Conflicting rates of increase in the sperm whale population of the eastern Caribbean: positive observed rates do not reflect a healthy population

Abstract

Observed rates of increase calculated from trends in the numbers of animals present in a population should generally agree with those estimated from life-history data. However, for a small population of individually identified sperm whales Physeter macrocephalus occupying the waters of the eastern Caribbean there is a discrepancy. Using a mark-recapture analysis that included heterogeneity in identification, the population, numbering about 156 adults (95% CI 126−195) in 1998, has been increasing at 3.4% yr −1 (95% CI: 1.0−5.7% yr −1 ). However, a 2-stage matrix population model including unweaned calves and adults (and excluding mature males), whose parameters were estimated directly from empirical data, gave a projected rate of increase of −2.7% yr −1 (95% CI: −5.4 to −0.4% yr −1 ). This estimate is primarily sensitive to calculated adult mortality. The discrepancy between the observed and projected rates of increase for this popula- tion may be explained by a high, probably anthropogenic, mortality of sperm whales in the east- ern Caribbean, coupled with immigration from surrounding regions, so the area becomes an attractive sink (ecological trap). The analysis emphasizes the fragility of sperm whale populations. More generally, our analysis of this population shows that a positive observed rate of increase is not necessarily a sign of a healthy population. This case study highlights the importance of analysing populations of endangered species using multiple methodologies and with a solid base of individual-level empirical data based on longitudinal monitoring.