Seasonal variation of striped dolphins, fin- and sperm whales' abundance in the Ligurian Sea (Mediterranean Sea)

In order to investigate seasonal changes in cetacean relative abundance, a series of surveys were conducted between the French mainland and Corsica. from February 2001 to February 2004, thirty similar transects were conducted monthly, using the same dedicated boat and a consistent sampling protocol, including visual observation and passive acoustic sampling. A total effort of 5759 km was sampled, conducted at the same speed and in good sighting conditions. Relative abundances of striped dolphins (Stenella coeruleoalba) and fin whales of striped dolphins peaked in May and September (> 1.3x10(-2) ind.km(-1)), while a consistent minimum value (< 0.6x10(-2) ind.km(-1)) was obtained from December to April. A maximum relative abundance of fin whales occured in August with 5.6x10(-2) ind.km(-1) and decreased to almost zero from November to January. for sperm whale (Physeter macrophalus), a long diver species, frequency and abundance indices were determined using acoustic sampling. The highest acoustic relative abundance was observed from August to October, with more than 2x10(-2) ind.km(-1). Environmental parameters (sea surface temperature and chlorophyll) were computer for the sampling area from remote sensing imagery and pooled on this monthly basis, to correlate with relative abundance of the three species.