Distribution of dusky dolphins (Lagenorhynchus obscurus (Gray, 1828)), in the southwestern atlantic ocean with notes on the size of herds

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

Aberdeen

Congreso; 48° Annual Meeting of the International Whaling Commissionn; 1996

International Whaling Commissionn

New records on the distribution of dusky dolphin, Lagenorhynchus obscurus, off the coasts of Argentina, in the southwestern south Atlantic  Ocean, are presented. Herd size and  sympatry  between dusky  and Peale´s dolphins, Lagenorhynchus australis, are  also discussed.  Information  was recorded by: a)  trained  observers aboard  oceanographic  and  fishing vessels  or  from  specimens entangled  in 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  sighting, 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°18´W), near Cape Horn. Dusky  dolphins are  sighted  from  the  coast to  almost  the  200nm  Exclusive Economic Zone border, but the present information do not  permit us  to  conclude if this species tends to be more  coastal  than offshore  or  viceversa  due  to the  bias  in  coastal  effort. Fishermen  detect  herds  of more than  50  individuals,  mostly between  noon and 4 pm, while the herds sighted  from  airplanes were of less than 20 individuals. The mean herd size  calculated for  aerial surveys was 3.85 individuals  (SD=6.5ind.),  clearly below  that of the published information (6 to 15ind.).  Several sightings  of  dusky  dolphins from 47° to 55°S  may  have  been actually  Peale´s dolphins, and few sightings of dusky  dolphins have been recorded near the Falkland (Malvinas) Islands and Cape Horn.  Dusky dolphins are very common from 42º to 47ºS, while Peale´s dolphins tend to be more frequent south of  47ºS.  From  the  information recorded  to date, there is no doubt that these two species  are not  allopatric, as suggested. On the contrary, a large area  of sympatry exists, even though they may use different habitats  in the marine enviroment.