Abstract:
Humpback whales found off west South Africa (WSA) are known to display an atypical migration that may include
temporary residency and feeding during spring and summer. At a regional scale there is uncertainty about how
these whales relate to the greater West African Breeding Stock B as a whole, with evidence both for and against
its division into two sub-stocks. A database containing sighting information of humpback whales intercepted by
boat in the WSA region from 1983 to 2008 was compiled. It included a total of 1,820 identification images of
ventral tail flukes and lateral views of dorsal fins. After systematic within- and between-year matching of images of
usable quality, it yielded 154 different individuals identified by tail flukes (TF), 230 by left dorsal fins (LDF), and
237 by right dorsal fins (RDF). Microsatellite (MS) matching of 216 skin biopsies yielded 156 individuals. By
linking all possible sightings of the same individuals using all available identification features, the periodicity and
seasonality of 281 individual whales were examined. Sixty whales were resighted on different days of which 44
were between different calendar years. The most resightings for one individual was 11 times, seen in six different
years, while the longest interval between first and last sightings was about 18 years. A resighting rate of 15.6% of
whales at intervals of a year or more indicates long-term fidelity to the region. Shorter intervals of 1 – 6 months
between sequential sightings in the same year may suggest temporary residency. The TF image collection from
WSA was compared to TF collections from four other regions, namely Gabon, Cabinda (Angola), Namibia and the
Antarctic Humpback Whale Catalogue (AHWC). Three matches were detected were between WSA (in late spring
or summer) and Gabon (in winter), confirming direct movement between these regions. The capture-recapture
data of four different identification features (TF, RDF, LDF and MS) from six successive subsets of data from
periods with the highest collection effort (2001 – 2007), were used to calculate the number of whales that utilise
the region, using both closed and open-population models. Since dorsal fins have never been used to estimate
abundance for humpback whales, the different identification features were evaluated for potential biases. This
revealed 9 – 14% incidence of missed matches (false negatives) when using dorsal fins that will result in an
overestimate, while variation in individual fluke-up behaviour may lower estimates due to heterogeneity of
individual capture probability, by as much as 57-69%. Taking into consideration the small dataset and low number
of recaptures, the most consistent and precise results were obtained from a fully time-dependent version of the
Jolly-Seber open-population model, with annual survival fixed at 0.96, using the MS dataset. This suggests that
the WSA feeding assemblage during the months of spring and summer of the study period numbered about 500.
The relationship of these whales to those (perhaps strictly migratory) that may occur here in other seasons of the
year, and their links to possible migratory routes and other feeding or breeding areas remain uncertain.
