A first estimate of white shark, Carcharodon carcharias , abundance off Central California

Abstract

The decline of sharks in the global oceans underscores the need for careful assessment and monitoring of remaining populations. The northeastern Pacific is the home range for a genetically distinct clade of white sharks (Carcharodon carcharias). Little is known about the conservation status of this demographically isolated population, concentrated seasonally at two discrete aggregation sites: Central California (CCA) and Guadalupe Island, Mexico. We used photo-identification of dorsal fins in a sequential Bayesian mark-recapture algorithm to estimate white shark abundance off CCA. We collected 321 photographs identifying 130 unique individuals, and estimated the abundance off CCA to be 219 mature and sub-adult individuals ((130, 275) 95% credible intervals), substantially smaller than populations of other large marine predators. Our methods can be readily expanded to estimate shark population abundance at other locations, and over time, to monitor the status, population trends and protection needs of these globally distributed predators.

Estimating apparent survival of sub-adult and adult white sharks (Carcharodon carcharias) in central California using mark-recapture methods

“…Though Chapple et al (2011) did not find evidence of temporary emigration (TE) during their shorter study period it was considered that females might have temporarily emigrated from the study area during our longer 6-year study period. This is important because when TE is non-random (Markovian), i.e., animals have a different probability of being a temporary emigrant in year t + 1 depending on whether they are already a temporary emigrant in year t or not, CJS estimates of ϕ can be biased (Kendall et al, 1997).…”

“…Chapple et al (2011) observed a ratio of 3.6:1 (69 males: 19 females). It should be noted that an additional 42 individuals were observed but could not be assigned to sex during the 3-year study making it impossible without further analysis to accurately assess the actual sample sex ratio.…”

“…Over the course of the study, the set of white shark dorsal fin photos obtained that met the identification standard described by Chapple et al (2011) were used to develop individual markrecapture encounter histories. These resighting histories indicated whether or not each individual shark had been seen or not in each year of the study and whether or not the sex of the animal was known or not when the animal was encountered.…”

DNA from historical and trophy samples provides insights into white shark population origins and genetic diversity

“…It is classified as 'Vulnerable' in the IUCN Red List, and in 2004 was placed on CITES Appendix II. Many populations have undergone dramatic declines (Baum et al 2003), and first estimates of white shark abundance in Californian waters seemed to suggest substantially smaller numbers than other large marine predators (Chapple et al 2011), prompting urgent calls for protection. However, a recent study refuted this, indicating a greater estimated population size in the eastern Pacific (Burgess et al 2014), while historic abundance trends in the western North Atlantic suggest recovering populations (Curtis et al 2014).…”

Tracking sharks without teeth: a non-invasive rigid tag attachment for large predatory sharks

“…Though Gleiss et al [10] found little effect from the tagging on the second dorsal fin of whale sharks, for white sharks, these biologging tags, which generally range in size from 100 to 500 g, must be placed on the first dorsal fin. These fins are biologically critical for stability while fast swimming [11] and are used scientifically as means to uniquely identify individuals across decades [12] and estimate abundance [13,14]. Additionally, some communities are dependent on white sharks for ecotourism, so there are sensitivities to the risk (or perceived risk) of damage to dorsal fins from attaching data loggers [15].…”