We hypothesized that the probability of a dugong being available for detection is dependent on water depth and that dugongs spend more time underwater in deep-water seagrass habitats than in shallow-water seagrass habitats. The dugong (Dugong dugon) is a bottom-feeding marine mammal and a seagrass community specialist. Accounting for covariates that reduce the probability of detection is important for obtaining robust estimates of the population abundance and determining its status and trends. The probability of an aquatic animal being available for detection is typically <1. This ability is likely to be particularly important under extrinsic constraints such as long-term environmental change. Nonetheless, we show that Northern Rockhopper Penguins demonstrate behavioural plasticity as a response to their changing energy requirements, which is a critical trait when living in a spatio-temporally heterogeneous environment. This prey switching behaviour may reflect preferential selection to account for the increased physiological needs of chicks or simply mirror changes in local prey abundance. We verified the previously shown dietary shift from zooplankton and cephalopods during the guard stage to a higher-energy fish-based diet during the crèche stage, which was reflected in a change in dive behaviour from shorter, shallower to longer, deeper dives. While no significant effects of breeding stage were detected on any foraging trip or dive parameters, stage/prey had a significant effect on feeding dive parameters, with dive duration, bottom time, and maximum depth explaining the majority of the dissimilarity amongst categories. Here, we investigated if this switch is reflected in their foraging behaviour, using time-depth recorders to study the diving behaviour of 27 guard and 10 crèche birds during the breeding season 2010 at Tristan da Cunha and obtaining complementary stomach contents of 20 birds. Recently, it was shown that Northern Rockhopper Penguins at Tristan da Cunha in the South Atlantic Ocean switch diet from lower to higher trophic level prey throughout their breeding cycle. Our procedure could be applied to other shallow-diving animals such as coastal dolphins and turtles.ĭuring the breeding season, seabirds must balance the changing demands of self- and off-spring provisioning with the constraints imposed by central-place foraging.
Frequency distributions of dive durations from TDRs and independent visual observations supported the selection. The comparison of these dive types indicated that a ZOC of 1 m and a dive threshold of 0.75 m were the optimum values for our dugong data as they gave the largest number of Plausible dives and smaller numbers of other dive types.
CONTROL C MINIMIZES ZOC TERMINAL SOFTWARE
The Plausible dives were further classied as Unrecognized dives if they were not identied by the software but were of realistic dive duration. We then determined the optimum values for each parameter by classifying dives identied by an open-source dive analysis software into Plausible and Implausible dives based on the duration of dives. We initially corrected the surface level using custom software. We used dive data from shallow-diving coastal dugongs (Dugong dugon) and visual observations from an independent study to develop and test a procedure that minimizes errors in characterizing dives. We present an empirical procedure to select optimum values that are critical to obtaining reliable results: the zero-offset correction (ZOC) and the dive threshold. The large volume of data acquired from TDRs can be analyzed using dive analysis software, however, the application of the software has received relatively little attention. Knowledge of the diving behaviour of aquatic animals expanded considerably with the invention of time–depth recorders (TDRs) in the 1960s.
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