Transit station or destination? Attendance patterns, movements, and abundance estimate of humpback whales off west South Africa from photographic and genotypic matching

摘要

Humpback whales found off west South Africa (WSA) are known to display an atypical migration that may includetemporary residency and feeding during spring and summer. At a regional scale there is uncertainty about howthese whales relate to the greater West African Breeding Stock B as a whole, with evidence both for and againstits division into two sub-stocks. A database containing sighting information of humpback whales intercepted byboat in the WSA region from 1983 to 2008 was compiled. It included a total of 1,820 identification images ofventral tail flukes and lateral views of dorsal fins. After systematic within- and between-year matching of images ofusable quality, it yielded 154 different individuals identified by tail flukes (TF), 230 by left dorsal fins (LDF), and237 by right dorsal fins (RDF). Microsatellite (MS) matching of 216 skin biopsies yielded 156 individuals. Bylinking all possible sightings of the same individuals using all available identification features, the periodicity andseasonality of 281 individual whales were examined. Sixty whales were resighted on different days of which 44were between different calendar years. The most resightings for one individual was 11 times, seen in six differentyears, while the longest interval between first and last sightings was about 18 years. A resighting rate of 15.6% ofwhales at intervals of a year or more indicates long-term fidelity to the region. Shorter intervals of 1 – 6 monthsbetween sequential sightings in the same year may suggest temporary residency. The TF image collection fromWSA was compared to TF collections from four other regions, namely Gabon, Cabinda (Angola), Namibia and theAntarctic Humpback Whale Catalogue (AHWC). Three matches were detected were between WSA (in late springor summer) and Gabon (in winter), confirming direct movement between these regions. The capture-recapturedata of four different identification features (TF, RDF, LDF and MS) from six successive subsets of data fromperiods with the highest collection effort (2001 – 2007), were used to calculate the number of whales that utilisethe region, using both closed and open-population models. Since dorsal fins have never been used to estimateabundance for humpback whales, the different identification features were evaluated for potential biases. Thisrevealed 9 – 14% incidence of missed matches (false negatives) when using dorsal fins that will result in anoverestimate, while variation in individual fluke-up behaviour may lower estimates due to heterogeneity ofindividual capture probability, by as much as 57-69%. Taking into consideration the small dataset and low numberof recaptures, the most consistent and precise results were obtained from a fully time-dependent version of theJolly-Seber open-population model, with annual survival fixed at 0.96, using the MS dataset. This suggests thatthe 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 theyear, and their links to possible migratory routes and other feeding or breeding areas remain uncertain.