21. May, 2017

Migrations of the great white shark

In the world of sharks the  great white  has always been an  outsider. It has a high media profile attention, and is the only shark that still carries that aura of brutal force, a ‘raw predator’  with its powerful body often  covered with scars and its fearless approach of human visitors.  It  prefers open and cold waters, with Australia/New Zealand, South Africa and the North Eastern Pacific as the mostly frequented locations. UW photographers that visit Guadalupe island in Mexico in autumn or early winter have a good opportunity  to get some nice pictures of this legendary shark. The  island is  the home base of the elephant seal which makes it a favorite place for foraging in that season.  Unfortunately, being  locked up in a cage is still the only safe  option for UW photographers to get close to the great white.  In South Africa at Seal island, another hotspot for meeting  great whites  visitors can even  join great white shark breaching trips  combined with cage  diving.  Great whites are attracted by a seal decoy  pulled behind the vessel and occasionally jump with lightning speed with unbelievable precision and accuracy to take the decoy.

Upper picture: Location data for four satellite-linked radio-telemetry-tagged female white sharks leaving from Guadalupe island during their offshore migration. Adapted from **

Lower picture: Site fidelity of all satellite tagged white sharks to three core areas including the North American continental shelf waters and the waters surrounding the Hawaiian Island Archipelago and the ‘White shark Café’.Yellow circles represent position estimates from light- and SST-based geolocations, red circles  satellite tag endpoint positions respectively.  Adapted from ****.

Apart from the white sharks disputed  reputation as a man eater and spectacular  sightings we know very little of its behavior:  its migratory routes, breeding grounds, socialising behavior, how it attracts a mate and what drives its search for a prey.  Getting to know how and were they travel  is one the challenges for  marine biologists  as well shark conservationists. The great white is on the red list of theathened animals of the World Conservation Union, and research that clarifies  their immigration patterns, mating and pupping grounds could indeed  have important implications for their conservation.  Are great whites  like other apex predators,  that often  alternate  long trips in the wide ocean  with visits to  specific aggregation areas?  

Bio-telemetry is the tool  that  has  shed more  light on these questions in the last decade. It implies  detection of  a shark from a certain distance  with an  electronic receiver that picks up a signal from a shark implanted  ‘tag’.  Often used are acoustic listening receivers placed at at the ocean floor at hot spots near the coastline. Whenever a tagged shark comes within a distance of  say 800  feet of a receiver, a code or ‘ping'' for that particular shark will be received and transmitted. Its a useful  tool to detect  the presence of a shark in specific areas. More advanced sattelite  based tools are  PAT (Pop-up Archival Transmitting) and SPOT (Shark Position and Temperature) tags.   PAT or Popup archival tags are normally active for maximal one year  and provide approximate location data, making  it most useful for tracking long-distance migrations.  These tags remain inactive as long as the shark is submersed,  until the tag detaches itself from the shark and floats to the surface and starts transmitting its collected data.  They do not have to be physically recovered for the data to be obtained. Popup tags are relatively non-invasive small transmitters. They are  placed with a tag pole  near the base  of  the dorsal fin, mostly  when the sharks come close  to a diver or takes a bait from  a small boat drifting with the shark on the surface.  In contrast, the  more expensive battery operated SPOT tags will transmit information for several consecutive years, making it the first choice  for  studying long term migration  periods. These tags do not work underwater but transmit a signal  to a satellite receiver whenever  the dorsal fin breaks the surface of the water. They are more difficult to deploy since the shark has to be caught and restrained while the tag is attached to the dorsal fin. Which requires hooking  the shark with a line and bait supended from a vessel, hauling it on board of the vessel, where the transmitter is attached by drilling  holes in the apex of the dorsal  fin and secured with metal or plastic bolts. It  certainly  involves some form of  mutilation of the shark, justified by the argument that it  provides a more complete picture of the sharks  migration routes  than  PAT tags.

GWs are pelagic as well as  philopatric Some  useful information has been collected with sattelite tagging  in the past years on its migration and  breeding sites. An important finding was that  great whites are pelagic (go out in the open sea)  as well as ‘philopatric’ (returning to the same sites). In the open ocean they often display long migration paths with  strange and irregular  patterns. Males and females also seem to differ  with respect to the duration of their migration phases.  Some more details are presented below.

