Demography and conservation of the Mediterranean fin whale (Balaenoptera physalus): what clues can be obtained from photo-identification data.

Abstract

Long-lived and slow reproducing species, such as cetaceans, are among the most critical conservation units: a demographic approach can be very useful for their management and conservation. In the present work, we examined, by demographic tools, the most exhaustive photo-identification database available for the Mediterranean fin whale (Balaenoptera physalus) population, recorded by Tethys Research Institute between 1990 and 2007 in the Western Ligurian Sea. A total of 31,782 km were covered and 548 fin whale sightings were recorded. The occurrence of the target species was uneven across the years and months: an anomalous reduction of sightings occurred between 2001 and 2004. Moreover, the target species is likely to concentrate in the study area in the summer months and probably moves to other areas in the early autumn. Using the photo-identification technique, 431 different individuals were photo-identified, but only 318 of them were sized: 6 calves (≤ 10 meters), 33 immature (10-15 m), 261 adolescent-adult (> 15 m) and 18 olds (≥ 20 m). For the first time the site of the fin whale sub-population in the Pelagos Sanctuary was obtained by photo-identification and mark-recapture techniques. These techniques were used to estimate the site of the sub-population composed by individuals > 15 meters as 539 fin whales (95% confidence interval = 345-732) over the period 1990-1999. The number of calves was likely underestimated, as the sighting period (late spring-summer) was shifted with respect to the peak of births (late autumn). To fill this gap of knowledge we propose a simple mathematical model for the yearly dynamics of calves. After correcting the number of calves, a static life history table for the period 1990-2007 was set out. Our results highlight the highest survival proportions between calf and immature (61.1%) and the minimum between adolescent-adult and old (2.5%) vital stages. The overall life expectancy is estimated to be 6.3 years while the life expectancy of individuals entering the adolescent-adult stage is 14.3 years. This paper aims at providing a contribution for improving the conservation efforts and the demographic knowledge on fin whales in the Pelagos Sanctuary.