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Friederichs S.J.,Thomas University | Zimmer K.D.,Thomas University | Herwig B.R.,Fisheries Research | Hanson M.A.,Wetland Wildlife Populations and Research Group | Fieberg J.R.,Biometrics Unit
Oikos | Year: 2011

We assessed the relative influence of total phosphorus and piscivore biomass on the abundance of benthivores, soft-rayed planktivores, spiny-rayed planktivores, zooplankton and phytoplankton in 69 shallow lakes in the prairie and parkland areas of Minnesota, USA. Piscivore biomass was the best predictor for three of these response variables, exhibiting a negative relationship with soft-rayed planktivores, a positive relationship with benthivores, and a weaker positive relationship with large-bodied cladocerans. Total phosphorus and piscivores comprised the best model for predicting spiny-rayed planktivores, while neither variable showed any strong relationship to small-bodied cladocerans. Total phosphorus was positively related to phytoplankton abundance, and was the best predictor among all candidate models. Moreover, contrary to predictions of trophic cascade theory, the relationship between chlorophyll a and total phosphorus did not differ between lakes with and without piscivores. Our results indicated top-down influences of piscivores extended through parts of two trophic levels, but failed to influence zooplankton - phytoplankton interactions, leaving phytoplankton abundance constrained largely by total phosphorus. Lack of a relationship between piscivores and phytoplankton was likely due to high densities of larval planktivores less susceptible to piscivory, as well as positive influences of spiny-rayed planktivores and benthivores on algal abundance. These results support the idea that top-down influences of piscivores on phytoplankton abundance may be reduced in more diverse fish communities where some prey species are less susceptible to piscivory. © 2011 The Authors. Source

Bajer P.G.,University of Minnesota | Parker J.E.,University of Minnesota | Cross T.K.,Fisheries Research | Venturelli P.A.,University of Minnesota | Sorensen P.W.,University of Minnesota
Oikos | Year: 2015

Although partial migration, a phenomenon in which some individuals in a population conduct seasonal migrations while others remain resident, is common among animals, its importance in facilitating biological invasions has not been demonstrated. To illustrate how partial migration might facilitate invasions in spatially complex habitats, we developed an individual-based model of common carp Cyprinus carpio in systems of lakes and winterkill-prone marshes in the Upper Mississippi River Basin (UMRB). Our model predicted that common carp are unable to become invasive in lakes of the UMRB unless they conduct partial migrations into winterkill-prone marshes in which recruitment rates are high in the absence of native predators that forage on carp eggs and larvae. Despite low dispersal rates of juveniles and higher mortality rates of migrants, partial migration was adaptive across a wide range of migration rates and winterkill frequencies. Partial migration rates as low as 10% and winterkill occurrence as infrequent as once in 20 years were sufficient to cause invasiveness because of carp's reproductive potential and longevity. Consistent with the results of our model, empirical data showed that lake connectivity to winterkill-prone marshes was an important driver of carp abundance within the study region. Our results demonstrate that biological invasions may be driven by a small, migratory contingent of a population that exploits more beneficial reproductive habitats. © 2015 Nordic Society Oikos. Source

Nolby L.E.,Thomas University | Zimmer K.D.,Thomas University | Hanson M.A.,Wetland Wildlife Populations and Research Group | Herwig B.R.,Fisheries Research
Freshwater Biology | Year: 2015

The classic island biogeography model (IBM) predicts highest species richness in large, connected habitats due to colonisation and reduced risk of extinction. Promoting large, connected habitats has subsequently become a common theme in conservation biology. However, the IBM does not account for direct and indirect interactions among species. For example, planktivorous and benthivorous fish may reduce biodiversity in shallow lakes by inducing shifts to a turbid-water lake state with low habitat complexity. We assessed relationships between species richness, landscape features, fish biomass and lake state in 104 shallow lakes in Minnesota, U.S.A. First, we tested whether lake size and connectivity influenced species richness of fish and biomass of planktivores and benthivores (fish biomass). We subsequently tested whether fish biomass affected the probability that lakes were in turbid versus clear-water states. Finally, we tested whether species richness of macrophytes and taxon richness of aquatic invertebrates showed stronger relationships with lake size and connectivity or with fish biomass and lake state. Fish richness and biomass both increased with lake size and were higher in connected basins. Fish biomass, in turn, increased the probability that lakes would be turbid. In contrast, macrophyte and invertebrate richness were unrelated to lake size or connectivity. Instead, macrophyte richness was best predicted by lake state, while invertebrate richness was predicted by lake state and fish biomass. Richness of both macrophytes and invertebrates was higher in clear lakes, and invertebrate richness was inversely related to fish biomass. Our results indicate the IBM poorly explains the diversity of macrophytes and invertebrates in shallow lakes, with diversity more strongly driven by biotic interactions and influences associated with fish. We suggest that ecological implications of increased connectivity and lake size should be considered in future conservation strategies for shallow lakes. © 2015 John Wiley & Sons Ltd. Source

