Long-term patterns of abundance, residency and movements of bull sharks (Carcharhinus leucas) in Sydney Harbour, Australia

Abstract

Bull sharks (Carcharhinus leucas) are known to frequent nearshore environments, particularly estuaries, resulting in interactions with humans. Knowledge of the behaviour of large individuals in temperate, estuarine environments is limited. This acoustic telemetry study reports on residency and movement patterns of 40 sub-adult and adult bull sharks in Sydney Harbour, a large temperate estuary, over seven years. Bull sharks exhibited clear seasonal patterns in their occurrence during the austral summer and autumn, with abundance peaking in January and February. This pattern was consistent between sexes and across all sizes. Bull sharks displayed weak diel differences in their spatial distribution, with individuals using areas further from the Harbour entrance more frequently during the day and at low tides. A diel pattern in depth use was apparent, with sharks utilising deeper water during daytime and moving shallower at night. Bull sharks had high individual inter-annual variability in their spatial distribution, however, when data were aggregated among all individuals and years, two locations of increased use were identified. Water temperature was the key predictor for seasonal movements and return behaviour to this estuary, suggesting that increasing water temperatures as a result of climate change may lead to higher shark abundance and possibly longer periods of residency in Sydney Harbour. Understanding the drivers for bull shark abundance and distribution will hopefully facilitate better education and shark smart behaviour by estuarine water-users, especially during summer and autumn months.

The effectiveness of Shark‐Management‐Alert‐in‐Real‐Time (SMART) drumlines as a tool for catching white sharks, Carcharodon carcharias, off coastal New South Wales, Australia

“…The exclusive deployment of SMART drumlines during daylight hours in the current programme could potentially explain the relatively small catch of bull sharks as they are more active and forage at night (Smoothey et al, 2016). Additionally, bull and tiger sharks are typically caught in warmer waters (Payne et al, 2018; Smoothey et al, 2019) during the austral summer and autumn.…”

“…It should be emphasised that, while maximum catch rates occurred in cooler months, white sharks were also caught during the austral summer and autumn, and so consistent, year‐round deployment of SMART drumlines is strongly recommended adjacent to popular swimming and surfing areas. Furthermore, bull and tiger sharks are key species observed within the study region and are likely to be more prevalent during these periods of warmer water (Holmes et al, 2014; Lee et al, 2019; Smoothey et al, 2019).…”

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

“…The large-scale movements of marine predators towards higher latitudes are often affected by seasonal changes in water temperatures (Block et al 2011;Smoothey et al 2016Smoothey et al , 2019, but the extent to which this is related to physiological constraints on metabolism (Payne et al 2018) or to seasonal presence of potential prey (Barnett et al 2010) is yet to be fully understood. At smaller geographical scales, for example, within estuaries, animal movements are influenced by a complex interplay of factors such as tide (Walsh et al 2013;Smoothey et al 2019) and freshwater runoff and their associated effects such as salinity levels (Childs et al 2008;Taylor et al 2014). Heavy rainfall may override regular behavioural patterns of estuarine teleosts, with fish using deeper areas and becoming more active at night following periods of intense rain (Payne et al 2013).…”

“…The bull shark is a euryhaline species capable of tolerating extreme salinity gradients (Pillans et al 2008). While juveniles tend to remain within their natal estuaries (Werry et al 2011), adults also make use of these ecosystems (Werry et al 2018;Smoothey et al 2019). Mature bull sharks can transit between freshwater and marine habitats within hours (McCord and Lamberth 2009) or remain in low salinity areas for days (Niella et al 2017).…”

“…> 83% contribution) of their diet. Along eastern Australia, bull sharks undertake seasonal migrations from the tropical Great Barrier Reef southward to temperate waters of Sydney during the summer (Smoothey et al 2016(Smoothey et al , 2019Lee et al 2019;Espinoza et al 2021). As oceans are gradually warming due to climate change, bull sharks are expected to spend more time at higher latitudes in future years, including the Sydney region (Niella et al 2020b).…”