Posted on Friday, 10 February 2012
Despite Seal Island False Bay being little more than 30 minutes drive from Cape Town, South Africa’s 3rd largest city, one of the world’s great wildlife phenomena had remained a secret up until 1997.
On the 22nd August 1997 from the vantage point of my 3,5m inflatable boat a colleague and I laid eyes upon the incredible sight of a breaching great white shark only a few meters away. The sight of one of the world’s most feared yet magnificent predators hurtling through the air was the catalyst that triggered my wife and I to spend the next 12 years recording non invasive data at Seal Island that would hopefully teach us more about these super sharks.
Today the iconic breaching great white shark image is well known to many yet despite more than 40 documentaries showcasing this behavior little has been said about why the great white sharks breach and even less about just how complex and sensitive the natural predation events actually are.
As with great culinary dishes needing key ingredients the secret to the unique behavior at Seal Island is as a result of the blend of these ingredients that is found at no other known location.
Seal Island is 400m long, 80m wide and crammed with over 60 000 seals, a great white shark supermarket bursting at it’s seams.
Come April ever increasing numbers of Great Whites congregate around Seal Island as they know the time is fast approaching when the inexperienced young seals will be leaving to forage on their own after months of indulgent suckling off their mother’s fat rich milk.
The great white sharks that patrol Seal Island are mostly sub adult sharks with an average length of 3,5m, the largest average size of all the locations in South Africa highlighting the specific size class of shark that is most reliant on seals as a primary food source over this time period.
You therefore have an abundance of relatively defenseless, high energy yielding prey being pursued by athletic competitive sharks in a setting perfect for ambush attacks.
Having the opportunity to witness this breaching great white shark behavior also makes it one of the most sort after locations for shark expeditions in South Africa and many eco tourists now flock to experience shark cage diving from Simon’s Town in False Bay.
To date we have recorded 5451 attacks and 50.8% of all these attacks have been successful.
The Great White Shark need to adapt to changing daily weather, con specific pressure and daily fluctuations in prey availability.
To do this the sharks appear to only hunt when optimal conditions exist and a premium is placed on energy conservation.
80% of all the predatory events that we witness take place in the half hour before sunrise until one and a half hours after sunrise. After that hunting dramatically tails off except for opportunistic events.
During the early part of each season at Seal Island larger numbers of great whites are found frequenting the Northern limits of the Island as the predominant S-SE winds, swell and currents wash any dead or injured seals off the Island in a Northerly direction. At this time of the year the sharks scavenge more than they hunt. By May large numbers of great white sharks are at Seal Island, the first young of the year seals are venturing offshore and the winds have switched to a predominantly Northerly orientation. The time for the sharks has arrived and now the concentration of these great predators is at the Southern end of the Seal Island.
By the end of June there is a regular exodus of young seals, maximum numbers of great white sharks and the feeding season is truly in full swing. We have observed days with up to 45 predatory events , many of them truly mind blowing as Great White Sharks breach fully out the water . Most days now play host to 10 or more events. Can you imagine seeing lions hunting game ten or more times a day?
As with all predator prey interactions the prey adapts to the pressure and we witness this most graphically during the intense predation periods.
The young seals quickly learn to depart in well organized groups comprised of
both adults and juveniles. Within these groups constant jostling, inverted seals looking down into the water, and a tapestry of ever changing shapes make for a difficult target if a great white shark is hoping to isolate a single small prey.
The seals also seem to have a sixth sense that they use just prior to a breaching attack. Often before a shark bursts clear of the water seals explode like ten pin skittles as they obviously pick up on the approaching torpedo.
Perhaps a pressure wave exists in front of the shark that the seals can detect but somehow they seem to dodge the bullet. Attacks on groups of seals only result in a successful kill rate of just over 15%.
When young seals return home during daylight hours the young of the year are often isolated having fallen off their groups which comprised of seals of all ages.
Returning alone or in small similar aged groups the young seals travel a regular path and unlike the adults which dive and regularly change course the young often porpoise the whole way to Seal Island making them an easy and predictable target to lock onto.
The sharks therefore choose incoming single young of the year seals in more than 75% of all attacks.
On days with intense predation older sharks have learnt that being close to Seal Island does not yield as great a success rate as being further offshore.
Although the density of prey is higher near to the island as the returning seals become more concentrated, like aero planes returning to an airport, the concentration of great white sharks is also higher .The result is that sharks that are close to the island launch sub optimal strikes to avoid another shark having a shot at a prey item they have discovered. The success rate of attacks close to the island in peak times is lower than the sharks which choose to hunt further away from the island.
The sharks hunting around the edges of the island, up to 3,5km away, are generally larger sharks. This would suggest that due to many a season spent at Seal Island, False Bay these larger sharks have learnt that it makes more sense to hunt less often but more successfully and have less chance of another shark stealing your kill.
As the daily winds in False Bay switch, so to does the preferred great white shark hunting area.
On days with a strong SE wind more activity is seen in the Western areas of the island as the seals return patterns change slightly and the sharks adapt accordingly.
Certain individual sharks also seem to have preferred hunting areas around the island and it is not uncommon to see the same shark hunting within a small area on any given day.
What we have also noticed is that certain individual sharks also are more successful hunters than others and often pursue their prey with a relentless determination. During our shark cage diving tours from Simon’s Town we have come to know some sharks very well. We have seen some sharks on more than 60 occasions having watched them grow from 2,5 meter babies to battleship like 5,0m adults weighing over 1500kg.
Whilst great white sharks will venture right into the 3m shallows of Seal Island to catch an un weary seal, sharks typically hunt in water 15m meters or more deep with the bulk being in around 25m.
From mid August the sharks take far longer to consume seals and this is probably as a result of decreasing numbers of sharks congregating around the island posing a smaller threat of a kill being stolen by another shark.
As the seals grow they too have learnt a great deal and now the 8-10 month old seals no longer return home alone so often, but usually in the company of similar aged seals which start to exhibit the same anti predatory defense as the well organized adult groups.
By mid September the massive Southern Right whales move into False Bay in great numbers and as the young seals have wizened and become more adept at eluding the great white shark so to do the sharks change their tack.
It is time to sadly bid our much loved sharks farewell as they move to the inshore environment of False Bay to patrol the nearby beaches in search of summer species of fish and other sharks which form the bulk of their summer diet.
Till the next autumns changing winds signal the return of these magnificent predators to Seal Island we can only wish them safe passage into seas filled with danger for the great fish that inspires such awe.
Till we meet again………………………………………………………………………