Matthew A. Gordos

Thermal plasticity of diving behavior, aquatic respiration, and locomotor performance in the Mary River turtle Elusor macrurus.

Locomotion is a common measure of performance used in studies of thermal acclimation because of its correlation with predator escape and prey capture. However, for sedentary animals such as freshwater turtles, we propose that diving behavior may be a more ecologically relevant measure of performance. Increasing dive duration in hatchling turtles reduces predator exposure and therefore functions as an ecological benefit. Diving behavior is thermally dependent, and in some species of freshwater turtles, it is also reliant on aquatic respiration. This study examined the influence of thermal acclimation on diving behavior, aquatic respiration, and locomotor performance in the endangered, bimodally respiring Mary River turtle Elusor macrurus. Diving behavior was found to partially acclimate at 17°C, with turtles acclimated to a cold temperature (17°C) having a significantly longer dive duration than hatchlings acclimated to a warm temperature (28°C). This increase in dive duration at 17°C was not a result of physiological alterations in metabolic rate but was due instead to an increase in aquatic oxygen consumption. Increasing aquatic oxygen consumption permitted cold‐acclimated hatchlings to remain submerged for significantly longer periods, with one turtle undertaking a dive of over 2.5 d. When burst‐swimming speed was used as the measure of performance, thermal acclimation was not detected. Overall, E. macrurus demonstrated a partial ability to acclimate to changes in environmental temperature.

Facilitating upstream passage of small-bodied fishes: linking the thermal dependence of swimming ability to culvert design

Fish passage through road culverts is poorly understood, particularly for small-bodied fishes, despite this information being integral to the restoration of waterway connectivity. We assessed the prolonged swimming performance of a small-bodied fish, empire gudgeon (Hypseleotris compressa; 3.2-7.7cm total length, TL), and juvenile Australian bass (Percalates novemaculeata; 3.5-7.8cm TL). Swimming trials were conducted in a hydraulic flume across a range of fixed and increasing velocities in response to acute and long-term thermal treatments. A new statistical approach (Tobit analysis) was used to relate the thermal dependence of swimming endurance to hydraulic characteristics of culverts, providing estimates of maximum water velocity allowing upstream fish passage. Reductions in water temperature of 10°C, similar to those caused by cold-water releases from dams, significantly impaired critical swimming speeds of both species. Traversable water-velocity models identified H. compressa as a weak swimmer, requiring very low water velocities (≤0.10ms-1 or 2.86 body lengths (BL) s-1) for unrestricted passage, whereas P. novemaculeata was predicted to traverse water velocities of ≤0.39ms-1 or 12.12BLs-1. Culvert designs can be improved by limiting water velocities to accommodate weak-swimming fishes and by accounting for the thermal sensitivity of swimming performance.

Effect of water depth and water velocity upon the surfacing frequency of the bimodally respiring freshwater turtle, Rheodytes leukops

SUMMARY This study examines the effect of increasing water depth and water velocity upon the surfacing behaviour of the bimodally respiring turtle, Rheodytes leukops. Surfacing frequency was recorded for R. leukops at varying water depths (50, 100, 150 cm) and water velocities (5, 15, 30 cm s-1) during independent trials to provide an indirect cost-benefit analysis of aquatic versus pulmonary respiration. With increasing water velocity, R. leukops decreased its surfacing frequency twentyfold, thus suggesting a heightened reliance upon aquatic gas exchange. An elevated reliance upon aquatic respiration, which presumably translates into a decreased air-breathing frequency, may be metabolically more efficient for R. leukops compared to the expenditure (i.e. time and energy) associated with air-breathing within fast-flowing riffle zones. Additionally, R. leukops at higher water velocities preferentially selected low-velocity microhabitats, presumably to avoid the metabolic expenditure associated with high water flow. Alternatively, increasing water depth had no effect upon the surfacing frequency of R. leukops, suggesting little to no change in the respiratory partitioning of the species across treatment settings. Routinely long dives (>90 min) recorded for R. leukops indicate a high reliance upon aquatic O2 uptake regardless of water depth. Moreover, metabolic and temporal costs attributed to pulmonary gas exchange within a pool-like environment were likely minimal for R. leukops, irrespective of water depth.

Substrate roughening improves swimming performance in two small-bodied riverine fishes: implications for culvert remediation and design

Disruption of riverine connectivity by artificial structures, such as culverts, can obstruct critical fish movements. We investigated the effectiveness of replacing smooth substrates with rough, naturalistic substrates (i.e. river stones) in improving fish swimming capacity and modelled fish passage through culverts.