The Wonder Down Under

The January/February 2016 issue of Brown University’s Alumni Magazine includes a feature of Cashes Ledge and Dr. Jon Witman, who is a professor of biology at the university and a Cashes Ledge expert. Having dived at Cashes Ledge for more than 30 years, Witman has seen the underwater mountain range evolve from a bountiful ecological environment to a still-productive but threatened habitat. Below is an excerpt from the article by Louise Sloan. Read the full version here

It’s not exactly a trip to the Statue of Liberty or Muir Woods. To get to Cashes Ledge, part of a proposed national monument in the Atlantic Ocean’s Gulf of Maine, you have to get in a boat and head to a spot about eighty miles east of Cape Ann, Massachusetts. After the four-hour trip, you drop an anchor near Ammen Rock, the tallest pinnacle in Cashes Ledge, a twenty-five-mile-long underwater mountain chain. Ammen Rock rises from the sea floor 720 feet below to within thirty feet of the water’s surface. Once there, divers set up a buoy marking the spot, the only clue to Cashes’s underwater marvels. Then they jump into forty-degree water that’s moving at a speed of two to three knots—about as fast as a class II rapids—and “swim like hell for the buoy,” says Professor of Biology Jon D. Witman, who has been conducting research at Cashes Ledge for more than thirty years.

As you pull yourself hand-over-hand down the buoy rope, Witman says, you slowly make out what looks like the ocean floor. But, as you get closer, you realize it’s moving. What you’re looking at is the canopy of an undersea jungle, a forest of kelp exponentially thicker than any you’ll find elsewhere in the coastal Gulf of Maine. Because of the distance between Cashes Ledge and the coast, where the water is clouded by runoff and other pollutants, sunlight penetrates deeply into the clear, cold water. As a result, the kelp grows as far down as 100 feet, and it grown unusually tall—up to fifteen feet.

. . .

Ten years ago, when the Conservation Law Foundation (CLF) asked Witman, who teaches Brown undergrads the basics of ecology, to recommend an ocean area to protect, Cashes was the obvious answer. Witman describes it as a “Disneyland of biodiversity” containing every kind of ocean bottom habitat, all in a concentrated space. Combined with the food pump provided by the waves, this dense habitat contains a rare proliferation of sea creatures representing an unusual variety of species. The complexity helps create more ecosystem stability and probably greater resilience to withstand such threats as climate change. With this range of creatures filtering water, removing carbon, producing oxygen, and providing all the other “ecosystem services” that the fish we eat depend on, Witman says, Cashes is a key to the health and productivity of the entire Gulf of Maine, including areas where commercial fishermen harvest cod.

Read the full article

The Ocean’s Top Predators

In “Ocean Soul,” world-renowned National Geographic photographer Brian Skerry says, “It’s been said that sharks have remained unchanged for hundreds of millions of years because they are perfect and that no further evolutionary change is necessary.”

Sharks have certainly reached the top of the food chain as the ocean’s apex predators. Apex is a word of Latin origin meaning peak or tip. In ecological terms, apex is used to describe an animal that has no natural predator within its ecosystem—terrestrial examples include wolves and cougars.

Although humans might find them terrifying, apex predators are crucial for maintaining healthy ecosystems. Sharks, for example, play a vital role in balancing ocean food webs. They aid in population control of smaller predators, so that prey species are able to exist at healthy levels. They also regulate the behavior and abundance of prey species, protecting habitat like seagrass beds and coral reefs from overgrazing, and promote biodiversity by preventing any single prey species from monopolizing resources.

Sharks also tend to prey on weak or sick members of a species, so as to minimize their caloric hunting expense. This natural tendency towards efficiency promotes healthier populations by advancing the transmission of strong genes, as weaker genes are taken out of the gene pool.

Despite the enormous amount of good we know sharks do for ocean ecosystems, humans hunt sharks for their meat, especially their fins. Sharks can also become collateral damage to commercial fishing for other species—accidental net entanglements or hooking often prove lethal. According to a 2013 report published in “Marine Policy,” humans killed approximately 100 million sharks in 2000, and 97 million in 2010. Annually, humans kill between 63 and 273 million sharks.

What happens when the oceans begin to feel the loss of sharks? Populations of smaller predator species boom, stressing habitats and depleting populations of prey species. These changes can be very detrimental to the fishing industry. For example, depleted great white shark populations have been linked to the boom in gray seal numbers in New England, and the voracious appetite of the seals may be harming the recovery of overfished cod. In North Carolina, a decline in large sharks because of overfishing led to an increase in cownose rays, one of the sharks’ prey species. Cownose rays eat shellfish, and so an increase in their population caused the bay scallop fishery to collapse.

Clearly, apex predators like sharks are absolutely critical to maintaining healthy marine ecosystems. The role of apex predators is better understood today than ever before, and now that we know how vital their role is, it is time to protect and respect the ocean’s top predators.

Photo via Mass. Office of Energy and Environment