Migration redefined? Seasonality, movements and group composition of humpback whalesMegaptera novaeangliaeoff the west coast of South Africa

Abstract

The migration of Southern Hemisphere humpback whales between their feeding and breeding areas has thus far been considered a highly predictable and seasonal event. However, p revious observations on the humpbacks that pass through the near-shore waters of the west coast of South Africa have revealed deviations from the behaviour and seasonality expected during a typical migration. This "anomaly" is hypothesised to be associated with prey availability in the region. Shore-based observations between July 2001 and February 2003 fro m North Head, Saldanha Bay yielded relative abundances that again did not support a classical migration pattern, with the highest sighting rates from mid-spring through summer. Movement parameters (actual swimming speed, direction and linearity) of humpback groups tracked by theodolite showed mid-spring to be a turning point in their behaviour, after which we observed a significant reduction in actual swimming speed, an increase in 'nondirectional' movement, and a distribution further fro m shore than in other seasons. Additional data on group composition and sex collected between 1993 and 2008, showed a significantly female-biased sex ratio during mid-spring, the first such recorded for any region. Direct observation of feeding on crustacean prey during spring and summer months further supports the notion that humpbacks may have more flexib le forag ing habits than previously appreciated, and that the Southern Benguela upwelling region may function as an important feeding area for these whales.

Does temporal and spatial segregation explain the complex population structure of humpback whales on the coast of West Africa?

“…genetic differences observed between samples from gabon and WSa could therefore be an evolutionary consequence of long-term-directed site fidelity to these specific feeding grounds. The different levels of resampling of individuals found in the two areas are consistent with this hypothesis; the overall resampling rate of photographically identified individuals in the WSa area is almost three times higher than in gabon, indicating high site fidelity, with several animals returning to the WSa region in multiple years (Barendse et al 2010(Barendse et al , 2011. Variations in site fidelity by humpback whales to breeding and feeding grounds have been shown elsewhere, with high annual return rates to feeding grounds (up to 90 %; Clapham et al 1993;acevedo et al 2006) whereas fidelity to breeding grounds appear to be more fluid, with whales roaming widely during the winter (Mattila et al 1994).…”

“…The other known feeding area is off the west coast of South africa (Best et al 1995;Barendse et al 2010) to which animals possibly take a coastal migration route between breeding and feeding grounds: whether this is the final or only temporary destination (with antarctic as final destination) is not known. genetic differences observed between samples from gabon and WSa could therefore be an evolutionary consequence of long-term-directed site fidelity to these specific feeding grounds.…”

“…Whales sampled off the coast of gabon were divided into 'early' season (e_gaB), including samples collected in July and august, and 'late' season (l_gaB), with samples collected in September and October. For WSa, the whales pass by during the northward (July/august) and southward (October/november) migrations, while some whales are present in the area until late summer (February; Barendse et al 2010). Here, the season was divided into 'regular' season (r_WSa), including samples collected between July and October, and 'late' season (l_WSa), with samples collected between november and February (late Spring and Summer).…”

“…The most recent available genetic data (mitochondrial Dna and microsatellites) suggest that there is some stock structure in this region: whales that overwinter in the greater gulf of guinea, here called the Tropical Southeast atlantic (TSa), differ genetically from those sampled off the West coast of South africa (WSa;rosenbaum et al 2009;Pomilla 2005). However, as the latter area appears to serve primarily as a migratory corridor and a feeding site, the origins of this difference, as well as the actual breeding site for a potential second sub-stock, are unknown (IWC 1998;rosenbaum et al 2009;Barendse et al 2010). Previous molecular studies using mitochondrial Dna only found indications of population sub-structuring, based on haplotype frequencies and low gene flow rates between the two regions (rosenbaum et al 2009).…”

“…On the West South african coast, recent observations indicate a regular and extended presence of whales as late as March including direct observations of feeding on euphausiids during the austral spring and summer months. Data from early twentieth century catches during the summer season off namibia suggest that this area, besides functioning as a migratory corridor, also serves as an important feeding ground for some members of the population (Best et al 1995;Findlay 2000;Barendse et al 2010Barendse et al , 2011Barendse et al , 2013. a number of questions regarding the definition of Breeding Stock B, the population structure and the precise geographic area it utilizes, remain unanswered.…”