Shark Demography and Population Dynamics

Common thresher shark (Alopias vulpinus) with satellite tag

Thresher (Alopias vulpinus), shortfin mako (Isurus oxyrinchus), and blue sharks (Prionace glauca) are captured in both commercial and recreational fisheries in the California Current. The California-based drift gillnet (CADGN) fishery is the commercial fishery which catches the greatest number of each of these species. Relative abundance trends for common thresher, shortfin mako, and blue shark in the CDGN have been investigated using data from fisher bridge logs, onboard observer records, and an NMFS fishery-independent relative abundance survey. Preliminary results indicate that local thresher shark stocks may be rebuilding after being overfished during the 1980s. Common thresher sharks have the greatest commercial value of sharks landed in California and are taken in both regional commercial and recreational fisheries. Pupping is believed to occur in the spring in nearshore waters ranging from Bahia San Vizcaino Bay in Baja California, Mexico, to at least as far north as Point Conception, CA. The Southern California Bight is a nursery area for neonate thresher sharks and as such, the commercial and sport catch of threshers off southern California consists largely of juvenile sharks.

Researchers use information on the unique life history characteristics of sharks and apply it to mathematical models to estimate and compare the rebound potentials of various shark species, i.e., their relative productivity and ability to rebound from fishing pressure or other sources of mortality. They have developed a method which does not require large data sets and incorporates a density-dependent population response, which they have used to examine the relative productivity of a range of shark species, including most U.S. West Coast shark species.

Trends in relative abundance of shortfin mako and blue sharks show a slightly decreasing trend in abundance along with decreased fish size in the catch over the same period but the extent to which this has been influenced by shifts in environmental conditions and fish distributions is not known. Efforts to determine abundance trends from commercial fishery data have been complicated by changes in regulations, targeted areas, and fishing methods over time. These changes have resulted in inconsistent capture rates and catch distributions that are difficult to interpret. Therefore, the SWFSC has been conducting a near shore longline survey in the Southern California Bight each summer since 2003 in order to define the core nursery areas of young-of-the-year common thresher shark pups. Additionally, a fishery-independent estimate of recruitment abundance is conducted and the data are currently being used in a collaborative thresher stock assessment with colleagues from Mexico. This complements the fishery-dependent data available through the nearshore small mesh net fisheries and offshore California drift gillnet fishery to provide measures of relative abundance of common thresher sharks for stock assessment models. In addition to providing important information on abundance and distributions, the thresher shark pre-recruit survey enhances other ongoing research at SWFSC, including age and growth, feeding, and habitat utilization studies.

The distribution of common threshers is very patchy and areas of high abundance are not consistent across years. In all years, however, a large percentage of the catch has been neonates, which were found in all areas surveyed. The resulting neonate index of abundance should mirror adult abundance because adult population and recruitment should be tightly linked in K-selected species such as sharks. K-selected species are those which mature slowly and live long lives, inhabit relatively stable environments, have long gestation periods (many months), and care for their young. In addition to  sharks, examples of K-selected species include primates, horses, and birds.

Check out ISC reports on shark demography and population dynamics.The previous link is a link to Non-Federal government web site. Click to review NOAA Fisheries disclaimer


Last modified: 5/17/2017