OCEAN TRENDS

As Great Whites disappear off the Cape’s Seal Island, sevengill sharks, ‘living fossils’, move in

By Shaun Smillie 13 February 2019
Caption
Waters surrounding Seal Island in False Bay are known for great white sharks breaching out the water in pursuit of seals (left image), but the recent disappearance of great whites from here has led to the emergence of another apex predator, sevengill sharks (right image) that now dominate the area. White shark image by Chris Fallows; Sevengill shark image by Neil Hammerschlag.

With the rapid decline of great white sharks in the waters surrounding Seal Island off Cape Town in 2017, a shark operator began noticing sevengill sharks for the first time. Since then their numbers have increased. Researchers believe sevengills have extended their range to Seal Island because great whites are no longer feeding on them.

In the waters surrounding Seal Island off the Western Cape Coast, there is a new sheriff in town. This sheriff has taken advantage of the sharp decline in the numbers of of an iconic predator — the great white shark – and this has researchers and tour operators worried.

The sevengill shark is famed for its close resemblance to the earliest sharks that appeared way before the dinosaurs. What it is not famed for is the flashy aerial displays of its predecessor, nor does it have the killer reputation that spawned a genre of movies from Jaws to Sharknado.

In fact, tour operators find that the sevengill just doesn’t have the appeal of its famous cousin, which was responsible for building R1-billion-a-year tourism industry.

Even the seals of Seal Island are having a chill time of late.

Once, this colony was so harried by great whites that scientists discovered that the seals were the most stressed in the world.

Sevengills clearly don’t have the same impact that the great white sharks had on the seal population, and the seals are adapting to this. They are changing their departure and arrival times and they are changing the way they behave around the island,” explains naturalist Chris Fallows.

A new study examining this ecological changing of the guard, titled “Disappearance of white sharks leads to the novel emergence of an allopatric apex predator, the sevengill shark”, has appeared in the latest issue of the online journal Scientific Reports.

The findings are part of a long-term collaborative study between shark researcher Professor Neil Hammerschlag from the University of Miami, and Fallows, who runs Apex Shark Expeditions.

Since 2000, the research team recorded 6,333 shark sightings and 8,076 attacks on seals. Then in 2015, great white sightings dropped off dramatically.

In 2017 and 2018, their numbers reached an all-time low, with great whites completely disappearing from our surveys for weeks and months at a time,” says study lead author Hammerschlag.

While the reasons for their decline and disappearance remain unknown, it provided a truly unique opportunity for us to see what happens to an ocean ecosystem following the loss of an apex predator.”

With the drop in numbers of the great white sharks in 2017, Fallows began noticing sevengills for the first time. Since then their numbers have increased.

Usually, explains Fallows, sevengill sharks are scavengers, the hyenas of the oceans. When needs be they will hunt, and the team did witness one attacking a live seal.

What usually happens is that they will feed on sick or injured seals,” says Fallows.

Before their move to Seal Island, sevengills were usually found about 18km away, feeding inshore among the kelp beds.

What researchers believe is that sevengills have extended their range to Seal Island because the seals are no longer fed on by white sharks.

This has affected the tourism industry in the area, which once had upwards of 100,000 visitors a year.

In Gansbaai, it is not sevengills that have replaced white sharks. Bronze whaler sharks now trail the tourist boats, filling the apex predator slot.

The irony is that you have a more protracted viewing experience with these (sevengill and bronze whaler) sharks as they are more interactive around the cages than great whites ever were,” says Fallows. “But when most people hear there are no great whites, they don’t want to go out.”

Fallows has tried to market the sevengill as a tourist attraction by offering “a dive with dinosaurs” experience.

It is the second-oldest shark species on the planet, and has been around for 180 million years. They are pretty remarkable creatures, with their seven gills and big head. So it is very much like diving with a primitive animal.”

No one knows just why the great whites are disappearing. Orcas have been blamed for killing them, but Fallows believes there is another reason.

Great whites don’t feed on seals only. Their diet includes smaller sharks and their numbers, according to Fallows, have been reduced through unregulated fishing.

Another mystery is whether this decline has to do with die-offs or if the sharks have simply moved to new feeding grounds.

It is like Africa losing its lions,” says Fallows. “The great white shark is an iconic and enigmatic animal and our coastline is so much poorer for not having them here in the same numbers.” DM

This article was edited for accuracy around Great White Shark patterns at 8.35am on 14 February,  2019.

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