Abstract:   Individual photo-identification mark-recapture study was performed to estimate the demographic parameters and socio-behavioural group dynamics of the Indian Ocean humpback dolphin (Sousa plumbea) in Plettenberg Bay, South Africa, during April 1999 to April 2004.  Apparent survival rates, recapture probabilities, and super-population size were quantified with the application of Cormack-Jolly-Seber and POPAN models.  Given that the majority of identified individuals were sighted only once, transience of animals was significant and these dolphins likely travel distances exceeding the size of the Plettenberg Bay region.  Higher survey intensity, however, is needed to improve the accuracy and robustness of population parameter estimations.

 

This study applied a comprehensive approach in the formulation of candidate models during the mark-recapture model selection process.  Various effects, such as the basic time-dependent (‘t’), constant over time (‘.’), transience (‘a2’), trap-dependence (‘*m’), seasonal (‘season’) and effort (‘effort’) effects were applied to test the likelihood of factors.  Incorporating and identifying these effects provided the means for more accurate and unbiased estimation of demographic parameters; e.g. separating “transient” individuals in the estimation of apparent survival rates limited the negative bias related to movement instead of true mortality, thus making the estimate closer to the true survival rates.

 

Socio-behavioural analyses revealed that humpback dolphins that use the coastal waters of Plettenberg Bay live in a dynamic fission-fusion society with fluid inter-individual associations.  There appears to be social differentiation between dolphins transiting through the area and those that use Plettenberg Bay more frequently, perhaps on long-term basis.  The transients, however, represent a predominant majority of the humpback dolphins seen in the Bay.  The degree of residency of the small number of more frequently seen individual remains undetermined.

 

The socio-dynamics of humpback dolphin groups in the Plettenberg Bay region meet the expectations of a recently proposed conceptual framework of delphinid social strategies, which suggests that group dynamics and ranging patterns of inshore delphinids are determined by the spatiotemporal predictability and availability of limited inshore resources.  The study in Plettenberg Bay confirms that humpback dolphins represent an intermediate-ranging pattern among coastal delphinids.

 

Although not free from technical and analytical limitations, which are discussed, this study provides important information on demographic parameters and socio-behaviour of humpback dolphins inhabiting the Plettenberg Bay region.  Such data carries considerable conservation implications and is informative to future management decision that may affect coastal waters of Plettenberg Bay and its mammalian inhabitants.