Blacktip shark

Many marine species distributions have shifted poleward in response to global climate change. Many environmental characteristics will be affected by climate change including temperature and phytoplankton concentration; yet, photoperiod will remain the same. It is imperative to gather baseline distribution data on migratory species so that these shifts can be measured and mitigated. Sex-specific differences in reproductive strategies cause sexual segregation, sex-specific differences in spatial distribution. Female blacktip sharks exhibit a synchronous, biennial reproductive cycle in which one year of reproduction is followed by a resting year. Acoustic telemetry can be used in conjunction with collaborative networks to track migratory species over great distances. However, the irregular spacing of acoustic receivers often results in sporadic detection data, which can lead to skewed distribution information. This project developed and tested an analysis process to regularize sporadic acoustic detection data. Those regularized data were then applied to cluster analyses to determine the seasonal spatial distributions of blacktip sharks, Carcharhinus limbatus, off the United States East Coast and corresponding environmental correlates of latitudinal movement. Sexes of this population were investigated separately and in combination. Differences in distribution were evaluated between sexes, and within females, between reproductive states. These data showed that the U.S. East Coast blacktip shark population distributes from Palm Beach County, FL to Long Island, NY and exhibits sexual segregation, in which females display a more truncated migratory pattern than males.

Sharks respond to low frequency pulsed sounds but are documented to lack the capacity to detect these sounds beyond the acoustic near field. The purpose of my study was to quantify the distance blacktip sharks (Carcharhinus limbatus) oriented to sound stimuli, and determine if responses occurred in the far field. Using an aerial drone and underwater speaker, C. limbatus were filmed responding to sound stimuli (100 – 200 Hz; 200 – 400 Hz; and 400 – 800 Hz). Upon detection, C. limbatus elicited a 20 – 160° turn from the speaker, and rapidly swam away. Sharks responded to all frequencies from at least 62 m, and 71.6% of all responses (n = 209) occurred in the far field. This indicates that blacktip sharks can detect and orient away from a sound stimulus at distances that extend beyond the acoustic near field, which suggests that they are not detecting sound using the inner ear otoconia.

Many marine species make use of the expanse of the ocean by traveling great distances to fulfill life history requirements. The blacktip shark, Carcharhinus limbatus, is a medium sized species associated near the coast found circumglobally in temperate waters. In the northwestern Atlantic Ocean, blacktips migrate in large aggregations along the eastern coast of the United States annually from the Carolinas in the summer months to a terminus in southeastern Florida in the winter. What daily movement patterns these adult sharks undergo during their multi-month residency at this southern terminus has largely been unquantified. I employed fin-mounted accelerometer tag packages to record high-resolution data from multiple blacktip sharks (n=8) during their winter residency in southeast Florida. Diel variation in behavior through depth range and the swimming kinematics were analyzed, while pseudo tracks were reconstructed to visually inspect post-release movement. Depth did not show significant variation between diel periods using a mixed model, with individual mean depths ranging from 1.43 – 39.20 m. Velocity was lower than expected for comparative studies of sharks, where overall mean values ranged from 0.15 – 0.93 m s-1. Diel period velocity comparison was also found to be insignificant. Tail beat frequency was found to have a group mean of 0.72 Hz, similar to other studies using different methods of the same population. The subset of tags analyzed for pseudo tracks (n=3) showed a swim away from the catch event towards the east. This data adds to the knowledge of blacktip shark movement, and contributes to the baseline comprehension of a population that is an important coastal fisheries species.