Overview of research methods

Surveys to monitor dolphins in the coastal waters of Greece are conducted from inflatable boats with fibreglass keels, normally between April and September.

Surveys to monitor dolphins in the Inner Ionian Sea archipelago were conducted from sailing vessels between 1991-1994, and since 1995 from inflatable boats with fibreglass keels.
Surveys were conducted ad libitum in early years. In recent years, pre-defined survey transects are also used for the long-term monitoring of the study areas. Research effort in the Inner Ionian Sea archipelago is mainly conducted in the warm season (June-September). Research in the Amvrakikos Gulf was carried out year-round between April 2006 and December 2008. Since 2009 the field work is done from April to October.

Individual photo-identification based on long-term natural marks on the dolphins’ dorsal fins is performed extensively with cameras equipped with 70-200mm f2.8 zoom lenses. Colour transparency film (ISO 100) was used in the early years of this study, and then digital photography was systematically adopted. Transparencies were scanned and turned into digital images. Both transparencies and digital photos were then cropped around the dorsal fin and visible part of the body and selected using consistent criteria (e.g. entire fin visible, right angle, appropriate sharpness and resolution). Following a quality-based selection, the total catalogue 1991-2011 includes 83,933 photos on which photo-identification and other analyses can be performed. Photo-identification allows us to obtain information on individual movement patterns, habitat use and preferences, reproductive success, social organization, population abundance and dolphin population trends in the two study areas.

Dolphin behaviour is sampled at regular intervals. These samples include position, group size and composition, group formation, directionality and speed of movement, surfacing pattern, dive duration, dolphin activity and behavioural events, presence of birds and bird data, and several other variables.

Marine species including sea turtles, monk seals, tuna, mobulas, swordfish and seabirds observed during the surveys at sea are recorded. Dead dolphins found stranded or adrift in the study area are inspected, and causes of death identified whenever possible. Findings are routinely reported to the Hellenic Ministry of Rural Development and Food and to the Pelagos Cetacean Research Institute, Athens. If possible, the animals are measured, sexed, and biological samples are taken. Occurrence of dolphin bycatch in fishing gear is also recorded. Drifting scales lost by fish prey following predatory events performed at the surface by the dolphins are collected by means of a dip net. The scales are subsequently analysed to identify the dolphin’s prey species.