Harasti D.,Australian Department of Primary Industries and Fisheries | Malcolm H.,Australian Department of Primary Industries and Fisheries | Gallen C.,Australian Department of Primary Industries and Fisheries | Coleman M.A.,Fisheries Research | And 2 more authors.
Journal of Experimental Marine Biology and Ecology | Year: 2015

Baited Remote Underwater Video (BRUV) has become a popular technique to survey fish assemblages for a wide range of purposes. BRUV methodology has, however, also varied greatly for a range of reasons. A major dichotomy occurs, in particular, in the time used to sample the fish assemblages i.e. the BRUV set time. The aim of this study was to determine whether differences in set time were likely to affect the conclusions reached by studies using different set times, and what might be the most appropriate and cost-effective set time to use to sample temperate reef fish assemblages. In this study, we test whether there were significant differences between the two main set times used (i.e. 30 and 60. min) in BRUV sampling for assemblage patterns, species diversity, and relative abundance of rocky reef fishes. In particular, we sampled fished and unfished rocky reefs to assess whether the different set times would lead to differences in the patterns or the interpretation of the subsequent data. We sampled fish assemblages on six rocky reefs (20-35. m) across two marine parks on the east coast of Australia. At each location replicate BRUVs were deployed for 30 and 60. min in a 'no take' area (marine sanctuary) and a fished area. The interpretations of the data across the fished and unfished zones were consistent with both set times indicating that the conclusions from both set time were comparable. Furthermore, there were no differences in fish assemblages or species richness between the set times. The relative abundances of the recreationally and commercially important snapper (Chrysophrys auratus) were greater in the longer set times, but the actual spatial patterns between zones and amongst locations were consistent. Piecewise regression analysis of the breakpoint times for species accumulation found that there were no significant differences between locations or between fished and unfished areas. Mean breakpoint, the time when species accumulation changes, occurred at 12. min ± 1.04. S.E. for all sites combined whilst the shorter set time was found to be less costly and require less field and laboratory times. This study quantifies that either 30 or 60. min will provide a reasonable estimate of rocky reef fish diversity and relative abundance for comparative purposes, on these shallow rocky reefs. © 2014. Source

Browne C.M.,University of Cape Town | Maneveldt G.W.,University of the Western Cape | Bolton J.J.,University of Cape Town | Anderson R.J.,University of Cape Town | Anderson R.J.,Fisheries Research
South African Journal of Botany | Year: 2013

Seagrasses support a great diversity of epiphytic organisms and new research has shown that non-geniculate coralline red algae are important occupiers of space on the fronds of seagrasses. Except for a few scant records, there are no detailed published accounts of non-geniculate coralline algae epiphytic on seagrasses in South Africa. The seagrass Thalassodendron leptocaule (previously known as Thalassodendron ciliatum) is unique among southern African seagrasses in that it occurs on exposed rocky outcrops along the Mozambique and north eastern South African coast; most other seagrasses are restricted to sheltered bays and estuaries. Here we present descriptions of three species of non-geniculate coralline red algae which we have identified growing epiphytically on this seagrass in northern KwaZulu-Natal: Hydrolithon farinosum, Pneophyllum amplexifrons and Synarthrophyton patena. Two of the corallines (P. amplexifrons and S. patena) were restricted to the seagrass' stems while the third (H. farinosum) occurred only on the leaves. Of the three coralline epiphytes, P. amplexifrons contributed most to the biomass (average wet weight per plant 0.6 ± 1.18. g); its wet weight, however, varied between habitats. H. farinosum and other smaller turf algae amounted to no more than 0.1. g (wet weight) per leaf. S. patena was far sparser and contributed less than 0.1. g (wet weight) per stem. P. amplexifrons and H. farinosum appear to be pioneer epiphytes and form additional surfaces onto which other seaweed epiphytes attach and grow. Distribution of these epiphytes is explained by the longevity of the stems and leaves of the seagrass. © 2013 South African Association of Botanists. Source

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