Pacific White Sharks
Adult white sharks tracked from several sites along the North American coast travel to a region in the northeastern Pacific, equidistant between Baja California and Hawaii, where they remain for up to six months. It remains unclear whether these represent breeding or feeding migrations
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Due to infrequent, yet often sensational interactions with people, white sharks have long captured the imagination of humans. Most of the studies of white sharks have centered around pinniped rookeries, where adult a white sharks feed on elephant seals and sea lions. Off the coast of northern California, the interactions between pinnipeds and white sharks have been studied at the Farallon Islands and Año Nuevo Island for decades (Ainley et al. 1985). White sharks are present at these islands predominately in the late summer through winter when they feed on young elephant seals and sea lions. Although pinnipeds are present throughout the year, white sharks are apparently only present for a portion of the year and their movement patterns after leaving remained a mystery for decades. With the advent of novel electronic tagging technologies, it has since been possible to track white sharks for periods of up to one year and shed light on their movement patterns after departing pinniped colonies.
To better understand the movements of white sharks throughout the year, a tagging program was initiated in 1999 off the Farallon Islands (Boustany et al. 2002, Weng et al. 2007a). Pop-up satellite tags are attached externally to marine animals allowing for attachment without the need for capture, which is useful when dealing with potentially dangerous animals such as white sharks. The tags collect pressure, temperature and light-level data while on the fish and then detach at a preprogrammed date and transmit summaries of stored data to satellites. Pressure readings reveal depth preferences of the tagged animal while light-level and temperature data can be used to reconstruct geolocated tracks. Much to the surprise of white shark researchers, sharks tagged off the coast of California all showed movement far offshore. All sharks showed highly directed movement (> 100 km/day) to the southwest in the winter or spring to the subtropical gyre equidistant between Baja California and the Hawaiian Islands. A small number of sharks maintained movement to the west, eventually ending up in the waters of Hawaii. All sharks remained in these areas until the late summer or early fall, at which point they showed return migrations to the coast of central California, oftentimes to the same small islands from which they were originally tagged. Pop-up tagging conducted at Isla Guadalupe, off the coast of Baja California, Mexico showed similar movement patterns, with most sharks migrating to the same offshore region and several sharks continuing on to the Hawaiian Islands (Domeier and Nasby-Lucas, 2008). While adult sharks from both Mexico and California travel to the same offshore regions, no individuals have yet been observed to travel between pinniped colonies in Mexico and the US.
Figure 1. Panels on left from Weng et al. 2007a showing seasonal densities of white sharks tagged off the northern California coast, USA. Panels on the right from Domeier and Nasby-Lucas, 2008 show seasonal densities of white sharks tagged off Baja California, Mexico.
While movement to the Hawaiian Islands is understandable, given the abundant marine mammal prey base, what attracts white sharks to the open ocean area between the continent and Hawaii is less understood. This area overlies a seafloor that is several kilometers deep with few seamounts or other physiographic features of note. Being subtropical, the surface waters are generally warm (22-25o C) with a deep thermocline and little in the way of productivity, eddies, fronts or strong temperature gradients. In addition, few fish or marine mammal resources are known in this area with commercial fish catches for tuna, billfish and sharks all being higher either to the north or to the south of this region. Likewise, whale, dolphin and pinniped densities are all higher, either to the north or south, on in the coastal areas of Hawaii or North America. While in this region, sharks exhibited frequent deep diving (> 700 m) experiencing a wide range of temperatures (4-27o C).
These offshore movements stand in contrast to migration patterns exhibited by juvenile white sharks. One to three year old white sharks electronically tracked off the North American coast showed only coastal movements, remaining near shore in the waters of Southern California and Northern Mexico (Weng et al. 2007b). These waters are known nursery areas for young of the year and juvenile white sharks where they feed on fish before making the transition to feeding on marine mammals as they mature.
