Aerial surveys of southern right whales Eubalaena australis in the neighbourhood of Península Valdés during the period 1999-2008

CRESPO, ENRIQUE A; PEDRAZA, SUSANA N; DANS, SILVANA L; COSCARELLA, MARIANO; SVENDSEN, GUILLERMO M

Puerto Madryn

Workshop; IWC Southern Right Whale Die-Off Workshop; 2010

CENPAT-IWC

Payne and colleagues carried out long term studies in Southern right whales since the early 70´s and estimated population size and other parameters derived from sighting data based on individual pattern of callosities. The rate of increase r was estimated close to 7%. Crespo et. Al., designed a monitoring area for Southern right whales in the neighbourhood of Península Valdés and developed a method for monitoring the population by means of aerial surveys, which could lead to study trend in numbers through years, seasonal changes within and through years, changes in distribution and the seasonal pattern of arriving and departing of whales in the area. Whales were classified as: a) mother-calf pairs (MC), b) solitary individuals (SI), c) breeding groups (BG) considering one female and n-1 males. Twelve aerial surveys were performed between May 1999-December 2000 and 29 between October 2004-November 2008, flying parallel to the coastline in a strip width of 1500m and an altitude of 500 feet, from the mouth of Chubut River (42°30´) to Puerto Lobos (42°) totalling a coastal strip of 620km. Around 95% of the whales are within the strip and the number of whales in the strip can be considered a measure of relative abundance. The period between flights ranged from 45-50 days which is beyond the average residence time of whales in the area, so ideally during each census new individuals were being counted. Every year a bell shaped curve was obtained increasing in May and declining in December when no whales remain in the area. Maximum number of whales is counted always during September and was as follows: 556 in 1999, 543 in 2000, 724 in 2005, 786 in 2006, 777 in 2007 and 673 in 2008. The rate of increase for the period 1999-2006 was estimated from the slope of the linear regression of the log-number of newborn calfs in the peak of the season through time (r= 4.4, Lower CI 95%= 0.5, Upper CI= 8.3; R2= 0.71, n= 6) and the log-number of whales in the peak of the season through time (r= 3.7, Lower CI 95%= 0.7, Upper CI= 6.8; R2= 0.74, n= 6). The rate of increase is lower and probably underestimated than that estimated by mark-recapture methods but with confidence intervals in the same order of magnitude. The cumulative number of calfs was estimated in 335 for 1999 and 553 for 2008, while the cumulative number of whales was estimated in 1318 for 1999 and 2507 for 2007. Golfo San Matías hosts at the same time more than 120 whales in the peak of the season which should be added to those of PV mainly composed by solitary individuals and copula groups with low number of calfs.