Large, tall hills occupy the top of the sea food chains, where they play important roles in maintaining diverse and healthy ecosystems. The loss of these predators can therefore have significant consequences for the ecosystems.
For a long time, sevengill sharks have taken the top of the food chain along with the more famous large white sharks in False Bay on the southern tip of South Africa. Both species feed on seals, dolphins, other sharks and fish.
But the structure of the False Bay food chain began to change significantly in 2015 with the appearance of a "new" predator, shark-eating killer whales.
The change was noted with the discovery of several dead sevengill sharks by divers from a popular dive site within the Table Mountain National Park marine protected area. This site was home to an exceptionally large group of sevengill sharks. Divers could dive with up to 70 sharks on a one-hour dive – no other place in the world had this many shingles sevengill sharks in one place.
The cause of death was originally a mystery because no dead sharks were recovered for examination. Speculations pointed to humans, large white sharks or killer whales. It was only months later after the discovery of even more dead sharks and a study of the carcass of scientists that it was revealed that whales were the sinners.
With this information in hand, we set out to review the literature on killer puppy walking, diet specialization and population definition globally and locally. Based on the review, we assumed that the attacks on broad-nose sevengill sharks in False Bay might be signs of the arrival of another subgroup – or eco-type – of killer whales in the bay that feeds on sharks.
Appearance of a Super Predator
Since 2009, there has been a steady increase in the number of killer whale observations and the number of pods in False Bay. Initially, the fines in the bay were observed to feed only on marine mammals, such as ordinary dolphins and occasionally Cape fur coatings.
First, it was assumed that the killer whales frequent False Bay and other coastal areas fed predominantly with mammals. So why did killer whales start killing sharks?
Evidence from our literature list points to the arrival of another killer whale, one that is aimed at sharks. It typically used to occur offshore. But it seems to have changed.
At the same time as the first discoveries of the dead sharks, a local whale-boat government documented the arrival of two new killer whales in the bay in January 2015. These individuals were easily identifiable by their characteristic bend over dorsal fins and were nicknamed "Port" and "Starboard" and was observed near the sevengill aggregation site at the time of both events in 2015 and 2016.
In 2017, it is assumed that these same two killer whales were also responsible for five large white sharks dying further up the coast in Gansbaai.
In the same way as the sevengill sharks, the wound pattern was the same, and the liver of the shark was missing. Investigation of carcasses by scientists showed that the sharks had large, gaping wounds between their pectoral fins and their liver was missing while the rest of the internal organs such as heart, stomach and reproductive organs were left behind.
There were obvious bites on the pectoral fins of the dead sharks. These evenly distributed circular tooth impressions were identified as most likely to be from a "flat-toothed" killer whale, which is rare in coastal waters. There were no bites elsewhere on the body, suggesting that the killer whale (or whales) had probably pulled on the pectoral fins to open the body cavity to remove the liver. The liver of the sharks accounts for up to one third of its weight and is rich in fat, a nutrient that killer whales seek.
The most important questions are, why have these shark-eaters moved to land and what influence will they have?
As part of our literature review, we looked at a case study from Alaska, USA, which gave some traces.
Killer whales began to target ocean otters and caused massive falls in their abundance. The decline was documented between 1990-1997 by population surveys conducted across the aleutic archipelago. This, on the other hand, had a groundbreaking effect, as sea chicken populations (hippo fever) exploded. As the sea chickens are primary food kelp, this increase in deforestation of kelp forests in the region.
Scientists wondered that the killer whales began to target the ocean's otters due to fall in predatory species in offshore areas.
This led us to deduce that the decline in offshore swaps in South Africa could therefore be one of the reasons why these whales have moved closer to the coast.
Currently, the once-popular sevengill aggregation site is abandoned, with only rare observations. Large white shark sightings have also fallen in False Bay, possibly partly due to the presence of these killer whales. There are significant gaps in our understanding of the killer whale's behavioral ecology in South Africa, but it is clear that the presence of these shark specialists could have profound and cascading effects on the ecosystem.
Due to the unique predator occupied by sevengill sharks in False Bay, the increased presence of these particular whale whales in False Bay can have profound effects throughout the ecosystem.
Alison Kock co-authored this piece with Tamlyn Engelbrecht, Ph.D. Students at the University of Cape Town. This article is re-released from the conversation under a Creative Commons license. Read the original article.