To our knowledge, this study is the first to attempt to associate environmental factors with the apparent colonization of a nursery habitat by a marine apex predator. Our results provide strong evidence that an increase in the use of Pamlico Sound as a Bull Shark nursery is associated with changing environmental conditions in the estuary, with temperature as the likely primary driver of Bull Shark presence. Though Bull Sharks were a rare catch overall in NCDMF surveys, the number of juveniles captured in Pamlico Sound increased from six from 2003–2011 (average of 0.75 sharks per year) to 53 captured from 2011–2016 (average of 8.8 sharks per year) with a corresponding increase in CPUE. This represents a dramatic increase in juvenile Bull Shark presence from previous assessments of shark nursery habitat use in North Carolina waters^20,24. Though 38.9% of the juvenile Bull Sharks in the survey were caught in 2012, their presence became more consistent beginning in 2011, with Age 1+ sharks caught every year and YOY sharks captured every year since 2012. A steady increase in juvenile Bull Shark CPUE was predicted when using the model generated with gillnet survey data to predict CPUE based on environmental measurements taken during the trawl survey, showing that at least two independent surveys have documented changing conditions in the estuary that are positively associated with shark abundance.

We found that the increased CPUE of juvenile Bull Sharks was most strongly associated with increasing water temperatures and salinities, which occurred in early and late summer, respectively. These environmental changes were consistent between both gillnet and trawl survey data, suggesting that increasing early summer temperatures and late summer salinities have been consistent trends in this system. While salinity was highly variable year-to-year, the increasing trend in water temperature became more consistent in trawl survey data after 2008. Generally, water temperature influences both large-scale seasonal migration and finer-scale habitat selection in sharks, while salinity is more strongly associated with localized movements and habitat use²⁷. In nursery habitats along coastal Florida, neonate and YOY Bull Sharks occupy a narrower salinity range than other demographic groups¹⁷ and move within estuaries to follow preferred salinity gradients¹³. However, Bull Sharks were not present at all at temperatures lower than 18–20 °C in the Indian River Lagoon¹⁷. Juvenile Bull Sharks remain within their nursery estuaries year-round, only leaving during cold temperature events, which can also cause cold stuns and mortality^17,28. Our data showed that increased CPUE of juvenile-sized Bull Sharks in Pamlico Sound associated with a minimum temperature of approximately 22 °C and greater than 9 ppt mean salinity. Pamlico Sound temperatures are considerably lower than this threshold during the winter and no Bull Sharks were captured in the survey from October – April, so it likely that juvenile Bull Sharks leave the estuary during this time. Therefore, the seasonal presence and possibly birth of juvenile Bull Sharks may be associated with temperature, while salinity likely influences spatial distribution within the estuary. However, the lack of a sharp increase in 2011 in the model-predicted CPUE suggests that there may be some other cause for the initial sudden increase in shark CPUE during this time, potentially a partruition or immigration event not directly associated with the environmental variables we analyzed.

In comparison with other areas of the U.S. Atlantic coast, northwestern Pamlico Sound represents the most likely nursery habitat north of the Indian River Lagoon based on the criteria of Heupel et al.¹¹: juveniles are more commonly encountered there than in other southeastern U.S. estuaries¹⁸ or parts of Pamlico Sound, are present in the system for the majority of the summer, and since 2011 have been documented every year. Bull Shark parturition occurs from May-August in the Indian River Lagoon^16,17, which coincides with the appearance of the smallest YOY Bull Sharks in Pamlico Sound. Previously, the Indian River Lagoon system in Florida was thought to be the northernmost functional Bull Shark nursery on the U.S. Atlantic coast¹⁷. However, sporadic captures of neonate/YOY Bull Sharks in estuaries in Georgia and the Carolinas^19,24,29 suggest that parturition may occur in areas north of the Indian River Lagoon on a limited, opportunistic basis if conditions allow, though studies with longer time series in these areas have not shown a consistent annual presence of juveniles. Captures of YOY Bull Sharks in 2004 and 2006 may represent opportunistic pupping during years with favorable conditions, while the consistent presence of juveniles since 2011 may be a result of consistently warm late spring/early summer temperatures allowing sharks previously born in Pamlico Sound to return. The size range of juveniles has also expanded to include larger individuals in the most recent survey years, which is further evidence of presence over multiple years. Bull Sharks show evidence of strong philopatry and both pregnant females and juveniles are likely to repeatedly return to primary nurseries²⁵, though if colonization of Pamlico Sound as a nursery is occurring then this would strongly suggest that straying is possible. The use of Pamlico Sound as a primary nursery may increase further if water temperatures remain warm and females born in the system reach maturity and return to give birth. The repeated return of individuals will need to be directly assessed to truly determine whether Bull Sharks are showing site fidelity or philopatry to Pamlico Sound, so the use of genetic and telemetry methods to verify repeated annual presence of individuals are necessary next steps.

The increase in juvenile Bull Shark abundance in Pamlico Sound coincides with a general northward shift in marine species distributions along the U.S. Atlantic coast^4,30,31. At least one other large shark, the Sand Tiger Shark (Carcharias taurus), has recently been observed using areas where it was historically rare as nursery habitat³². There is evidence that Bull Shark populations are recovering from previous declines^33,34, which may cause apparent range expansion as the species returns to parts of its historical range, but the lack of evidence for nursery habitat in Pamlico Sound extends to years prior to documented shark population declines in U.S. Atlantic waters²⁰. Another possible explanation is a currently undescribed decadal-scale trend in Bull Shark population dynamics similar to one recently discovered among Lemon Sharks (Negaprion brevirostris) in Bimini³⁵. Such a trend could not be inferred from the only long-term study on Bull Shark catches in North Carolina waters due to a lack of standardization between survey and observational data²⁰. Changes in the catch of all shark species combined have been observed in the University of North Carolina shark survey near Cape Lookout, North Carolina but factors causing or influencing these changes are unclear³⁶. With limited evidence for other explanations, we conclude that the increase in juvenile Bull Shark abundance in Pamlico Sound is related to increasing water temperatures and salnities during the time of parturition.

Many wide-ranging large coastal sharks depend on specific regions or sites as nursery habitat³⁷, and changing environmental conditions at these sites may have a disproportionate effect on their populations³. Our results suggest that Bull Sharks have the potential to colonize new nursery habitats, which may make their populations resilient to large-scale disturbances such as climate change. A local increase in Bull Shark presence may exert new top-down effects on Pamlico Sound communities⁸, but interspecific interactions involving Bull Sharks in this estuary have not been assessed. Additionally, Bull Sharks are considered dangerous to humans³⁸ and have been implicated in depredations on species targeted by recreational fisheries within North Carolina estuaries (Bangley, personal observation), so localized population increases may affect the frequency and nature of local human-shark interactions. Further monitoring will be needed to determine the ecological effects of Bull Shark nursery expansion into Pamlico Sound and to predict further range expansion.