Inferring population trends for the world's largest fish from mark–recapture estimates of survival

Abstract

Summary 1.Precise estimates of demographic rates are key components of population models used to predict the effects of stochastic environmental processes, harvest scenarios and extinction probability. 2. We used a 12-year photographic identification library of whale sharks from Ningaloo Reef, Western Australia to construct Cormack-Jolly-Seber (CJS) model estimates of survival within a capture-mark-recapture (CMR) framework. Estimated survival rates, population structure and assumptions regarding age at maturity, longevity and reproduction frequency were combined in a series of age-classified Leslie matrices to infer the potential trajectory of the population. 3. Using data from 111 individuals, there was evidence for time variation in apparent survival ( φ ) and recapture probability ( p ). The null model gave a @ of 0·825 (95% CI: 0·727-0·893) and p = 0·184 (95% CI: 0·121-0·271). The model-averaged annual @ ranged from 0·737 to 0·890. There was little evidence for a sex effect on survival. 4. Using standardized total length as a covariate in the CMR models indicated a size bias in φ . Ignoring the effects of time, a 5-m shark has a @ = 0·59 and a 9 m shark has @ = 0·81. 5. Of the 16 model combinations considered, 10 (63%) indicated a decreasing population ( λ < 1). For models based on age at first reproduction ( α ) of 13 years, the mean age of reproducing females at the stable age distribution ( A ) ranged from 15 to 23 years, which increased to 29-37 years when α was assumed to be 25. 6. All model scenarios had higher total elasticities for non-reproductive female survival [ E ( s nr )] compared to those for reproductive female survival [ E ( s r )]. 7. Assuming relatively slow, but biologically realistic, vital rates ( α = 25 and biennial reproduction) and size-biased survival probabilities, our results suggest that the Ningaloo Reef population of whale sharks is declining, although more reproductive data are clearly needed to confirm this conclusion. Combining relatively precise survival estimates from CMR studies with realistic assumptions of other vital rates provides a useful heuristic framework for determining the vulnerability of large oceanic predators for which few direct data exist.

Long-Term Photo-Identification Reveals the Population Dynamics and Strong Site Fidelity of Adult Whale Sharks to the Coastal Waters of Donsol, Philippines

“…data). Despite regulations and bans on direct hunting in several countries, the late onset of sexual maturity (Bradshaw et al, 2007) hinders population recovery and leaves whale sharks susceptible to overexploitation and anthropological impacts .…”

Individual Recognition

“…Photo-identification of natural markings is increasingly being used to collect data on individual animals for application to demographic studies (Thompson et al, 2000;Lusseau et al, 2006;Bradshaw et al, 2007). Although information obtained from photo-identification is highly valuable (Hammond, 1990;Blackmer et al, 2000), processing data in large catalogues can be labour-intensive and subject to human error (Kelly, 2001).…”

“…The ability to recognize individuals from naturally occurring features has many advantages over conventional marking techniques including: animals are not physically captured, identifiable characteristics are stable over time, and the behaviour of the animal is less likely to be affected by the identification system (Hammond, 1990;Blackmer et al, 2000). Photographic identification, in which researchers photographically capture these natural markings to identify and re-identify individuals, has proven to be a useful tool in long-term monitoring of wild animal populations (Thompson et al, 2000;Forcada and Aguilar, 2003;Bradshaw et al, 2007). For example, whisker patterns are used in lions, facial scale patterns in sea turtles, pelage spot patterns in whale-sharks, fin shape and scarring in dolphins (Richardson et al, 2000;Thompson et al, 2000;Arzoumanian et al, 2005;Ogutu et al, 2006).…”

“…The ability to identify individuals within a population is often a starting point for ecological and conservation studies (Thompson et al, 2000;Sibly et al, 2005;Lusseau et al, 2006;Bradshaw et al, 2007;McMahon et al, 2007). Realistic estimates of population size and life history parameters are central to effective wildlife management, but are often difficult to measure in long-lived and elusive migratory marine vertebrates (Caughley, 1994).…”

Occupancy estimation of marine species: dealing with imperfect detectability

“…Katsanevakis 2009, Ronconi & Burger 2009) and mark− recapture (e.g. D 'Anna & Pipitone 2000, Bradshaw et al 2007, have been used in the marine environment. However, methods for estimating occupancy of marine species from presence− absence surveys that take into account imperfect detectability have not been applied yet.…”