Role of Crassicauda sp. in natural mortality of pantropical spotted dolphins Stenella attenuata: a reassesment

Evaluating the effect of parasites on population size is essential for designing management and conservation plans of wild animal populations. Although knowledge in this area is scarce in cetaceans, current evidence suggests that species of the nematode genus Crassicauda may play an important regulatory role in some populations. In the present study, a semiparametric regression technique is applied to a previously published dataset to re-examine the role of Crassicauda sp. in natural mortality of pantropical spotted dolphins Stenella attenuata. The resulting model indicated parasite-induced mortality at ages between 6.5 and 9 years and at roughly 12 years. The maximum mortality estimates obtained could represent 2-4% of natural mortality in dolphins 6-8-year old. This estimate is substantially smaller than previously published values, but in contrast with previous research, the present model provides clear statistical evidence for parasite-induced mortality because the bootstrapped 95% confidence intervals of the estimated mortality rates excluded the zero value. The present study also evaluates, through simulations, how potential sampling biases of infected dolphins could overestimate parasite-induced mortality. Small differences in sampling selectivity between infected and uninfected animals could substantially reduce the mortality estimates. However, the simulated models also supported the notion of statistically significant mortality in juvenile dolphins. Given that dolphins older than 16 years were poorly represented in the dataset, further research is needed to establish whether Crassicauda sp. causes meaningful mortality for population dynamics among adult individuals