Abstract:
Estimating numbers of whales present in an area from recorded call rates could be a useful conservation
tool. We recorded southern right whale Eubalaena australis vocalisations and presence in Walker
Bay on the south coast of South Africa. In all, 45 sessions with synchronous acoustic and visual data
were analysed to determine call rates directly (overall call rate, OCR) and in relation to the number of
whales sighted (call rate per whale, CPW) and number of groups sighted (call rate per group, CPG).
The OCRs were examined in the presence of varying numbers of whales, using a log-linear model to
investigate the dependence of the call rate on whale density. The number of whales present exerted
a strong quadratic effect on the OCR, which peaked at around 15 whales and decreased to a low rate
as whale presence approached a maximum, for all calls combined and for four of their constituent 13
call types: a quadratic trend was present to varying degrees among the remaining call types. Both
quadratic and linear trends were absent when OCR was assessed against number of groups present,
possibly because group size increased with increasing density of whales. A linear regression on the
CPW and CPG suggested that there was a negative inverse relationship with the number of whales
and groups present respectively. These findings are important in that, while they preclude the estimation
of absolute numbers from call rates, they imply that, under the conditions prevailing in Walker
Bay, southern right whales were aware of the presence, arrival and departure of other animals in their
vicinity and adjusted their vocal behaviour accordingly.
