1. Body mass is a key determinant of diving performance in endotherms. In air-breathing ectotherms however, this paradigm occurs with considerably less force. Here, through remote recordings of dive behaviour over a wide size range (5-42 kg body mass, n = 17) of freshwater crocodiles (Crocodylus johnstoni), we demonstrate why body mass is such a poor determinant of dive duration for ectothermic divers. 2. Crocodiles were released into the wild with a time-depth-recorder attached to their dorsal scutes, and a movement activated radio-tag attached to their tail. Over 15 days, 652AE6 ± 58AE4 (mean ± SE, n = 17) dives were recorded, with all individuals exhibiting two specific divetypes. These were, a resting-dive (62AE7 ± 5AE4% of total dive no.), where no activity occurred during the dive, and an active-dive (37AE1 ± 6AE3% of total no.) associated with swimming. 3. The durations of resting-dives (12 min) were similar for all crocodiles. Smaller crocodiles (6AE3 ± 0AE7 kg, mean ± SE, n = 9) exhibited a significant correlation between dive duration and post-dive surface-interval, whilst larger crocodiles (17AE9 ± 3AE75 kg, mean ± SE, n = 8) did not. This demonstrated that aerobic dive duration was mass-specific during resting-dives, but other mass specific factors, presumably ecological, determined dive duration. 4. The durations of active-dives were never >1 min, showed no relationship with body mass and no correlation with the post-dive-surface interval. In crocodiles, aerobic metabolic scope is independent of body mass but anaerobic capacity is mass dependent, suggesting that active-dive duration was determined by sustained activity and dives were terminated before anaerobic metabolism became significant. 5. All individuals showed similar diel phase shifts in dive duration, type and depth, illustrating the overwhelming influence of the external environment on dive behaviour. Dive durations which resulted in significant anaerobic debt occurred rarely, but were undertaken in response to a potential threat. 6. Body mass was a poor predictor of diving in C. johnstoni because the external environmental and ecological factors exerted a greater influence on dive duration than oxygen reserves.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The University of Chicago Press ABSTRACTLocomotion 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 17ЊC, with turtles acclimated to a cold temperature (17ЊC) having a significantly longer dive duration than hatchlings acclimated to a warm temperature (28ЊC). This increase in dive duration at 17ЊC 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.

This study compared the critical swimming speed (U crit ) and endurance performance of three Australian freshwater fish species in different swim-test apparatus. Estimates of U crit measured in a large recirculating flume were greater for all species compared with estimates from a smaller model of the same recirculating flume. Large differences were also observed for estimates of endurance swimming performance between these recirculating flumes and a free-surface swim tunnel. Differences in estimates of performance may be attributable to variation in flow conditions within different types of swim chambers. Variation in estimates of swimming performance between different types of flumes complicates the application of laboratory-based measures to the design of fish passage infrastructure.

The purpose of this study was to determine whether a relationship existed between the diel surfacing trends of the bimodally respiring freshwater turtle Rheodytes leukops and daily fluctuations in specific biotic and abiotic factors. The diel surfacing behaviour of adult R. leukops was recorded over four consecutive seasons (Austral autumn 2000 – summer 2001) within Marlborough Creek, central Queensland, Australia, using pressure-sensitive time–depth recorders. Additionally, diurnal variations in water temperature and aquatic PO2 level, as well as the turtle's behavioural state (i.e., active versus resting), were monitored. In autumn and summer, surfacing frequency increased significantly during the daylight hours, with peak levels normally occurring around dawn (0500–0700) and dusk (1700–1900). However, no consistent diel surfacing trend was recorded for the turtles in winter or spring, owing to considerable variation among individual R. leukops. Diurnal surfacing trends recorded for R. leukops in autumn and summer are attributed to periods of increased activity (possibly associated with foraging) during the daylight hours and not to daily variations in water temperature or aquatic PO2 level. Turtles generally remained at a depth greater than 1 m throughout the day, where the effect of diel fluctuations in water temperature (<0.5 °C) and aquatic PO2 level (<15 mm Hg (1 mm Hg = 133.322 Pa)) was considered to be negligible.

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.