Número y tendencia poblacional de la Ballena Freanca Austral (Eubalaena Australis) en la vecindad de Península Valdés durante el período 1999-2013 por medio de reconocimientos aéreos y náuticos

ENRIQUE A. CRESPO; SUSANA N. PEDRAZA; DANS SILVANA; MARIANO A. COSCARELLA; GUILLERMO M. SVENDSEN; MARIANA DEGRATI

Puerto Madryn

Workshop; Reunión: Mortandad de Ballena Franca Austral; 2014

During the last fifteen years the Marine Mammal Lab. (CEHPAT-CONiCET) developed a method for monitoring the population based on aerial surveys, which could lead to study seasonal changes wlthin and through years. A monitoring area was defined around Peninsula Valdes totalling a coastal strip of 350 nm (620km) flying the coastal zone parallel to the coastline at an altitude of 500 feet. Mother-calf pairs, solitary individuals and breeding groups were counted. A total of 55 aerial surveys were carried out in 1999,2000. and from 2005 to 2013. The rate of increase was calculated using the number of whales and number of calves at the peak of the breeding season ot those years. It was estimated from the slope of the linear regression of the log-number of whales through time. The rate of increase, for the period 1999-2013 {taking out 2008 to 2010) was estimated from the slope of the linear regression of the lognurnDer or me total numaer or wnales (r= o . ~r,o wer LI y5a= 4.2, Upper Cl= 8.1; RE= 0.93, n= 7j and trom the newborn calves (r= 6.6, Lower CI 95%= 4.2, Upper Cl= 9.0; R2= 0.82, n= 6) in the peak of the season through time. When the analysis was done considering all the years without excluding those of suboptimal surveys, the rate of increase of total number of whales was estimated in 4.2 (C1 1.1-7.3) and the increase of calves was 5.2 (CI 2.8-7.6). The rate of increase was also estimated frorn Generalized Linear Models using the full data set. Both Poisson and negative binomial models were applied to data of censuses of SRW. As predictor variables we included the Year and the Month, considered as continuous variables (Month 1-12; Year 1999-2013). Monthly variation in number of whales was modeled using also the ~onth´, allowing the models to explore a non-linear relationship between numbers of whales and temporal variables. The selected model for the census of all of SRW in Peninsula Valdks from 1999 to 2013 indicates that population has increased in 4.579b annually (95% IC= 2.84 - 8.82%). The selected model suggests the number of calves of SRW born in Peninsula ValdCs increased by 7.01% per year (95% C1= 3.5 - 11.49% per year) from 1999 to 2013. The distribution of SRW regards of distance to the coast was studied by means of a binomial nonparametric test. The proportions of whales observed between the two sides of the aircraft (coastal - offshore side) was compared with a theoretical random distribution with p=0.5. In the surveys carried out from 2005 to 2010 the proportion of whales on the ofFshore side of the strip became to increase, this was observed for all the groups involved. Density was also estimated inside of the monitoring zone as the total number of whales in the monitoring area. It was remarkable the high density shown by Ef Doradillo in 1999 where more than 6 whales per km2 was estimated. Other areas were between 1 and 2 whales per km2. Between 2005 and 2007 high density areas were between 2 and 4 whales per km2. In spite of the fact that there are three areas where density of whales in the coastal zone is very high, the trend of the average density inside of the monitoring zone showed a steadily increase frorn 1999 to 2007 almost doubling the density in the former years. Given also that there is an unknown number of whales out of the monitoring zone in the coastal area, it was decided to estimate this figure by means of nautical surveys in deeper areas of Golfo Muevo and Golfo San JosC. Random transects were carried out by a research boat from January 2001 to August 2011 on both gulfs. SRW density (Dl was estimated in deeper areas using the standard distance sampling methods applied to individual animals. The selected model was Hazart Rate with series cosine {orders 2,3). The C W was rather high and the low encounter rate (n/L) was the most important component of the variance (93.1%). Density (D) obtained in offshore areas was 2.61 whales/km2, and the effective strip width was about 73m. Several surveys have been carried out in the coastal zone of Golfo San Matias, to the north of Peninsula Valdes around the peak oi the breeding season. Mother-calf pairs, breeding groups and solitary individuals were counted with higher concentrations around Bahia de San Antonio and Puerto Lobos. In summary, the trend of the population is positive, irrespectively of me rnernoa useu to esrlmate tne trena. Also aenslty is increasing, and the whole area seems to be expanding. Considering Peninsula Valdk as the optimum habitat, once it becomes saturated, the rate of growth in the area should decrease, and that the whales should start to move to other regions, less dense and in which the rate of growth could be higher. There are clues that indicate that this could be the case, as shown by the number of whales occupying deeper waters in Peninsula Valdh, and the increasing number of whales spotted in Golfo San Matias and other areas.