Life History of Risso’s Dolphin (Grampus griseus) (G. Cuvier, 1812) in the Faroe Islands

Abstract

Risso’s dolphins (Grampus griseus) were taken for the first time by the opportunistic drive fishery in the Faroe Islands in two separate incidents: three in September 2009 and 21 in April 2010, with in total 16 females and eight males. Their sizes ranged from 193 to 308 cm in length and 60 to 395 kg in weight for females, and 186 to 323 cm in length and 70 to 505 kg in weight for males; the maximum weights are heavier than previously documented for this species. The smallest mature female was 277 cm long, while the youngest and also lightest mature female was 8 y old and weighed 280 kg. Sperm competition and a promiscuous mating system were suggested for the species based on large testicular masses. The diet was composed of cephalopods from both the water column (Todarodes and Loligo) and the ocean floor (Eledona and Todaropsis). Although both schools landed showed a mixed diet, the September school diet centred on a pelagic squid (Todarodes sagittatus), while the April school diet centred upon a benthic octopod (Eledona cirrhosa). Since August 2009, Risso’s dolphins have been observed on five occasions in waters around the Faroese north of 61o 34' N, the northernmost observation situated at a latitude of 62o 23' N. Sightings of the species off Shetland occur mostly between April and September, with a peak in August and September, the observations in Faroese waters (2 in April, 1 in August, and 2 in September) falling within this period. While the species had not previously been observed in this area north of the Shetland-Faroe Channel, these observations in Faroese territorial waters indicate a likely northward extension of the known range of the species.