Species richness and geographical variation in assemblage structure of the freshwater fish fauna of the Wet Tropics region of northern Queensland

A field survey of the freshwater piscifauna of the Wet Tropics region of northern Queensland was undertaken in August, September and October 1993. Sixty-six species of fishes, only two of which were exotic, were collected during the study, and reference to a further 12 species was found in the literature. The region contains two-thirds of the continent's fish genera and 40% of species. If the comparison is restricted to only those genera and species found in northern Australia, then the proportions increase to 80% and 55% respectively. Several species not previously recorded from Australia and one undescribed species from the family Percichthyidae, were collected. Ordination of the data set revealed a strong latitudinal gradient in assemblage structure, with the Bloomfield River and streams of the Cape Tribulation area and the Cardwell area containing the most distinctive faunas. The remaining seven drainages formed a homogenous group with little inter-basin separation, probably because they share similar hydrologies, water quality and contain a similar array of habitats. Discontinuities in river profile had a marked effect on assemblage structure

Brownian Dynamics Simulation of Polydisperse Hard Spheres

Standard algorithms for the numerical integration of the Langevin equation require that interactions are slowly varying during to the integration timestep. This in not the case for hard-body systems, where there is no clearcut between the correlation time of the noise and the timescale of the interactions. Starting from a short time approximation of the Smoluchowsky equation, we introduce an algorithm for the simulation of the overdamped Brownian dynamics of polydisperse hard-spheres in absence of hydrodynamics interactions and briefly discuss the extension to the case of external drifts

Phase Transitions of Hard Disks in External Periodic Potentials: A Monte Carlo Study

The nature of freezing and melting transitions for a system of hard disks in a spatially periodic external potential is studied using extensive Monte Carlo simulations. Detailed finite size scaling analysis of various thermodynamic quantities like the order parameter, its cumulants etc. are used to map the phase diagram of the system for various values of the density and the amplitude of the external potential. We find clear indication of a re-entrant liquid phase over a significant region of the parameter space. Our simulations therefore show that the system of hard disks behaves in a fashion similar to charge stabilized colloids which are known to undergo an initial freezing, followed by a re-melting transition as the amplitude of the imposed, modulating field produced by crossed laser beams is steadily increased. Detailed analysis of our data shows several features consistent with a recent dislocation unbinding theory of laser induced melting.Comment: 36 pages, 16 figure

Spatial and temporal variation of fish assemblage associated with aquatic macrophyte patches in the littoral zone of the Ayapel Swamp Complex, Colombia

ABSTRACT: Aim: The purpose of the present study was to examine spatial and temporal variation in fish assemblage structure associated with aquatic macrophytes in the littoral zone of the ASC. Methods: Specimens were caught between January 2008 and February 2009, over four limnimetric moments, using both cast net and seine net. Data on the temperature, electrical conductivity, pH and dissolved oxygen was recorded for the characterization of the water mass in the sites. Results: A total of 34,151 specimens from 44 species were collected. The most abundant species were Eigenmannia virescens, Astyanax caucanus, Astyanax fasciatus, Roeboides dayi and Cyphocharax magdalenae, which together accounted for more than 75% of the sample. Temporal and spatial comparisons showed variation in the environmental conditions and highlighted the existence of heterogeneous abiotic conditions (p0.05) regarding the fish assemblage structure. The multivariate analysis showed no significant relationship between existing environmental conditions and the fish assemblage (p=0.04). The analysis also showed the absence of a relationship between the fish assemblage and environmental variables with respect to the flood pulse and sampling sites (p>0.05). Conclusion: The uniformity of the fish communities that inhabit aquatic macrophyte patches in the littoral region of the ASC may be related to the availability of suitable habitat in structural terms, that probably supports a more abundant and varied wildlife

Non-local rheology in dense granular flows -- Revisiting the concept of fluidity

Granular materials belong to the class of amorphous athermal systems, like foams, emulsion or suspension they can resist shear like a solid, but flow like a liquid under a sufficiently large applied shear stress. They exhibit a dynamical phase transition between static and flowing states, as for phase transitions of thermodynamic systems, this rigidity transition exhibits a diverging length scales quantifying the degree of cooperatively. Several experiments have shown that the rheology of granular materials and emulsion is non-local, namely that the stress at a given location does not depend only on the shear rate at this location but also on the degree of mobility in the surrounding region. Several constitutive relations have recently been proposed and tested successfully against numerical and experimental results. Here we use discrete elements simulation of 2D shear flows to shed light on the dynamical mechanism underlying non-locality in dense granular flows

Trophic polymorphism and water clarity in northern Australian Scortum (Pisces: Terapontidae)

The diets and relative intestinal length of two typically herbivorous terapontid fish from contrasting high and low clarity environments were compared. Fish of both species collected from high clarity habitats were almost exclusively herbivorous, whereas conspecifics from low clarity habitats were omnivores. Relative intestinal lengths in both species were significantly shorter in low clarity environments. Pronounced contrasts in water transparency have the apparent capacity to produce dietary niche shifts in these two species that in turn induce and maintain trophic polymorphisms in a riverine environment

Climate change and its implications for Australia's freshwater fish

Freshwater environments and their fishes are particularly vulnerable to climate change because the persistence and quality of aquatic habitat depend heavily on climatic and hydrologic regimes. In Australia, projections indicate that the rate and magnitude of climate change will vary across the continent. We review the likely effects of these changes on Australian freshwater fishes across geographic regions encompassing a diversity of habitats and climatic variability. Commonalities in the predicted implications of climate change on fish included habitat loss and fragmentation, surpassing of physiological tolerances and spread of alien species. Existing anthropogenic stressors in more developed regions are likely to compound these impacts because of the already reduced resilience of fish assemblages. Many Australian freshwater fish species are adapted to variable or unpredictable flow conditions and, in some cases, this evolutionary history may confer resistance or resilience to the impacts of climate change. However, the rate and magnitude of projected change will outpace the adaptive capacities of many species. Climate change therefore seriously threatens the persistence of many of Australia's freshwater fish species, especially of those with limited ranges or specific habitat requirements, or of those that are already occurring close to physiological tolerance limits. Human responses to climate change should be proactive and focus on maintaining population resilience through the protection of habitat, mitigation of current anthropogenic stressors, adequate planning and provisioning of environmental flows and the consideration of more interventionist options such as managed translocations

Distribution of the Lake Eacham rainbowfish in the Wet Tropics region, North Queensland

The Lake Eacham rainbowfish, Melanotaenia eachamensisAllen & Cross, 1982, was thought to be extinct in the wild until recentresearch demonstrated the presence of wild populations in a few tributaries ofthe upper Johnstone River and the upper Barron River, north Queensland, byusing the technique of DNA sequencing. We present the results of amultivariate analysis of a range of morphological and meristic characters ofrainbowfish collected from rivers of the Wet Tropics region, particularly theJohnstone River, that demonstrate that M. eachamensis iswidespread in the upper and lower reaches of the North and South Johnstonerivers and tributaries of the upper Tully River.M. eachamensis was most often the dominant species inthose locations where sympatry withM. splendida splendida was observed.M. eachamensis should be considered a stream-dwellingspecies rather than a lacustrine species although a significant lacustrinepopulation (Koombooloomba Dam) was detected