Urban sharks: residency patterns of marine top predators in relation to a coastal metropolis

Abstract

Understanding and ultimately predicting how marine organisms will respond to urbanization is central for effective wildlife conservation and management in the Anthropocene. Sharks are upper trophic level predators in virtually all marine environments, but if and how their behaviors are influenced by coastal urbanization remains understudied. Here, we examined space use and residency patterns of 14 great hammerheads Sphyrna mokarran, 13 bull sharks Carcharhinus leucas, and 25 nurse sharks Ginglymostoma cirratum in proximity to the coastal metropolis of Miami, Florida, using passive acoustic telemetry. Based on the terrestrial urban carnivore literature, we predicted sharks would exhibit avoidance behaviors of areas close to Miami, with residency patterns in these urban areas increasing during periods of lower human activity, such as during nocturnal hours and weekdays, and that dietary specialists (great hammerhead) would exhibit comparatively lower affinity towards highly urbanized areas relative to dietary generalists (bull and nurse shark). However, we did not find empirical support for these predictions. Space use patterns of tracked sharks were consistent with that of ‘urban adapters’ (species that exhibit partial use of urban areas). Modeling also revealed that an unmeasured spatial variable was driving considerable shark residency in areas exposed to high urbanization. We propose several hypotheses that could explain our findings, including food provisioning from shore-based activities that could be attracting sharks to urban areas. Ultimately, the lack of avoidance of urban areas by sharks documented here, as compared to terrestrial carnivores, should motivate future research in the growing field of urban ecology.

From Extermination to Conservation: Historical Records of Shark Presence during the Early and Development Phase of the Greek Fishery

“…Although [ 28 ] underlines a direct link between shark attacks and the increasing density of sea-users, we must further consider the possibility that some shark species have shifted their distribution as a result of the increasing human pressure on the coastal ecosystems and urbanization (increase in population in the two biggest cities in Greece by more than 130% between 1951–2021: [ 29 ]). Such changes have been described elsewhere [ 23 , 30 ], while the recent continuous observations of blue shark sightings in ports and beaches in the Mediterranean (including Greece) during the COVID lockdowns (pers. Comm.…”

Both environmental conditions and intra- and interspecific interactions influence the movements of a marine predator

“…This is not surprising as nautical human activities and density are spread fairly evenly along the west coast of the island where the network of receivers was deployed. In addition, it is now recognized that bull sharks can adapt to urbanized areas and do not especially avoid these high human density areas and their activities (Hammerschlag et al, 2022; Werry et al, 2012). The majority of large-bodied terrestrial carnivores tend avoid high human densities and activities (Tucker et al, 2018, 2023).…”

“…In addition, it is now recognized that bull sharks can adapt to urbanized areas and do not especially avoid these high human density areas and their activities (Hammerschlag et al, 2022;Werry et al, 2012). The majority of large-bodied terrestrial carnivores tend avoid high human densities and activities (Tucker et al, 2018(Tucker et al, , 2023.…”

“…acoustic receivers) attributes as explanatory variables Robins, Snijders, et al, 2007;Snijders et al, 2006). These models account for the non-independence of the data expressed as movements between sites acting as sources or sinks (Silk & Fisher, 2017) Bull shark movements are known to be influenced by both biotic and abiotic factors (Lee et al, 2019;Lubitz et al, 2023;Niella et al, 2022;Werry et al, 2018), and anthropogenic factors (Hammerschlag et al, 2022;Werry et al, 2012), but less is known about the importance of con-and heterospecifics in movement decisions. Even if most studies on the movements of these marine predators have identified environmental factors as most influential to their movement ecology, we expect that social and competition factors may be just as important in explaining individual movements.…”

Animal migration in the Anthropocene: threats and mitigation options

“…That said, habitat alterations, such as urbanization, that introduce food subsidies could attract migratory animals, altering natural behaviour and nutritional physiology/energetics [e.g. sharks (Hammerschlag et al ., 2022 a ); birds (Oro et al ., 2013)].…”