The Basking Shark, A Modern Marine Mystery
This week, we’re diving in with an elusive gentle giant, the Basking Shark.
Sharks have been ruling the media lately. From Great White sightings off of Cape Cod to mysterious appearances in wooded backyards to the recent string of encounters along the coast of North Carolina, it seems that sharks have been everywhere, just in time for the annual explosion of shark media – the always entertaining, awe-inspiring, and extraordinarily sensationalist Discovery Channel Shark Week.
Last week, sharks, a diverse and ecologically vital clade, have been labeled as “ninjas,” “monsters,” and “serial killers” – kind of aggressive descriptions, right? While some species do serve as apex predators, maintaining delicate ecosystems from the top of the food web, it’s important to remember that some sharks, such as this week’s feature, also maintain ecological balance by feeding at the base of the food web. Basking sharks aren’t interested in seals or tuna. They’ve got much smaller prey on their menu—tiny fish, fish eggs, and zooplankton.
It may be difficult to believe that a creature often mistaken for a Great White is actually a filter feeder. These gentle giants are estimated to grow up to 12 meters (exceeding 30 feet), but they survive on itty bitty prey consumed in massive quantities. Named for their tendency to “bask” on the surface of the water, basking sharks swim open-mouthed, passively feeding as their gill rakers act as sieves, preventing prey from passing through their gill slits.
Basking sharks are still a bit of a mystery to scientists. We know they roam the cooler waters of the Northern Atlantic and Pacific during the summer months, but for decades, the world’s second largest fish disappeared every winter.
In 2009, Gregory Skomal from the Massachusetts Division of Marine Fisheries and colleagues discovered that, during winter months, New England basking sharks travel south at depths between 200 and 1,000 meters (600 and 3,000 feet). While some tagged sharks stopped in Florida, others traveled as far south as the Caribbean Sea, or even the mouth of the Amazon River! The migration has Skomal questioning previous beliefs about basking shark population structure: “What were thought to be regional stocks may in fact belong to a single, oceanwide population.”
Why do the sharks make this annual trek? Skomal suggests that they follow plankton to warmer waters in the winter months. “But why do they move all the way to Brazil?” Skomal asks. “There is plenty of food for them in northern Florida.” One possibility is that they migrate south to find nursery grounds. “We still have no idea where they give birth,” says Skomal.
Endangered Mammoth Migrators
While we still have a lot to learn about basking sharks, we do know that their populations are dwindling. In 2000, the International Union for the Conservation of Nature listed the North Pacific and Northeast Atlantic populations as endangered and the species itself as vulnerable.
The main reason for population decline is fishery overexploitation. For centuries, basking sharks were caught for their liver oil (to be used for lighting and industry), their skin (to be used as leather), their flesh (for food and fishmeal), and their fins (which are highly valuable in international trade, especially in East Asia). The basking shark’s exceptionally slow recovery rate – females are believed to sexually mature between 16 and 20 years old – makes them extremely vulnerable to overfishing.
Today, almost all basking shark fisheries around the world are closed. The only significant trade is in bycatch from New Zealand blue grenadier fisheries. Basking sharks are protected by the Convention on the Conservation of Migratory Species, the European Union, the International Council for the Exploration of the Sea, the Convention on International Trade in Endangered Species of Wild Fauna and Flora, and numerous national governments, including the United States federal government.
You can help!
Basking sharks are a common sight in our waters. Dr. Jon Witman from Brown University has spotted the sharks at Cashes Ledge, and there have already been reports this year of sightings off the coasts of Massachusetts and Maine.
Also, the New England Basking Shark and Ocean Sunfish Project is working to better understand the biology and ecology of these mammoth migrators, and they need help from citizen scientists like you! If you spot one of these gentle giants, be sure to report your sighting. The more data collected, the more we can learn and help protect!
Our Fish Feature series shares posts from New England Ocean Odyssey’s weekly “Fish Fridays.” New England Ocean Odyssey uses breathtaking images and captivating stories to highlight the amazing riches and diversity of New England’s ocean and bring attention to the issues facing it.