Our data demonstrate the presence and shifts in movement of Bonnetheads along barrier islands in the northern GOM following an anthropogenic alteration to natural coastline. These findings indicate a relatively rapid shift in Bonnethead movement following the construction of the rock-rubble berm that closed off the passageway of Katrina Cut, suggesting that an anthropogenic modification affected Bonnethead movements.
Barrier islands shift and naturally change geomorphology, often returning to their previous state before a devastating event, such as a hurricane . For example, Dauphin Island, AL was previously fragmented into two islands in the early 1900s with an 8.5-km-wide channel that eventually filled due to longshore sediment transport during several years without large storms [18, 28, 29]. Human intervention to accelerate or facilitate natural processes often comes with unanticipated consequences that may affect marine taxa. For example, closing off a corridor between the GOM and Mississippi Sound may negatively affect species using the corridor as a means to ingress into shallower and more protective waters for biological or reproductive strategies. Bonnetheads using Katrina Cut as a means of moving into and out of Mississippi Sound may have been prohibited from reaching an area of preferred habitat, and may have thus shifted movement patterns.
When Katrina Cut was open, Bonnetheads generally remained in a small area near that passageway. The highest proportion of detections occurred at Katrina Cut when it was open, both before and during construction of the rock-rubble berm, and then decreased in the proportion of detections once the cut was closed. As we observed a relatively rapid decline in the proportion of detections at Katrina Cut following the installation of the rock-rubble berm, we suggest that Bonnetheads were using this passageway as a means to enter Mississippi Sound and that the area near Katrina Cut was a preferred habitat. However, the high proportion of detections at Katrina Cut may reflect seasonal habitat use. Bonnetheads have been shown to use estuarine waters more frequently during the summer months  and thus our high proportion of detections may be a function of season. In addition, Bonnetheads have been shown to have a high degree of site fidelity to specific estuaries . Since 2010 was the first year of this study, we were unable to examine the degree of site fidelity Bonnetheads exhibit to Katrina Cut. However, we suggest that there is likely some plasticity to Bonnethead site fidelity and that Katrina Cut was a preferred location for Bonnetheads to reside in this region.
Changes in salinity offer the most intuitive explanation for the changes in habitat use shown by tagged Bonnetheads, in line with previous findings for this species [4, 6, 15]. We found that Bonnetheads generally remained at Katrina Cut while the cut was open and made a general westward shift once the cut was closed, as seen from the IDW analysis. The IDW analysis was based on our subset of tagged individuals, and thus areas with higher proportions of use could be biased by the five individuals that we focused on; however, this analysis is a good visual representation of the areas with the highest proportion of use for these individuals. Modeled salinity for 2010, when Katrina Cut was open, demonstrates high-frequency fluctuations in both surface and bottom salinities. Clearly, the presence of Katrina Cut allowed for higher salinity GOM water to be advected into Mississippi Sound by flooding tidal currents, creating high-frequency tidal fluctuations in both surface and bottom salinity. It is important to note that in 2010, salinity was the highest despite the largest freshwater discharge, likely due to the tidal connection with the GOM providing a source of higher salinity water. While we lack a complete set of data to test this relationship, relatively small fluctuations in salinity due to freshwater discharge and tidal cycles may be sufficient to alter Bonnethead movements in our study area.
Salinity may have indirectly altered the distribution of Bonnetheads, through a redistribution of their preferred prey. Previous studies have indicated the importance of prey availability in determining the distribution of coastal sharks [30, 31]. Bonnetheads have been widely documented to consume crustaceans, particularly blue crabs (Callinectes sapidus [32, 33]). It has been suggested that Bonnetheads use estuaries along the east coast as summer feeding grounds, likely due to the high availability of blue crabs . Seagrass is an important habitat for blue crabs  and changes in habitat may play a role in prey redistribution. For example, when Katrina Cut was open, shoal grass (Halodule wrightii) was present in large abundance stretching over 6 km in length for several years while the cut was open (KL Heck unpublished observations). The closure of Katrina Cut was followed by a marked decline in shoal grass (KL Heck unpublished observations), which could potentially alter habitat use by blue crabs. Diet analysis of Bonnetheads in the current study region confirms that Bonnetheads primarily consume portunid crabs, notably blue crabs [Kroetz, unpublished data]. While not directly tested in the current study, a shift in blue crab distribution due to changes in habitat, salinity, dissolved oxygen, or temperature generated by the closure of Katrina Cut may be a factor in the shift in Bonnethead habitat use observed in this study.
Water depth has been identified as an important driver of coastal shark distribution, and is likely species specific . Long-term fisheries independent gillnet surveys conducted in Texas estuaries have shown that the probability of capturing Bonnetheads was highest near tidal inlets with access to deepest waters (>1 m deep, ). Katrina Cut was a tidal inlet that was approximately 3 m deep in the middle of Dauphin Island allowing for swift currents to flow between the two land pieces . Closing off Katrina Cut effectively terminated the tidal inlet and thus reduced the swift currents as well as the higher salinity tidal connection to the GOM. The closure of the tidal inlet may be another driver for the shift in movement that we observed with acoustically tagged Bonnetheads.
During the construction of the rock-rubble berm, cranes, boats, and trucks were continuously present for several months, and these disturbances may have impacted Bonnethead habitat use. In addition to the physical water column disturbance caused by these machines, there was potential for pollution and habitat degradation during the berm construction, and these factors have been shown to affect shark resilience and abundances [5, 35]. While anthropogenic impacts were not found to influence Bonnethead occurrence in FL estuaries , Bonnetheads in our study area may have been affected by the continuous, unnatural disturbance.
Knowledge of acoustic detection range when using passive telemetry is critical for the interpretation of animal movements within an array . Many variables can affect detection range (see  for a comprehensive review), including properties of the water body such as salinity, temperature, and suspended particles and substrate . A portion of this study included monitoring movements of Bonnetheads during construction of the rock-rubble berm. During construction, sediment, mud, and silt were likely suspended in the water column, which can affect detection range of receivers . For the duration of the construction period, four Bonnetheads were detected at Katrina Cut. The proportion of detections that we received during the construction period decreased by half from the proportion we detected before construction at the same receivers. It is possible that Bonnetheads may have been present in the area for longer periods, but decreased acoustic detection range may have precluded their identification. We suggest that future work examining the movements of this species include models that incorporate other potential predictors of movement, such as detection range, temperature, and dissolved oxygen.