A suspended migration of humpback whales Megaptera novaeangliae on the west coast of South Africa

Abstract

Between 5 October and 19 November 1993, 62 sightings of 155 humpback whales were made from the lighthouse at Cape Columbine South Afnca, in 3 episodes about 9 d apart For 59 groups tracked by theodolite the average d ~s t a n c e offshore peaked at 2 to 2 5 km Independent information on offshore distribution was o b t a ~n e d from 1112 km searched by a ski-boat in the vlclnity of Cape Columbine from which the proportions of humpback lvhale gioups In 2 strata (0 to 5 and > 5 km) fiom the shore as seen from the lighthouse did not difter from those evpected from the boat data Net directions of movement fol 51 gioups were dlstnbuted equallv to all 4 quadrants of the compass and those gioups showing a concerted directionallty of movenient were headed equally to the north and to the south Of the same 51 groups 70'X) trdvelled at net speeds of less than 1 5 km h ' Migratory indices for individual groups (average speed/net speed) ranged as high ds 82, and those ivith the lowest indices (corresponding to those most l~kely to be migrating) were distributed equally between noltherly a n d southerly di~ections Individual identlficatlon photographs taken of 27 humpback whales over 20 d revealed only 10 individuals 5 of w h ~c h were reslghted on more than 1 occasion and up to 20 d apart Apparent feedlng behavloui by huinpback whales was seen on 10 occasions over 38 d and the production of reddlsh particulate faeces indicative of reccnt f e e d ~n g was seen on 7 occaslons at a defecat ~o n rate of 0 22 whale ' h ' Faecal samples collected contalned euphausiid ~e m a i n s (possibly Euphausla lucens) on 2 occaslons and amphipods on another Overall the data indicated that the southward migration of humpback whales expected at t h ~s time of year on the west coast of South Africa had been suspended probably in response to locally abundant prey