Number of southern right whales Eubalaena australis and population trend in the neighbourhood of Península Valdés during the period 1999-2011 by means of aerial and boat surveys

CRESPO, E.A.,; PEDRAZA, S.N.; DANS, S.L.; COSCARELLA, M.A.; SVENDSEN, G.M.; DEGRATI, M.

Buenos Aires

Workshop; Workshop on Southern Hemisphere Right Whales; 2011

International Whaling Commissionn

During the last thirteen years Crespo and colleagues (RW4) developed a method for monitoring the population, which could be considered a measure of relative abundance. However, an unknown number of whales is in transit in deeper waters in the gulfs and need to be estimated by other methods. Aerial surveys were carried out from the mouth of Chubut River (42°30´) to Puerto Lobos (42°) totaling a coastal strip of 350 nm (620km) flying the coastal zone parallel to the coastline at an altitude of 500 feet. Whales from the air can be distinguished in three groups: a) mother-calf pairs (MC) which are one adult female and a calf, b) solitary individuals (SI) which can be either adult males or females or subadult individuals, c) breeding groups (BG) which are usually formed by one adult female and n-1 males. The groups and the number of whales in each group was recorded. The rate of increase was calculated using data collected in 1999, 2000, and from 2005 to 2011. However, given that the surveys carried out on the peak of the season of years 2008, 2009 and 2010 were done on bad weather conditions were taken out of the analysis. From the surveys it is clear that there was an increase in the number of whales between the surveys 1999-2000 to those carried out from 2005 to 2007. Maximum number of whales is counted always during September (RW4, Fig. 3, Table 1). In 2011 (year incomplete) a high number of whales was counted in mid-August (1263 whales), the maximum number ever counted in a single survey. This could indicate a potential higher number to be counted in September. With regards to the number of calves a similar shape in any given year is obtained. As in the number of individuals the calves showed a clear increase in 2007 with regards to 1999-2000 (RW4, Fig. 4, Table 2). A maximum of 360 was counted in 2011. The rate of increase was estimated from the slope of the linear regression of the log-number of whales through time. Two variables were used: a) the total number of whales in the peak of the season, and b) the number of calves in the peak of the season. The rate of increase for the period 1999-2011 was estimated from the slope of the linear regression of the log-number of the total number of whales (r= 7.0, Lower CI 95%= 4.6, Upper CI= 9.3; R2= 0.93, n= 6) and from the newborn calves  (r= 6.5, Lower CI 95%= 2.8, Upper CI= 10.1; R2= 0.82, n= 6) in the peak of the season through time without the years of 2008 to 2010. The rate of increase is in the same order of magnitude than that estimated by resightings of adult females with mark-recapture methods but with confidence intervals in the same order of magnitude. 7.6 (SE 1.7%) (Payne et al., 1990), 6.9 (SE 0.7%) (Cooke et al., 2001).