Acoustic telemetry of shark movements and residency near artificial habitats in the northern Gulf of Mexico

Abstract

In the present study, acoustic telemetry was used to monitor the movements and residency of seven Sandbar Sharks Carcharhinus plumbeus, five Atlantic Sharpnose Sharks Rhizoprionodon terraenovae, four Bull Sharks Carcharhinus leucas and two Nurse Sharks Ginglymostoma cirratum around artificial reefs in the northern Gulf of Mexico. Sharks were tracked for periods of one to 449 days. Residency indices (RIs) were not significantly different among species (GLMM: F3,14 = 2.29, P > 0.05), however there was a significant effect of season on residency for C. plumbeus (GLMM: F3,83 = 7.54, P < 0.001), with greater residencies in fall. In addition, there was a significant, positive relation between temperature and residency for C. plumbeus (MLM: F1,82 = 12.90, R2 = 0.41, P < 0.001), C. leucas (MLM: F1,53 = 5.42, R2 = 0.12, P < 0.05) and G. cirratum (MLM: F1,20 = 12.19, R2 = 0.35, P < 0.01). A significant negative relation was also detected between dissolved oxygen and residency for C. leucas (MLM: F1,53 = 12.08, R2 = 0.22, P < 0.001). Minimum convex polygon (MCP) areas (ANOVA: F3,14 = 1.81, P > 0.05) and spatial evenness indices (ANOVA: F3,14 = 0.67, P > 0.05) were not significantly different among species. One C. plumbeus and one G. cirratum showed long-term (repeated over 3 years) preferences for specific sites. When detected within the present study area, mean distance traveled was 5.5 km day-1for R. terraenovae, 6.3 km day-1 for C. leucas, 8.8 km day-1 for G. cirratum and 8.1 km day-1 for C. plumbeus. Long-distance migrations away from the study area were observed for one C. leucas (843 - 858 km), one G. cirratum (230 - 856 km) and three C. plumbeus (229 – 1,894 km). Specifically, one C. leucas was detected in the present study area in the summer after two annual winter migrations to the Florida Keys. One G. cirratum made three migrations (February - April) to the Florida Keys and returned each year to the northern Gulf of Mexico in late summer and stayed through fall. One female C. plumbeus was detected off Tampa, Florida one spring (May), and one male C. plumbeus was detected over three winters (January, February) along the Florida panhandle and further south to Tampa, Florida. Interestingly, one additional female C. plumbeus migrated around the southern tip of Florida and up the Atlantic coast and was detected in May 2017 off the South Carolina coast. All of these sharks subsequently returned to the study area after each migration. The long-term residencies and homing behavior observed in the present study indicate that the artificial reefs off coastal Alabama provide important habitat for shark species, with foraging as their most probable function. These homing behaviors, combined with the occurrence of regular long-distance migrations, indicate that the artificial reefs off coastal Alabama may have far-reaching effects that extend as far as the northwestern Atlantic Ocean.