Reproductive parameters and factors influencing calf survival of bottlenose dolphins that engage in a unique foraging cooperation with fishermen

Abstract

A subset of the bottlenose dolphin Tursiops truncatus gephyreus population in Laguna, southern Brazil, specialize in foraging cooperatively with fishermen. In this study, we describe reproductive parameters for these dolphins and investigate whether this specialized tactic generates reproductive advantages for females that frequently engage in this unusual behavior. We analyzed photo-identification data collected during 233 boat-based surveys during 2007–2009 and 2013–2017. From 27,808 high-quality photographs, we identified and tracked the fate of 24 reproductive females and 45 of their calves. Calving was found to be seasonal, with most births occurring in late spring/summer. The average crude birth rate was 0.09, and estimated fecundity was 0.17. The mean inter-birth interval was 2.09 (for all calves) or 2.43 years (for surviving calves only). Survival to 1 and 2 years estimated by the Kaplan–Meier method was 0.78 (95% CI 0.65–0.92) and 0.65 (95% CI 0.51–0.83), respectively—which represents a survival rate in the second year of 0.83. We investigated the potential influence of birth timing, resource availability, and maternal foraging tactic, home range size and frequency of interaction with fishermen on calf survival. Timing of birth was a significant predictor of calf survival. Giving birth close to the local mullet season would provide lactating females with increased seasonal prey resources, leading to increased calf survival. Due to our sample size (n = 9 cooperative and 15 non-cooperative females), we could not conclude whether or not the cooperative foraging tactic influences calf survival and female reproductive success. We emphasize the importance of long-term monitoring of populations to understand regional life history characteristics and provide accurate information for viability analyses.