The species we know more about fall into two categories: those typically smaller species, such as leopard, horn, and angel sharks, that do not stray far from their home territory; and larger species with commercial value. The Southwest Fisheries Science Center (SWFSC), a regional research branch of the National Oceanic and Atmospheric Administration (NOAA), publishes fact sheets about 16 species of shark, but the thrust of active research is devoted to three species: shortfin mako and common thresher sharks, which, considered good eating, come to market typically as bycatch in the swordfish fishery; and blue sharks, which are also caught as bycatch in gillnets but are then discarded at sea, having no commercial value.

In 2006, SWFSC reported that in Pacific Ocean waters thresher shark stocks were possibly rebuilding after being overfished during the 1980s, while shortfin makos and blues showed a slightly decreasing trend in abundance, with decreased size as well. No crash, but trends that bear watching.

Large sharks are highly migratory, and spend at least some time far from shore. Scientists are gaining a certain amount of knowledge of their movements from satellite tagging studies and analysis of catch patterns. "Marrying . . . satellite imagery to the animals’ tracks, we’ve started to identify the ocean equivalent of desert oases or the watering holes of African savannahs, where the animals gather to feed and to breed," states the website of Tagging of Pacific Predators (TOPP), a project that is part of the Census of Marine Life and is comanaged by NOAA’s Pacific Fisheries Ecosystems Lab, Stanford’s Hopkins Marine Lab, and University of California, Santa Cruz’s Long Marine Laboratory. A map showing "near real-time" tracks of individual mako sharks over the course of a year can be viewed at the TOPP website (http://topp.org/species/mako_shark); sometimes they are clustered off San Diego, and at other times they scatter widely. TOPP has also tagged blue and great white sharks.

The tags used on blue and mako sharks, which often swim close to the surface, are called SPOT (smart position or temperature transmitting) satellite tags, and are attached to the animal’s dorsal fin. When the antenna breaks the surface, it sends data--including water pressure, water temperature, and travel speed--to a satellite. The animal’s location is estimated by calculating the Doppler shift in the transmission signal in successive transmissions. When the animal goes beneath the surface, a saltwater switch turns off the tag.

Another sort of tag, the pop-up archival tag (PAT), is more suitable for animals that don’t spend a lot of time in surface waters, such as thresher and great white sharks. Divers insert PAT tags into the animal using a small surgical titanium anchor. At a preset time, such as 30, 60, or 90 days after the tag is attached, the battery triggers the tag’s release and it floats to the surface. The tag then sends samples of its data to a polar-orbiting Argos satellite for about two weeks. If the tag is later found, the entire data set can be downloaded and analyzed. The information collected includes pressure (to determine dive depths), ambient light (to estimate location), and internal and external body temperature.

All this information tells us quite a bit about where these fish go. The data also show which fish stray over national borders--an area of concern for fisheries management--or out into longline-infested open-ocean waters.

Blue sharks, for example, have been tracked from the West Coast of North America westward to the Hawai’ian Islands and Midway. Thresher sharks are thought to have a seasonal north-south migration between Baja California and Oregon and Washington, with pupping taking place in southern California in early spring. The SWFSC description of shortfin mako shark movements, meanwhile, is full of words like "appear to," "estimated," and "presumably." Apparently, they start off in southern California waters, then move offshore or to the south. Many tagged fish have been recaptured in southern California, but some have been taken as far north as Point Arena, as far south as Acapulco, and as far west as Hawai’i.