Estimating survival and abundance in a bottlenose dolphin population taking into account transience and temporary emigration

Abstract

Knowledge of demographic parameters of most cetacean populations is scarce because of problems associated with sampling open populations of wide-ranging animals. In recent years, capture-recapture models have been developed to address these problems. We used a photo-identification dataset collected from a population of bottlenose dolphins Tursiops truncatus between 1999 and 2004 around 2 islands of the Azores archipelago, to demonstrate the use of some of these methods. A variety of open models and Pollock's robust design were applied to estimate population size, survival probability and emigration rates. Using only the estimates with the lowest coefficients of variation, the annual abundance of adult dolphins varied between 202 (95% CI: 148 to 277) and 334 (95% CI: 237 to 469), according to the Jolly-Seber method, and between 114 (95% CI: 85 to 152) and 288 (95% CI: 196 to 423), according to the robust design. The number of subadult individuals varied from 300 (95% CI: 232 to 387) to 434 (95% CI: 316 to 597) based on the Jolly-Seber method. The open models yielded estimates of adult survival (0.970 ± 0.029 SE) that were significantly higher than those for subadults (0.815 ± 0.083 SE). Movement patterns of dolphins in the Azores seem to follow a Markovian model, in which dolphins seen in the study area in 1 yr show higher probability of emigrating in the following year. Despite some limitations, this is the first study to model transience and temporary emigration in a dolphin population.

Estimates of abundance and apparent survival of coastal dolphins in Port Essington harbour, Northern Territory, Australia

“…Studies of dolphins often ignore the possibility that some members of the populations are wide-ranging and may emigrate temporarily from the study area (Silva et al 2010). Study areas rarely encompass the whole range of the animals and, even though we could not model it here, estimates of temporary emigration are very useful in accounting for movements between the overall range of the population and the area under study.…”

“…Movements could be due to local-scale shifts in prey distribution (Irvine et al 1981;Fazioli et al 2006;Silva et al 2010;Speakman et al 2010), seasonality (Smith 2012) or phases in reproductive activity. Further investigations into individual movement patterns by the three species are required and might be addressed by multi-state models (Williams et al 2002;Nicholson et al 2012), whereby data are collected simultaneously in several close-by study sites.…”

Abundance, survival and temporary emigration of bottlenose dolphins (Tursiops sp.) off Useless Loop in the western gulf of Shark Bay, Western Australia

“…Silva et al 2009;Speakman et al 2010;Cantor et al 2012), combines the use of closed and open population models under a nested sampling framework. Short bursts of sampling, with longer gaps in between the bursts, mean that closed models can be used for the bursts, whereas open models are needed for the whole study.…”

“…The photo-identification method is widely applied with capture-recapture methods to estimate abundance and other demographic parameters, such as survival and temporary emigration rates, of small cetaceans (e.g. Wilson et al 1999;Read et al 2003;Parra et al 2006;Silva et al 2009). The main advantage of using photo-identification data for capture-recapture analyses is that animals do not need to be physically captured to be marked (Hammond 1986(Hammond , 1990.…”

Report of the Working Group on Population Parameters and Demography of Tursiops truncatus in the Southwest Atlantic Ocean

“…Survival estimate in Cagarras Island is among the lowest for wild bottlenose dolphins worldwide (Fortuna, 2006;Currey et al, 2008;Silva et al, 2009;Mansur et al, 2012;Daura-Jorge et al, 2013;Fruet et al, 2015b). Lodi et al (2014) discuss that such a low survival probability could represent both a gradual abandonment of the study area or a high mortality outside this area.…”