Distribution and abundance of humpback whales, Megaptera novaeangliae, off the coast of Mozambique, 2003

Abstract

Humpback whales within the southwestern Indian Ocean undertake annual migrations from summer Antarctic/Southern Ocean feeding grounds towinter breeding grounds in the tropical and sub-tropical coastal waters of Mozambique, Madagascar and the central Mozambique Channel Islands.Little is known of the inter-relationship of humpback whales on each of these wintering grounds, or the inter-relationship of these wintering groundswith the summer Antarctic feeding grounds.A line-transect survey of cetacean species was carried out in Mozambique coastal waters between Cabo Inhaca (26°00’S, 33°05’E) and just northof Mozambique Island (14°26’S, 40°53’E) and between the 20 and 200m isobaths, over the period 26 August to 7 September 2003. The majority(98.1%) of 951.8 n.miles of search effort carried out on this survey was in passing mode due to the high densities of whales encountered. Humpbackwhales were the only large whales to be identified and the distribution of 691 sightings of an estimated 1,130 individual humpback whales and 132sightings of an estimated 154 large unidentified whales show distribution throughout the survey region. Two sightings of individual small whaleswere made in the region of Inhambane.In general, higher than expected sighting densities (based on survey effort) were recorded in the region between Cabo Inhaca and Xai-Xai, andin the region of the Pantaloon and David Shoals to the north east of Quelimane. Lower than expected sighting densities were recorded over theSofala Banks. No distribution trends could be ascribed to environmental parameters, apart from whales being distributed in waters of higher salinitiesthan expected, possibly due to turbidity associated with low salinity water arising from river input. Groups containing a cow and calf pair weredistributed across the entire region surveyed.Analyses of unstratified data result in a total abundance estimate of 6,808 (CV = 0.14) humpback and unidentified whales in the 14,029.5 n.mile2area surveyed. As a result of the differences in width of the coastal shelf area along the coast of Mozambique, the line transect survey data werefurther analysed in four strata. Pooling of estimates over these four strata results in a total abundance of 6,664 whales (CV = 0.16), with highestdensities in the southernmost stratum and the lowest densities in the narrow shelf region across the Sofala Banks. Similar analyses of humpbackwhales only resulted in abundance estimates of 5,930 (CV = 0.15) (unstratified data) and 5,965 whales (CV = 0.17) (data analysed by four strata).Although not directly comparable due to differing survey platforms, these estimates indicate the population to have increased since previous surveysin the early 1990s.