Humpback Whale Migrations
Beginning in the late Summer and early Autumn, we at Ocean Blue Adventures are always excited in anticipation of seeing Humpback Whales passing Plettenberg Bay on their annual breeding migration. Initially, the whales can be seen heading north as they travel to the tropical and sub-tropical waters of northern Mozambique, southern Tanzania and Madagascar to breed. After an initial period of consistently northward travel, we begin to see some whales heading in a southerly direction (normally around late June) and this gradually becomes a wholly southward movement (from September onward), culminating in the end of the migration, in this area, around early January as the whales leave our shores to return to the feeding grounds in the Antarctic Circle.
The Humpbacks complete the longest migration of any mammal, travelling up to 25 000 km (16 000 miles) annually as they range between their feeding grounds in the polar latitudes (approximately 70 degrees north and south of the equator) to the near-equatorial waters at around 10 degrees of the equator. It is thought that the whales can complete the migration in as little as a month.
The reason for the migration is twofold; the northerly leg takes the whales to warm water conducive to favourable breeding conditions, while the southerly leg returns to the whales to the nutrient rich polar waters. For breeding purposes, it is believed that warm water improves sperm motility, and hence increases the chances of the males impregnating females. Additionally, when the calves are born (after a 12 month gestation period), they have little or no body fat on them. As such, if they were born in the frigid polar waters, they would likely die of exposure.
In terms of their feeding requirements, Humpback Whales require prodigious amounts of food; either plankton or bait-fish. In the southern Summer, the polar circle receives almost 24 hours of sunshine per day. This abundant light leads to massive blooms of algal growth, which in turn is fed on by zoo-plankton (such as krill), whose numbers in turn proliferate and are fed on by the whales. Consequently, by timing their feeding season to coincide with this abundance, the whales are able to source and consume in excess of one ton of food per animal per day. It is vital that they absolutely engorge themselves during the feeding season, as they eat little or no food during the course of their migration. Hence, they are able to store huge resources of nutrients in their fatty, blubber layer which will sustain them through this period. It has been estimated that a 45-ton whale might return to the feeding grounds after the migration, having lost up to a third of its body weight.
Quite how the whales navigate their way on this colossal migration remains something of a mystery. One theory is that they are able to read the earths’ magnetic field, much the way a competent map reader can deduce terrain by looking at a topographic map. Within in the magnetic field, there are zones of concentrated density, commonly called way-points, and it has been theorized that the whales simply travel from one way-point to the next. Apparently there are several of these in close proximity to Plettenberg Bay. From the north, there is thought to be one just off Nature’s Valley and another one between Keurbooms Village and the Keurbooms River mouth. There also appears to be one about 3 miles south of Robberg Point and finally, another waypoint somewhere in the vicinity of The Island (on the southern side of Robberg Peninsula).
This theory seems to be borne out by the pattern of sightings that we enjoy. The whales appear to follow distinct lines along these routes and it is conceivable that they are able to pass on knowledge of which way-points to select to successive generations. Over the last few months, we have enjoyed considerable success by taking our guests to a point roughly 3 miles off the point and drifting until we see whales surfacing. It is uncanny how they can be found consistently in this area – and we invite you to join us in witnessing this very special annual spectacular as soon as possible.