Environmental Drivers of Fine-Scale Predator and Prey Spatial Dynamics in Sydney Harbour, Australia, and Adjacent Coastal Waters

Abstract

Greater Sydney is the largest coastal city in Australia and is where bull sharks (Carcharhinus leucas) are present every summer and autumn. A decade of acoustic telemetry data was used to identify drivers of space use for bull sharks and their potential prey, according to standardised 6-h intervals using dynamic Brownian bridge movement models. Influences of environmental, physical, and biological variables on the areas of space use, location, and predator–prey co-occurrence were investigated with generalised additive mixed models. Rainfall in the catchment affected space use for all animals (i.e. teleost species and both sexes of sharks), with varying temporal responses. Male sharks responded most promptly to high rainfall moving upstream in < 1 day, followed by teleosts (2 to 7 days), and female bull sharks after 4 days. Environmental luminosity affected male shark dispersal and space use, possibly indicating use of visual cues for foraging. Physical characteristics of habitat were important factors driving spatial overlaps between predator and prey in estuarine areas. In sandy embayments < 10-m deep, males and female bull sharks overlapped with different species, whereas males and silver trevally (Pseudocaranx georgianus) co-occurred in deep holes (> 30 m). Shark size influenced overlap between sexes, with smaller females less likely to co-occur with larger males (~ 50 cm). Variability in space use suggests spatial segregation by sex and size in bull sharks, with individuals targeting similar prey, yet either in different areas or at different times, ultimately enabling them to exploit different resources when in the same habitats.

Environmental drivers of yellowtail kingfish, Seriola lalandi, activity inferred through a continental acoustic tracking network

“…Kingfish are seasonal residents of the estuaries studied, remaining in Coffin Bay during summer and autumn (from September to April) and Sydney Harbour in spring and winter (from May to September) (Niella et al, 2022). Despite a difference in the timing of these seasonal aggregations, the water temperature was similar when kingfish were present in Coffin Bay and Sydney Harbour (16–18°C), with kingfish leaving Coffin Bay when the water temperature drops below approximately 16°C and leaving Sydney Harbour when the water temperature exceeds approximately 18°C.…”

“…Morrongiello et al, 2014) or, in some cases, can have negative outcomes, such as reduced resource availability, unfavourable thermal conditions, and increased energetic demands (Magnuson, Meisner & Hill, 1990; Sorte et al, 2013; Johansen et al, 2015). Whereas the environmental drivers of spatial movements and the temporal persistence of some species have been identified (Ackerman et al, 2000; Block et al, 2001; Udyawer et al, 2015; Niella et al, 2022), there remains a paucity of information on the effects of environmental conditions on the energetic status of free‐ranging animals. Identifying how environmental variability influences the activity of marine animals can therefore contribute towards understanding the effects of natural and anthropogenic‐driven changes on physiology (Udyawer et al, 2015; Niella et al, 2022).…”

“…Using this deeper and protected area might provide an opportunity to recover from being more active in the morning, while remaining within a more consistent thermal environment, compared with shallow areas like Kellidie Bay, where kingfish might be exposed to greater diel temperature fluctuation. Kingfish are seasonal residents of the estuaries studied, remaining in Coffin Bay during summer and autumn (from September to April) and Sydney Harbour in spring and winter (from May to September)(Niella et al, 2022). Despite a difference in the timing of these seasonal aggregations, the water temperature was similar when kingfish were present in Coffin Bay and Sydney Harbour (16-18 C), with kingfish leaving Coffin Bay when the water temperature drops below approximately 16 C and leaving Sydney Harbour when the water temperature exceeds approximately 18 C. The similarity in water temperature when kingfish reside in these distant estuaries suggests temperature-driven movements into shallow estuaries.…”

The Relative Abundance and Occurrence of Sharks off Ocean Beaches of New South Wales, Australia

“…Bull sharks had the highest abundances recorded at Anna Bay and Ballina, although Anna Bay and Scotts Head had the highest bull shark observation days to total flight days. Bull sharks are widely distributed in tropical and sub-tropical coastal waters, typically undergoing seasonal migrations along the east coast of Australia [ 45 , 46 , 47 ]. Not all bull sharks, however, exhibit these migration tendencies, with some individuals remaining in the same region for extended periods [ 25 , 45 , 48 ].…”

Coastal seascape variability in the intensifying East Australian Current Southern Extension

“…The impacts of the intensification of the EAC southern extension on coastal systems are already evident and include prolonged marine heatwaves (Oliver et al, 2017;Schaeffer et al, 2017;Huang et al, 2021), modification of nutrient loading regimes (Harris et al, 1987;Harris et al, 1991) migration of typically subtropical species poleward (Edyvane, 2003;Pittock, 2003;Thresher et al, 2003;Vergeś et al, 2014;Niella et al, 2021) and shifts in species assemblages, the distributions of pelagic fish and planktic species, and trophic interactions (Carroll et al, 2016;Carroll et al, 2017;Hobday et al, 2011;Johnson et al, 2011;Kelly et al, 2016;Larsson et al, 2018;Thompson et al, 2009) that are projected to continue in coming decades (Cetina-Heredia et al, 2015). While many ecological patterns have been investigated in the context of EAC variability via correlations with temperature and current velocity, there has been little work investigating how EAC variability (or western boundary current variability more broadly) influences seascape characteristics over multiple spatiotemporal scales simultaneously, including how relationships between geostrophic and ageostrophic processes link offshore and coastal dynamics, and impact biological activity.…”