Summer fin whales (Balaenoptera physalus) distribution in relation to oceanographic conditions: implications for conservation

Fin whales are commonly observed in the Western Mediterranean Sea, mainly in summer in the Northern part of the basin. Sightings and observation efforts for the region were extensively recorded from 1993 to 2001, allowing to map their cumulative summer abundance in this area. Because of environmental spatial structure and data characteristics, a geostatistical approach was developed to estimate fin whale density. The distribution was modelled using a Poisson law, often used in independent random event count. Over the study period fin whale density maps obtained by Krigging indicate that fin whales were aggregated in patch within the North-Western Mediterranean sea. Available data about regional oceanographic parameters like bathymetry and measured from space like sea surface temperature, chlorophyll concentration, sea level and the associated gradients were used to investigate the statistical relationships between the climatalogy of the distribution of fin whales abundance index and the climatology of oceanographic factors over the Northern part of the western basin. Data were processed using multivariate geostatistical techniques. The occurrence of fin whales was found to be negatively related to SST and primary production gradient at a 80 km spherical scale, but positively related to primary production gradient, and negatively related to sea surface temperature gradient and sea level anomaly at a 160 km spherical scale. This distribution seems to reflect areas were sinking of dense water is taking place during winter and early spring, a process which stimulates primary production through water mixing.