Global Population Structure of Thresher Sharks (Alopias spp.) Based upon Mitochondrial DNA Sequence Data
Tonatiuh Trejo Email address: ttrejo@mlml.calstate.edu

Skyrocketing international demand for shark fins and other shark products is driving a huge increase in shark fishing around the world. Sharks are particularly vulnerable to exploitation because of their relatively long life spans and low reproductive rate. Many sharks do not reach sexual maturity until late in life and produce only a small number of offspring. Consequently, shark populations are slow to respond when depleted by overfishing.

Thresher sharks include the common (Alopias vulpinus), bigeye (A. superciliosus), and pelagic (A. pelagicus) species, and are distinguished by large upper caudal fins, which make up almost 50 percent of the total length. They can use the caudal fin as a whip to stun or kill prey, and are often caught tail-hooked on longlines.

The common thresher shark (A. vulpinus) is the largest of the three species (up to 6.1 meters total length). It is found in all warm and temperate areas of the world’s oceans and feeds primarily on schooling fish and cephalopods, such as herring, sardines, anchovies, and squid. The common thresher is an active, strong-swimming shark, and has been witnessed leaping out of the water (see pictures below).

The bigeye thresher shark (A. superciliosus) is distinguished by its large eyes and a deep groove that runs along the top of the head, and is also found in all warm and temperate areas of the world’s oceans. It ranges from the surface to at least 500 meters depth, but is typically found below 100 meters, where it uses its large eyes to hunt for pelagic fishes and squids, including lancetfishes, herring, mackerel, and small billfishes.

The pelagic thresher shark (A. pelagicus) is the smallest of the thresher sharks (up to 3.65 meters total length), and ranges in depth from the surface to at least 152 meters. It is often confused with the common thresher, but can be most easily distinguished by its rounded pectoral fins (which are pointed in the common thresher). In contrast to the worldwide distribution of the common and bigeye threshers, the pelagic thresher is only found in the Pacific and Indian Oceans.

Thresher sharks are directly targeted in commercial and recreational fisheries around the world, primarily because of their high quality meat, but also for fins, livers, and hides. Large-scale commercial fisheries for thresher sharks have operated in the northwestern Indian Ocean, the western North Atlantic, and the central and western Pacific, typically using long-line gear (Compagno, 2001). Beginning in the late 1970s, a gill net fishery for threshers operated off the Pacific coast of the United States but declined rapidly as a result of overfishing (Smith and Aseltine-Neilson, 2001). Because only the United States and New Zealand report commercial catch statistics of these sharks, little information is available on fisheries statistics for thresher sharks worldwide (Compagno, 2001). However, a recent analysis of logbook data for the U.S. pelagic longline fleets targeting swordfish and tunas in the Northwest Atlantic indicates that catches of thresher sharks have declined by 80% since 1986 (Baum et al., 2003), a trend that likely reflects their global status.

Effective conservation and management strategies for thresher sharks require a fundamental understanding of their population structure. If thresher shark populations are genetically distinct, then management guidelines aimed at large geographic areas could be inappropriate, leading to the permanent loss of regional populations and decreasing their genetic diversity and evolutionary potential.

Like tunas, marlin, and swordfish, tagging data indicate that thresher sharks are capable of large-scale oceanic movements. A female common thresher shark tagged in the equatorial zone of the Indian Ocean in 1972 was recaptured 2 years later, in which time it had covered a distance of 840 miles (Gubanov, 1976). Recently, the National Marine Fisheries Service tagged a common thresher shark in Santa Monica Bay, California, and tracked it to an area 540 miles southwest of La Paz, Baja California just 210 days later (Smith and Aseltine-Neilson, 2001).

However, these data have proven difficult to obtain due to the inherent difficulties of working in the marine environment with elusive animals such as thresher sharks. Thus, defining the boundaries that limit individuals and populations, and measuring the amount of interchange among regions is a challenge.

Molecular genetic data can be used to infer population structure in thresher sharks by quantifying the degree of genetic relatedness between geographically disjunct populations. Although mitochondrial DNA (mtDNA) comprises less than 1% of the total genome in each cell, its rapid rate of sequence evolution and relative simplicity of amplification using “universal” primers make it an excellent marker for population genetic studies. Mitochondrial DNA was first suggested to evolve at a rate 5-10 times faster than the evolution of nuclear DNA (Rand 1994). However, not all regions of mtDNA change at the same rate. For example, the control region has been found to have a substantially higher nucleotide substitution rate than other mtDNA genes (Attardi, 1985). It is, therefore, the most commonly used mtDNA marker in population studies, and has been used to detect population structure in a number of large mobile marine organisms including the bigeye tuna (Alvarado-Bremer et al., 1998), swordfish (Reeb et al., 2000), North Atlantic right whale (Malik et al., 1999), humpback whale (Baker et al., 1993), and leatherback turtle (Dutton et al., 1999).

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