There are several hypotheses regarding why white sharks exhibit offshore migrations to this specific region of the northeast Pacific. It is possible that this area serves as a pupping ground, where female white sharks give birth in an area that is warm and low in predators, increasing survivorship of young sharks. These young white sharks could then travel towards the nursery areas along the North American coast sometime in their first year of life. Both male and female sharks travel to this region, suggesting that it does not constitute only a pupping ground, as males would have no need to migrate there if that was the case. It remains possible that this area serves as both a pupping and breeding ground, providing the gestation period is approximately 12 months long. In such a scenario, males would mate with females shortly after the females give birth. It also remains possible that this is a foraging area for adult white sharks. The extended period of time that sharks remain in this region, up to 9 months, and extensive diving patterns observed while sharks are there, suggest foraging. With few marine mammal resources known from this region, it remains unclear what prey items white sharks would be targeting. As such, even after a decade of tracking white sharks to this area, it is still unknown why they travel to this region. However, given the fact that all adult sharks tagged off the North American coast travel to and spend a considerable amount of time in this area, it can safely be stated that this region is important for the ecology of white sharks in the North Pacific.
How the EBSA was Identified
Both studies used diving and movement data returned from the tags to discern different behavioral patterns exhibited by the sharks. Diving patterns changed abruptly as sharks moved from the coastal waters and began their migrations to the west. Daily dive depths increased from less than 70 m to greater than 500 m as sharks left the pinniped rookeries in the early winter. During the migration phase, time spent at the surface also changed dramatically. While hunting for seals and sea lions nearshore, sharks spent very little time (<25%) in the top 5 m but as they moved offshore time spent in the surface waters increased dramatically (>50%), possibly to allow for celestial geolocation by the sharks (Weng et al. 2007a, Domeier and Nasby-Lucas, 2008). In addition to diving behavior, horizontal movement patterns were also used to delineate different regions and behaviors in the seasonal cycle of white sharks. Both studies used longitudinal movements to identify when sharks began their offshore migrations and when they arrived to the presumed offshore foraging area. It was therefore possible to identify migratory corridors as well as foraging regions.
In addition, it is possible to calculate utilization distributions for white sharks for both the nearshore and offshore phases of their lives. Weng et al (2007a) used the kernel density method (Silverman 1986) to identify specific areas of high use by the tagged sharks. Tracks were regularized to one position per day and all the resulting points were broken down into meteorological seasons. Spring and summer seasons corresponded with the offshore component of the white shark’s annual cycle. Both studies also calculated 25, 50 and 95% utilization distributions, with the 25% contour identified as the core region of habitat utilization. Utilization distributions were calculated using the home range extension (Rodgers & Carr 1998) for ArcView 3.2 (ESRI).
Sources of Data
Although animal observation data can be difficult and expensive to collect for open-ocean species, several sources exist to serve these data after they have already been collected. The Tagging of Pacific Predators project (TOPP) is one of the largest electronic tracking projects in the world. To date, researchers associated with this project have tagged over 2000 individual animals in the North Pacific Ocean. Twenty-two different species, including elephant seals, white sharks, leatherback turtles, Humboldt squid, albatross and sooty shearwaters have been tracked and data can be viewed online through the TOPP website (http://www.topp.org/). Beyond serving only electronic tag data, several programs are currently consolidating all types of marine animal observation data, including data from surveys, fishery catch data and tagging data. The Ocean Biogeographic Information System (OBIS) has set up an online portal where researchers can upload and download any marine species observation data (http://www.iobis.org/). They currently have over 19 million individual sightings from over 106,000 marine species from 660 databases. While the OBIS program deals exclusively with marine species, the Global Biodiversity Information Facility (GBIF) performs a similar service for both marine and terrestrial species (http://www.gbif.org/). The GBIS program currently has over 180 million records from over 8,000 individual datasets. From all these websites, it is possible to contact individual researchers in order to gain access to data for scientific purposes.