Partial migration of the nurse shark, Ginglymostoma cirratum (Bonnaterre), from the Dry Tortugas Islands

Abstract

Nurse sharks have not previously been known to migrate. Nurse sharks of the Dry Tortugas (DRTO) mating population have a highly predictable periodic residency cycle, returning to the Dry Tortugas Courtship and Mating Ground (DTCMG) annually (males) or bi- to triennially (females) during the June/July mating season. For 23 years we have followed the movements of 76 recaptured adults of a total of 115 tagged adults. Telemetry detections of 40 females tagged with acoustic transmitters show that most tagged and presumably post-partum females are continuously present in the DRTO in the fall, winter and early spring following the June mating season but these females depart in late March to early May. Detections reveal these females avoid the DTCMG completely during the next mating season, returning from late summer to fall. Telemetry records of nine of 17 adult males that co-habited with these females in the DTCMG depart DRTO waters every July. Both sexes may overwinter in the DRTO. Between 2011 and 2016 three males and five females with transmitters were detected to move up the west coast of Florida outside of the mating season as far north as the waters off Tampa Bay (335 km). Six others were only detected in the lower Florida Keys (292 km). Nine sharks returned to DRTO; one returned six times. Some overwintered and some resumed courtship in June, demonstrating both resident and migratory contingents within their population, partial migration and an ability to navigate with high spatial and temporal precision.