Abstract
Adult survival is arguably the most important demographic parameter for long-lived species as it has a large impact on population growth, and it can be estimated for cetacean populations using natural markings and mark-recapture (MR) modelling. Here we describe a 26-year study of a genetically discrete, resident population of bottlenose dolphins in the Shannon Estuary, Ireland, conducted by an NGO using multiple platforms. We estimated survival rates (SRs) using Cormack-Jolly-Seber models and explored the effects of variable survey effort, multiple researchers, and changes in camera equipment as well as capture heterogeneity induced by changes in marks and site fidelity variation, all common issues affecting longitudinal dolphin studies. The mean adult SR was 0.94 (±0.001 SD) and thus comparable to the estimates reported for other bottlenose dolphin populations. Capture heterogeneity through variation in mark severity was confirmed, with higher capture probabilities for well-marked individuals than for poorly marked individuals and a “transience” effect being detected for less well-marked individuals with 43% only recorded once. Likewise, both SR and capture probabilities were comparatively low for individuals with low site fidelity to the Shannon Estuary, and SR of these individuals additionally decreased even further toward the end of the study, reflecting a terminal bias. This bias was attributed to non-random temporal migration, and, together with high encounter rates in Brandon Bay, supported the hypothesis of range expansion. Our results highlight the importance of consistent and geographically homogenous survey effort and support the differentiation of individuals according to their distinctiveness to avoid biased survival estimates.
Highlights
For effective conservation of animal populations, regular assessment of demographic parameters such as population size and life history traits is essential (Berrow et al, 2012; Arso Civil et al, 2019)
The number of surveys in the Shannon Estuary varied considerably between years ranging from only two rigid-hulled inflatable boat (RIB) surveys in 2004 to a combined 102 surveys in 2016 from both research RIB and commercial dolphin watching vessels (Figure 2)
Covariates with an overall Akaike weight of ≥0.001 are shown in this summary, the remaining covariates can be found in Supplementary Table 8
Summary
For effective conservation of animal populations, regular assessment of demographic parameters such as population size and life history traits is essential (Berrow et al, 2012; Arso Civil et al, 2019). Survival Rates and Capture Heterogeneity for any changes, information about those parameters driving population size are required (Lindberg and Rexstad, 2002; Arso Civil et al, 2019; Schleimer et al, 2019). In cetaceans, which are highly mobile and elusive animals (Pace et al, 2017), mark-recapture (MR) modelling has become well-established as tool to assess survival rates (SRs) (Currey et al, 2009; Pace et al, 2017; Bertulli et al, 2018; Arso Civil et al, 2019). After the reconstruction of encounter histories of individuals over time, MR-models allow for the estimation of both survival and capture probabilities through maximum likelihood approaches (Lebreton et al, 1992; Corkrey et al, 2008; Cooch and White, 2019a)
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