Basking Shark (Cetorhinus maximus) Movements in the Eastern North Pacific Determined Using Satellite Telemetry

Abstract

To fill data gaps on movements, behaviors and habitat use, both near-and offshore, two programs were initiated to deploy satellite tags on basking sharks off the coast of California. Basking sharks are large filter-feeding sharks that are second in size only to whale sharks. Similar to many megafauna populations, available data suggest that populations are below historic levels. In the eastern North Pacific (ENP) Ocean, the limited information on basking sharks comes from nearshore habitats where they forage. From 2010 to 2011, four sharks were tagged with pop-off satellite archival tags with deployments ranging from 9 to 240 days. The tags provided both transmitted and archived data on habitat use and geographic movement patterns. Nearshore, sharks tended to move north in the summer and prefer shelf and slope habitat around San Diego, Point Conception and Monterey Bay. The two sharks with 180 and 240 days deployments left the coast in the summer and fall. Offshore their paths diverged and by January one shark had moved to near the tip of the Baja Peninsula, Mexico and the other to the waters near Hawaii, USA. Vertical habitat use was variable both within and among individuals and changed as sharks moved offshore. Nearshore, most time was spent in the mixed layer but sharks did spend hours in cold waters below the mixed layer. Offshore vertical movements depended on location. The shark that went to Hawaii had a distinct diel pattern, with days spent at ∼450-470 m and nights at ∼250-300 m and almost no time in surface waters, corresponding with the diel migration of a specific portion of the deep scattering layer. The shark that moved south along the Baja Peninsula spent progressively more time in deep water but came to the surface daily. Movement patterns and shifts in vertical habitat and use are likely linked to shifts in prey availability and oceanography. Data collected indicate the potential for large-scale movements and the need for international dialogue in any recovery efforts.

Drivers of Spatial Distributions of Basking Shark (Cetorhinus maximus) in the Southwest Pacific

“…Basking sharks have been shown to dive as deep as 1,264 m and have been regularly documented at depths of 600-1100 m (Francis and Duffy, 2002; Gore et al, 2008;Doherty et al, 2017). The species has also been shown to follow distinct water masses at depth, remaining at depths of 250 m or more for months without coming to the surface (Braun et al, 2018;Dewar et al, 2018). Basking sharks are known for complex diel vertical movements, which are thought to be influenced by shifts in prey availability and oceanography (Sims et al, 2005;Dewar et al, 2018).…”

“…The species has also been shown to follow distinct water masses at depth, remaining at depths of 250 m or more for months without coming to the surface (Braun et al, 2018;Dewar et al, 2018). Basking sharks are known for complex diel vertical movements, which are thought to be influenced by shifts in prey availability and oceanography (Sims et al, 2005;Dewar et al, 2018). In wellstratified deep waters, basking sharks exhibit normal diel vertical movements (shallow depths at night, deeper depths during daylight), while sharks occupying inshore, inner-shelf areas near thermal fronts conduct reverse diel vertical movements (shallow depths during the day, deeper depths at night) (Sims et al, 2005).…”

“…It is the second largest fish in the world after the whale shark (Rhincodon typus), reaching an maximum size greater than 10 m total length (Weigmann, 2016). Basking sharks are known for their slow surface swimming behavior but may also spend months at mesopelagic depths, and may dive to at least 1,264 m depth (Gore et al, 2008;Skomal et al, 2009;Dewar et al, 2018). The species also engages in long distance migrations and has been recorded crossing the Atlantic Ocean both from east to west and from north to south (Skomal et al, 2009;Braun et al, 2018;Dewar et al, 2018;Johnston et al, 2019).…”

“…Basking sharks are known for their slow surface swimming behavior but may also spend months at mesopelagic depths, and may dive to at least 1,264 m depth (Gore et al, 2008;Skomal et al, 2009;Dewar et al, 2018). The species also engages in long distance migrations and has been recorded crossing the Atlantic Ocean both from east to west and from north to south (Skomal et al, 2009;Braun et al, 2018;Dewar et al, 2018;Johnston et al, 2019). Recent genetic analysis suggests high gene flow and weak genetic structuring across the Atlantic and Pacific Oceans (Lieber et al, 2020).…”

“…Distributions of basking shark are known to be highly variable between years, with up to 20 years between sightings reported from some Northern Hemisphere locations (Dewar et al, 2018). Their distribution and occurrence appears to be strongly linked to zooplankton/prey abundance at smaller spatial scales, but the drivers of broad scale distribution patterns are largely unknown (Sims, 2008).…”