Long-term photo-identification study of fin whales in the Pelagos Sanctuary (NW Mediterranean)
M. Zanardelli, S. Airoldi, M. Berube, J.F. Borsani, N. Di-Meglio, A. Gannier, P. Hammond, M. Jahoda, G. Lauriano, G. Notarbartolo di Sciara, and S. Panigada
International Whaling Commission
Long-term photo-identification was used to assess presence and site fidelity of fin whales (Balaenoptera physalus) in the Pelagos Sanctuary (Western Mediterranean Sea), to estimate survival rate, population size, rate of change, sex ratio, and to investigate vessel collision injuries. Data from the Tethys Research Institute were collected during dedicated cruises on 18 consecutive summers (1990-2007). A total of 435 individuals was identified, 74 (17%) of which were resighted at least once during the study period: 47 resightings occurred in different years, 19 within the same season and eight both in the same season and in different years. Individuals were resighted up to six times in different years, with a maximum interval of 17 years. The number of resightings within-season ranged from one to four, over periods from 1 to 90 days, indicating that at least some whales use the Pelagos Sanctuary over the entire summer. To estimate population size and survival rate, photo-id catalogues from other Institutes (99 identified individuals, 29 resightings) were merged with the original catalogue. The estimates of population size using a simple closed two-sample estimator were most consistent and most precise for years 1993-1994 and 1994-1995: 1,070 (CV=0.30; 95% CI=598-1,914) and 1,133 (CV=0.29; 95% CI=652-1,969) animals, respectively. A Jolly-Seber open population model was used to estimate apparent survival rate (0.88, 95% CI=0.76-0.94), population size in 1990 (980, 95% CI=670-1,437), and rate of population change (0.99, 95% CI=0.92–1.07). The apparent survival rate is unexpectedly low and may be linked to temporary or permanent emigration, or mortality due to ship strikes. However, the estimated population rate of change implies a stable population over the study period. Genetic analyses of remotely collected skin biopsies allowed sex determination of 154 whales, 88 females (57%) and 66 males (43%), suggesting no sexual segregation in the Pelagos Sanctuary. Twenty individuals showed evidence of vessel collision, a well-documented cause of death for Mediterranean fin whales, underlining that only few animals survive a strike, usually lethal for large whales. These results confirm the existence of persistent site-fidelity to this feeding area and provide sound information to support the conservation of this unique population in the Sanctuary area and in the entire Mediterranean. Mark-recapture abundance estimates are discussed and compared with available line-transect distance sampling ones and demonstrate the usefulness and robustness of this method.