Recent advances in satellite tracking technology allow scientists to essentially follow large pelagic fish on their ocean journeys without getting wet. There are two kinds of satellite tags that are commonly used to study the movements of sharks. First are the pop-off satellite archival tags (PAT tags). These are attached to the fish’s back where they record data on water temperature, swimming depth, and light levels. After a programmed period of time the tags pop off the fish, float to the surface and transmit their data to satellite. From light level data (which tells us when the sun rises and sets) and sea surface temperatures recorded by the tag, it is possible to estimate the latitude and longitude and recreate the track of the animal. A second type of satellite tag, the SPOT (Smart Position and Temperature) tag, is attached to the dorsal fin and transmits the position of the shark when it is at the surface. The SPOT tags have allowed us to track sharks for almost two years although most tracks are shorter. For many species, the satellite tags have provided, in just a few years, more data on movements and behaviors than had been collected over previous decades.
The SWFSC HMS Group started using satellite tags in 1999 to study the movements of sharks and was one of the first groups to mount SPOT tags to a shark’s dorsal fin. In the first years of research, efforts focused primarily on mako (Isurus spp.) and thresher sharks (Alopias spp.). In more recent years, NOAA Fisheries has been collaborating with the Tagging of Pelagic Predators (TOPP) program and Mexican colleagues at the Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE) to deploy satellite tags on blue (Prionace glauca), shortfin mako (Isurus oxyrinchus) and common thresher sharks (Alopias vulpinus) in the California Current. The goals of the project are to document and compare the movements and behaviors of these species and to link these data to physical and biological oceanography. This approach will allow us to both characterize the habitats sharks most frequently utilize or prefer and subsequently to better understand how populations might shift in response to changes in environmental conditions. This type of information is critical for fisheries management, especially in the face of climate change.
The SPOT and PAT tags have provided some exciting insights into the biology of the blue, mako and thresher sharks. To date, over 200 tags have been deployed on the three species. While all species utilize the coastal waters in the Southern California Bight and regularly cross the border into Mexico, key differences among years and species are apparent. For example, over the period covered by the satellite tracks, the blue sharks tagged have traveled the farthest offshore to waters south of Hawaii. Movement patterns for mako sharks show distinct differences among years. For example, in 2004 movements were much farther offshore than in 2003 and 2005. Interestingly, 2004 was an El Niño year and the shift in distribution likely reflects a change in the availability of food near-shore.
An examination of daily dive patterns reveals some additional differences among the three species. In comparison to the mako sharks, the blue and thresher sharks show a distinct diurnal pattern. The greatest difference in day and nighttime depth distributions is apparent in thresher sharks, which spends more time at depth during the day than either of the other two species. Stay tuned for more exciting results on vertical habitat use and geographic movement patterns in the years to come. Future efforts will focus on deploying satellite tags on larger sharks over a broader geographic range. To date, most sharks have been tagged in the southern California Bight and are juveniles.