Number of southern right whales Eubalaena australis and population trend in the neighbourhood of Península Valdés during the period 1999-2012 by means of aerial and boat surveys

CRESPO, ENRIQUE ALBERTO; PEDRAZA, SUSANA NOEMI; DANS, SILVANA LAURA; COSCARELLA, MARIANO ALBERTO; SVENDSEN, GUILLERMO MARTIN; DEGRATI, MARIANA

Congreso; 15º Reunión de Trabajo de Expertos en Mamíferos Acuáticos de América del Sur; 2012

CENPAT-UNPSJB

On the base of needing a quick and cheap methodology to estimate relative abundance and population trend of Southern Right Whales we developed a method based on direct counts of whales on the coastal zone from Chubut River to Puerto Lobos (620km of coastal zone). The objectives of this work were: 1) to develop the method, 2) to estimate relative abundance in the monitoring zone every year, 3) to estimate rate of increase, 4) to document changes in the distribution of whales through time, and 5) to study the seasonal pattern of arriving and departing of whales in the area. Given that not all the whales are located in the coastal zone, additional objectives were: 6) to estimate density out of the coastal zone, 7) to have an idea of the total number of whales circulating in the area every year. During the last 13 years we decided to survey the area with aircrafts for monitoring the population. Around 95% of the whales is found within the coastal strip. However, an unknown number is in transit in deeper waters. We counted a) mother-calf pairs (MC, b) solitary individuals (SI) and c) breeding groups (BG) which are usually formed by one adult female and n-1 males. The rate of increase was estimated from the slope of the linear regression of the log-number of whales or the number of calves through time in the peak of the season. Whale density (D) in deeper areas was estimated using the standard distance sampling methods applied to individual animals. The rate of increase for the period 1999-2011 was estimated from 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. Density (D) obtained in offshore areas was 2.61 whales/km2, and the effective strip width was about 73m and corresponds to whales are thought to be in transit from the mouth of the gulfs to the coastal zone through deeper. A rough estimation of the surface covered by nautical surveys can be around 200 km2, therefore another 500 whales can be moving in deeper waters and added to at least another 1300 whales in the coastal zone during the peak of the season.