South Africa In South Africa, great white sharks in False Bay do not stay at Seal Island year-round, although it provides their favorite food, cape fur seals*. Most of the PAT  tagged white sharks revealed at least three different movement patterns, including wide-ranging coastal migrations up and down the eastern side of South Africa. Often they leave the island for long trips in the Ocean, even  extending over  thousands of miles and crossing oceans separating continents.  Most notorious case is  female shark Nicole (named after Nicole Kidman)  that was tagged in Gansbaai South Africa   and  migrated 11,000 km to Western Australia.  Nicole was first tagged in November 2003, near the Western Cape of South Africa where researchers affixed a pop-up archival satellite transmitting tag to her dorsal fin. This is not a unique event;  there have also been a few other recorded incidents of great whites from South Africa migrating to Australia and back within a year. This also included  return of females mating in Australia to give birth in South Africa.

Pacific white sharks: Guadalupe island migrations Most detailed  information on great whites migratory patterns has been collected  from  northeastern Pacific white sharks, revealing a seasonal migration between a vast offshore region and coastal aggregation sites. The sharks spend roughly half of their time in the deep-ocean environment, sometimes traveling as far as the Hawaiian Islands before returning to the continent. Hawaii is likely to be an important foraging area for  great whites. A  study  by investigators  of the Marine Conservation Science Institute used  satellite-linked radio-telemetry (SLRT) tags with a multi-year battery capacity on a limited group of sharks.** (see insert:  upper figure for aggregated data of 4 females).  It confirmed a strong seasonal philopatry of GWs  to one of two aggregation sites in the northeastern area: one off central California, USA, and the other at Guadalupe Island. Mexico. There were also  interesting  sex differences. Females showed a  2-year migration period that  rougly consisted of a offshore gestation period phase which began when the females departed Guadalupe island. They spent around 18 months patrolling  in the great ocean between Baja California, Mexico and Hawaii before migrating back to the coastal regions of Mexico during the pupping season.  During this phase, females remained in the coastal waters of Baja California, after which  they returned to Guadalupe  again for the autumn and winter months, probably with a double purpose: feeding on the seals and mating.

White shark ‘café’ Tracking data  of this study also indicated that during the offshore phase mature males and females were spatially strictly segregated, but joined again during  their concurrent seasonal presence at Guadalupe Island.  For males the offshore migration phase lasted only one year. Curiously, males in particular had a strong preference to hang around at a location  approximately halfway between the coast of Baja California, Mexico and the Hawaiian Islands, also called  shared offshore foraging area (SOFA) or White shark Café. This area lies within the eastern boundary of the North Pacific Gyre. Their preference for this destination remains a mystery because it is presumed to have very little food for the animals; researchers describe it as the shark equivalent of a desert (see also ***)

North Pacific Californian coastline Another study was carried out on a much larger group of North Pacific sharks by biologists from  the Department of Biology, Stanford University. **** They mainly  focused on the migratory routes of great whites of  the California coast,  using satellite PAT tags in combinaton with  passive acoustic monitoring via listening stations near the coastline. The Californian  sharks also  alternated  site fidelity at hot spots along  the Californian coastline with  long-distance migrations to and from defined oceanic core areas (see insert: lower figure for aggregated data).  This included the  same foraging  area  (White shark café) visited  by the Guadalupe  island sharks. “What we know," said Salvador Jorgensen, who co-authored the study, "is that all of them leave the coast in the winter and all of them end up either in the cafe or offshore in Hawaii."  By comparing overlap between male an female sharks  the study suggests that mating might also occur at the Cafe. Observed overlap was minimal near Hawaii and  ‘’no direct or indirect evidence of copulation at North American coastal sites has ever been reported,  despite decades of observation’. So the issue of mating sites still remains a bit  of an open question  in both studies of  north Pacific whites.

Sources  and links

* Bonfil R, Meyer M, Scholl MC, Johnson R, O’Brien S, Oosthuizen H, Swanson S, Kotze D, Paterson M: Transoceanic migration, spatial dynamics, and population linkages of white sharks. Science 2005, 310:100–103

**Domeier ML, Nasby-Lucas N: Migration patterns of white sharksCarcharodon carcharias tagged at Guadalupe Island, Mexico, and identification of an eastern Pacific shared offshore foraging area. MarEcol Prog Ser 2008, 370:221–237.

***Domeier ML, Nasby-Lucas N, Palacios DM: The Northeastern Pacific white shark Shared Offshore Foraging Area (SOFA): A first examination and description from ship observations and remote sensing. In Global Perspectives on the Biology and Life History of the White Shark. Edited by Domeier ML. Boca Raton: CRC Press; 2012:147–158.

****Jorgensen, SJ; Reeb, CA; Chapple, TK; Anderson, S; Perle, C (2010), "Philopatry and Migration of Pacific White Sharks", Proceedings of the Royal Society B277: 679–688

https://en.wikipedia.org/wiki/White_Shark_Caf%C3%A9

http://www.whitesharktrust.org/migration.html

https://www.niwa.co.nz/coasts-and-oceans/research-projects/white-sharks