Big Threats to the Bigeye Thresher

Defenders calls on NMFS to protect the bigeye thresher under the Endangered Species Act

As the name suggests, the bigeye thresher shark (Alopias superciliosus) has a very distinctive look. Its extremely large eyes are unusual among sharks, and placed in sockets that allow them to rotate upward to help spot prey above. Like all thresher sharks, it has an extremely long tail that it can use, whip-like, to stun its prey. In the case of the bigeye, its tail is almost as long as its entire body, allowing it to grow up to 15 feet long!

The bigeye thresher shark is a “pelagic shark,” which means it is found in the open ocean and can swim to great depths. It is a highly migratory species of shark, and is found all over the world, including the waters off the eastern coast of the United States. However, the bigeye thresher shark population is vulnerable worldwide and at great risk in the Atlantic region.

The Problem(s)

The most critical threat to the bigeye thresher shark comes from commercial and recreational fishing. In certain areas of the world, the shark is targeted for its fins. In other areas, it is caught as bycatch (meaning, caught accidentally) by commercial fishing boats targeting other species, such as tuna. Sadly, its own natural habits make the bigeye thresher particularly vulnerable to becoming bycatch.

The shark spends its days in deep water, sometimes up to 500 feet down, and comes to the surface at night to feed. Traveling through that range of depth exposes the shark to many types of fishing gear, from long lines to drift nets. And since the sharks are so large, they can easily get stuck in fishing nets that are often left for days in the open ocean.

Even though it is illegal to possess a bigeye thresher shark in the United States, some recreational fishermen continue to target this species. In 2011, nearly 2.7 million sharks were caught recreationally in U.S. waters alone and the bigeye thresher shark is one of the most sought after. Even when recreational fishermen catch the sharks, they don’t always keep them. And sharks that are caught and released are often too injured to survive. In the Atlantic region, all thresher shark populations have declined by at least 80% since just the early 2000s. Compared to how abundant thresher sharks were historically, the decline is even greater.

Bigeye thresher sharks take a very long time to reproduce. In fact, they have the lowest reproductive rate of almost all shark species. Not only does this shark take 9-13 years to mature, but even then females only give birth to a maximum of two pups. This means that it can take up to a decade to replace every shark that is lost to bycatch, finning or other causes. Right now, the sharks are being killed far too quickly to keep up.

What Can Be Done?

There is no comprehensive international protection for the bigeye thresher. State shark finning bans, domestic and foreign national laws, and international fishery management agreements have not been able to effectively stop the decline of bigeye thresher populations. In November 2014, the bigeye thresher shark was listed as a protected species under the Convention on Migratory Species – an environmental treaty under the United Nations. Unfortunately, even this effort falls short because many of the countries in the bigeye thresher’s range haven’t signed on to this treaty – including the United States.

Because of the threats to the bigeye thresher, its alarming decline, and the power the U.S. has to protect these sharks, Defenders is filing a petition asking the U.S. National Marine Fisheries Service to list the species under the Endangered Species Act. Though few sharks are currently protected under the ESA, there has been some recent progress. The scalloped hammerhead shark became the first to receive protection under the ESA last summer. And just last month, the common thresher shark was determined to be a candidate species. We are eager to get the bigeye thresher shark the protection it needs to survive and recover.