Distribution and school size of dusky dolphins Lagenorhynchus obscurus (Gray, 1821) in the Southwestern South Atlantic Ocean

CRESPO ENRIQUE ALBERTO; PEDRAZA SUSANA NOEMI; COSCARELLA, MARIANO ALBERTO; GARCÍA NÉSTOR ANÍBAL; DANS SILVANA LAURA; IÑIGUEZ BESSEGA, MIGUEL; REYES, LAURA MARINA; KOEN-ALONSO MARIANO; SCHIAVINI ADRIÁN CARLOS MIGUEL; GONZÁLEZ, RAÚL

Aberdeen, Escocia

Workshop; Scientific Committee International Whaling Commission SC48; 1996

International Whaling Commission

New records on the distribution of the dusky dolphin. Lagenorhynchus ohscurus, off the coasts of Argentina, in the southwestern south Atlantic Ocean, are presented. School size and sympatry between dusky amI Peale's dolphins, Lagenorhynchus australis. are also discussed. Information was recorded by: (a) trained observers onboard oceanographic and fishing vessels or fram specimens entangledin fishing gear; (b) trained observers during aerial surveys; (c) fishermen on fishing vessels; (d) less-trained observers and previous records in the literature. The information gathered included: date; location of sightings; entanglement or stranding; latitude; longitude; number of individuals; sex; sea state; weather conditions; etc. The northernmost record was at La Lucila del Mar, north of Buenos Aires Province (36°19'S), and the southernmost record was at Bahía Nassau (55°26'S 67° lWW), near Cape Horn. Dusky dolphins have been sighted fram the coast to just before the 200 n.miles Exclusive Economic Zone border, but the present information does not allow us to condude whether this species' distribution tends to be more coastal than offshore or vice versa due to the bias in coastal effort.Fishermen detected schools of > 50 individuals, mostly between noon and 1600hrs, while the schools sighted fram airplanes were of < 20 individuals. The mean school size calculated for aerial surveys was 3.85 individuals (SD =6.5 individuals). considerably lower than that of the published information (6-15 individuals). Several sightings of dusky dolphins fram 47° to 55°S may have actually beenPeale's dolphins. and few sightings of dusky dolphins have been recorded near the Falkland (Malvinas) Islands and Cape Horn. Dusky dolphins are common fram 42° to 47°S, while Peale's dolphins tend to be more frequent south of 47°S. Fram the information recorded to date, there is no doubt that these two species are not allopatric, as suggested. On the contrary. a laige area of sympatry exists. eventhough they may use different habitats in the marine enviranment.