Abstract
AbstractUnderstanding population status and trends is important for developing and evaluating management and conservation actions for threatened species. Monitoring population status of marine organisms is especially challenging. Because sea turtles come ashore to lay their eggs and nests are easily counted, these counts are commonly used as an index of abundance and population trends. Nest counts do not provide a direct index of adult female population abundance because females typically lay more than one nest per year and most do not reproduce every year. This study attempts for the first time to investigate the likelihood that observed fluctuations of nest counts represent inter‐annual changes of the adult female population by accounting for uncertainty in reproductive rate parameters. We analyzed 30 yr of reproductive data from the largest nesting loggerhead sea turtle population worldwide, breeding in Florida (USA), and for the three Recovery Units and seven Management Units therein. Nest counts followed a general non‐monotonic trend with wide fluctuations that corresponded to decreasing and increasing trends during short intervals. When we accounted for uncertainty in both clutch frequency and remigration interval, there was no evidence for an increasing or a declining trend in the breeding female population across the entire period. Despite extensive conservation efforts and protections for loggerheads in Florida and the wider USA, we did not find evidence of a strong population recovery. We recommend maintaining a high level of protection, addressing persistent anthropogenic threats, continued collection of rigorous nest‐count data, and monitoring reproductive parameters to better link nest counts to adult female population abundance. Our results demonstrate the need for caution in using nest counts as a direct proxy for adult female population status, as it may lead to unsupported conclusions potentially detrimental to conservation. Therefore, we recommend to always translating nest trends to at least adult female trends, including uncertainty in reproductive parameters. Our approach can be exported to other populations, even where reproductive parameters are not available. Applying high parameter uncertainty obtained from other populations can help identifying unequivocal population changes; that is, nest trends unlikely justified by uncertainty and poor knowledge of reproductive parameters.
Highlights
Over the last five centuries, sea turtle populations have been so drastically reduced by anthropogenic activities that they may no longer fulfill their historical ecological roles (Jackson et al 2001, Bjorndal and Jackson 2003)
We explore the extent to which estimates of adult female trends derived from nest counts are influenced by the uncertainty in two reproductive parameters—remigration interval and clutch frequency
It is important to stress that the uncertainty we have addressed in c and r is based on simulations
Summary
Over the last five centuries, sea turtle populations have been so drastically reduced by anthropogenic activities that they may no longer fulfill their historical ecological roles (Jackson et al 2001, Bjorndal and Jackson 2003). Because sea turtles take decades to reach sexual maturity (Heppell et al 2003b), any fluctuations observed in nest counts (and by extension, adult females) are the cumulative result of events (either threats or conservation measures) that occurred prior, either on the nesting beach or in the water. These limitations notwithstanding, nest counts have always been the most common, if not sole, index of sea turtle population abundance and trends (Carr et al 1978, Meylan 1982, Schroeder and Murphy 1999, National Marine Fisheries Service and U.S Fish and Wildlife Service 2008, Witherington et al 2009, Wallace et al 2013b, Casale and Tucker 2017, Mazaris et al 2017). Nest counts can be widely and systematically obtained, compared among study locations, and tracked over long time series
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