Number of southern right whales Eubalaena australis and population trend in the neighbourhood of Península Valdés during the period 1999-2013 by means of aerial and boat surveys

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

Workshop; 65th Meeting of the International Whaling Commission Scientific Committee; 2014

In synthesis, the trend of the population is positive, irrespectively of the method used to estimate the trend. Also density is increasing, and the whole area seems to be expanding. The estimates of the increasing rates varied from 4.57% to 6.2% for the whole population and from 6.6% to 7.01% for the calves. The estimates obtained from GLM are more conservative, but the IC reaches an upper boundary of 8.82%, higher than the 8.1% estimated by the linear regression. For the calves, the linear regression yielded a 6.6%, while the GLM estimated a 7.01%, with an upper boundary of the IC of 9% and 11.49%. Nevertheless, each of the GLM presented models consistently had a higher estimate of the increasing rate for the calves. This could be due to a non-stable age structure within this population (Caughley, 1977). These differences could also be explained by the fact that whales that are not calving could be moving far away to other peripheral areas as shown by the disproportionate number of breeding groups and solitary individuals sighted in Golfo San Matías (Table 7). Also, the proportion of mother-calf pairs is still higher than expected on the coastal side. If the protection given by the shallow water is a resource used by this kind of groups, the solitary individuals and the breeding groups are to be found in deeper waters (Fig.6). In either case, the increasing rate for each type of group should be estimated separately in the area to test the hypothesis that in the next few years we will start finding proportionally more mother-calf pairs in the Península Valdés area. The surveyed area for the southern right whale includes most of the population off the coast of Argentina, and could be considered as the optimum habitat for the species on its breeding grounds. The increase rate estimated using the maximum of whales and calves by means of a linear regression can be considered as the maximum increase rate for the optimum breeding area in the South Atlantic Ocean at this moment. The estimates using GLM can be viewed as a moderate increasing rate for the same area, regarding the errors associated to the methods used (aerial survey). But considering that Península Valdés as the optimum habitat, once it becomes saturated, the rate of growth in the area should decrease, and 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 Península Valdés, the increasing number of whales spotted in Golfo San Matías, Buenos Aires, Uruguay and Santa Catarina in southern Brazil (Groch, et al 2005, IWC. 2010). The actual population rate of increase would be a combination of the growth in the optimal habitat and the rate of expansion to more peripheral areas.