however, positively selected when present in higher proportions. In the presence of Hexarthra mira, Daphnia similoides or the mosquito larval instar-IV as an alternative prey, the mosquitofish preferred fish larvae regardless of prey proportions. In the medium where either mosquito larval instar-I or chironomid larvae were given as alternative prey, the mosquitofish either rejected or randomly selected the carp larvae. Given a multispecies prey combination, mosquitofish primarily selected the larvae of L. rohita and mosquito larval instar-I. We also found a prey switching ability of mosquitofish in relation to varying abundances of prey species in the environment. Conclusions: The present results suggest that mosquito immatures are not the preferred food of mosquitofish when fish larvae are present in their natural habitats. Since mosquitofish and carp larvae have overlapping natural habitats and prey preferences are the invasive mosquitofish may have a substantial impact on native communities of invertebrates and fish. This way, they are equally important for extensive fisheries and conservation management.

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