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 300 days, 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 800 km 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.

First records of the seven‐gilled Notorynchus cepedianus and six‐gilled Hexanchus griseus sharks (Chondrichthyes: Hexanchiformes: Hexanchidae) found in the Galápagos Marine Reserve

“…As N. cepedianus is thought to predominantly inhabit shallow coastal waters (Ebert, 2002), finding this species 1000 km from the continental N. cepedianus has also been found in the remote archipelago of Tristan da Cunha, >2700 km from the nearest South African coastline (Caselle et al, 2018). These findings, together with those of the present study, indicate that N. cepedianus also occurs in offshore habitats, supporting the notion that this species is capable of undertaking large-scale movements (Ketchum et al, 2017;Williams et al, 2012) and spends more time in deeper waters (Stehfest et al, 2014) Currently, very little is known about H. griseus and N. cepedianus populations in the ETP apart from data gleaned from by-catch reports in Peru and Ecuador's coastal region. Thus, documenting and defining their occurrence and ranges is critical to formulating informed conservation strategies and identifying key habitats for their survival (García et al, 2008;Hearn et al, 2014).…”

Use of a hydrodynamic model to examine behavioral response of broadnose sevengill sharks (Notorynchus cepedianus) to estuarine tidal flow

“…The San Francisco Bay is the most geographically expansive estuary along the California coastline, with a surface area of 1240 km 2 (Conomos et al 1985). Tides are mixed and semidiurnal (i.e., two unequal high and low tides per day), with a high-flow channel where currents reach peak velocities (approximately 2.0 m/s; Bennett et al 2002) through the narrow passage underneath the Golden Gate Bridge (Conomos et al 1985;Ketchum et al 2017). The mean tidal range is approximately 1.7 m (NOAA, National Ocean Service 2019), and during the study period, tidal amplitude ranged from 1.5-4.0 m. The mean depth of the entire estuary is less than 10 m, but the central channels (such as that beneath the Golden Gate Bridge) can reach depths of over 100 m (US Geological Survey 2014).…”

“…During the spring and summer, the San Francisco Bay estuary serves as a foraging and pupping ground for one such species, the sevengill shark (Notorynchus cepedianus; Ebert 1989; Barnett et al 2010aBarnett et al , 2012Ketchum et al 2017). Classified as data deficient by the International Union for Conservation of Nature (Compagno 2009), sevengill sharks are widely distributed in temperate coastal regions, and diet studies have shown that they may play a significant ecological role through regulation of mesopredator populations in these habitats (Barnett et al 2010b).…”

“…This species tends to occupy bays and estuaries throughout its range, moving seasonally between inshore and offshore waters (Ebert 2003;Last and Stevens 2009;Barnett et al 2010aBarnett et al , 2012Ebert and Compagno 2012). Sevengill sharks also show a preference for high-flow areas (Ketchum et al 2017), like many other mobile marine predators such as cetaceans, pinnipeds, and seabirds that exploit tidally energetic environments for foraging opportunities (Benjamins et al 2015;Lieber et al 2018). It is therefore likely that their movements are affected by the prevailing tidal flow in the deep narrow channel underneath the Golden Gate Bridge.…”

“…We combine tidal current simulations generated at a~50 m resolution with active tracking data from three sharks. In the estuary, sevengill sharks show a preference for high-flow areas, such as the channel underneath the Golden Gate Bridge (Ketchum et al 2017). We first hypothesized that sevengill sharks would minimize energy expenditure by consistently moving in the direction of tidal flows to travel within the high-flow channel.…”