Residency and long-distance movements of sevengill sharks (Notorhynchus cepedianus) tagged in San Francisco Bay

Abstract

Background: Sevengill sharks are common inhabitants of estuaries and coastal areas and particularly abundant in San Francisco Bay (SFB). There is limited knowledge about the level of residency and migratory movements of this species. We describe the degree of residence of sevengill sharks in SFB and nearby locations, using ultrasonic tags and automated listening stations. Results: Sevengills showed a high degree of residency in SFB, particularly to the Golden Gate (GG) area where they have a strong site preference, both seasonally and inter-annually. Site fidelity was also the highest at the GG. In sharks with deployment times longer than 300days, we found that >80% of visits of all mature males were at GG, 80% of visits of a single mature female occurred there, >85% of visits of most juvenile males, and >70% of visits of most juvenile females. At GG, sharks moved into the range of the receiver day and night without a strong preference for a particular time of the day, which implies movements up and down the area day and night. At most other locations within the bay, sharks were present during daytime hours, while at Point Reyes during nighttime hours. Sharks moved into SFB during early spring and summer, and moved out of SFB to Point Reyes during late spring and fall. Three sevengill sharks migrated roughly 800km southeastward along the coastline from SFO, one making three trips back and forth between the sites. Conclusions: There was strong evidence of residency of sharks within SFB, and sharks repeatedly returned to the same region of the bay with some sharks making long-distance movements.