Estimating bottlenose dolphin population parameters from individual identification and capture-release techniques

Abstract

Field studies begun in 1970 and continuing to date have identified at least three adjacent resident populations (or communities) of bottlenose dolphins along the central west coast of Florida. We have used photo-identification, mark-recapture techniques, behavioral observations, radio-tracking and brief captures for biological sampling to examine the structure and dynamics of these populations. Population designations are based on consideration of individual home ranges, social association patterns and genetics. Although the populations are relatively discrete in terms of ranges and associations, electrophoretic analyses of blood samples indicate that genetic exchange occurs between populations. Males travelling between populations appear to be the probable vectors for genetic exchange. Most field effort has concentrated on the Sarasota dolphin population. Most of its members are identifiable from natural marks or tagging efforts over the last 20 years. This population consists of about 100 individuals. For the analyses presented here, we considered 116 dolphins identified during 1980-1987. Of these, the sex was known for 90 dolphins and the age has been estimated for 79 dolphins. The long time span of the study and the high proportion of identifiable individuals has allowed us to estimate vital rates for this population. An annual recruitment rate to age 1 of 0.048 was countered by a minimum mortality rate of 0.010 and a mean annual loss rate from other causes of 0.029 (e.g., emigrations, mortalities for which carcasses were not recovered or undocumented changes in identifying characteristics). Immigration was infrequent, with a mean annual rate about 0.02. The mean fecundity rate was 0.144. Knowledge of maternal relationships allowed comparisons of the percentage of calves observed in the field vs the percentage of young of the year. Because of the prolonged period of association between mothers and calves, there were nearly six times as many mother-calf pairs as mothers with young of the year. To test the effectiveness of photo-identification techniques, we compared the number of correct identifications made in the field 'by eye' against the number identified from photographs. We identified 89% of these well-marked dolphins correctly by eye. Because virtually all the dolphins were marked in the most-heavily surveyed portion of the study area, it was also possible to test the accuracy and precision of mark-recapture methods. Both the Petersen and Schnabel methods underestimated the known population size, although the Schnabel estimate was less biased. This bias was likely due to heterogeneity of sighting probabilities; different age-sex classes were shown to have different sighting probabilities.