Abstract
Species conservation relies on understanding population demographics, yet this information is lacking for many species and populations. Four stocks of common bottlenose dolphinsTursiops truncatusinhabiting the waters surrounding the main Hawaiian Islands (USA) are exposed to anthropogenic disturbances including fisheries interactions, tourism, naval activities, ocean noise, and contaminants. Although these stocks are managed under the Marine Mammal Protection Act, a demographic assessment has not been undertaken since 2006, and there is currently no information on population trends. We combined regular survey effort with citizen science contributions to estimate apparent survival and annual abundance within each stock using photographs collected between 2000 and 2018. Over this time period, we collected 2818 high-quality identifications of 765 distinctive individuals across all 4 stocks. Analyses of inter-annual movements indicated that individuals exhibit restricted habitat use within stocks, which contributed to non-random sampling. Annual abundance estimates ranged from the 10s to the low 100s. Apparent survival ranged from 0.84 to 0.9, with lower-than-expected estimates in all stocks. Annual abundance estimates declined in 3 of the 4 stocks; however, this decline was not significant for the Kaua‘i/Ni‘ihau and O‘ahu stocks, and may be an artifact of sampling design in all stocks. Given the small population size for these stocks, it is important to closely monitor trends in abundance as a first step in mitigating negative effects of anthropogenic activities. Future efforts should focus on consistent geographic coverage in all stocks to decrease model uncertainty and improve trend assessment.
Highlights
Modern strategies for the conservation and management of species rely primarily on the knowledge of population demographics, such as abundance, recruitment, survival, migration, and trends in these over time
It is especially important to monitor these trends in species or populations that regularly come in contact with anthropogenic stressors, in order to understand whether those stressors threaten the population or species
In the USA, population demographics of most cetaceans are assessed by the National Marine Fisheries Service (NMFS) using observational line-transect surveys conducted from a variety of platforms (e.g. Bradford et al 2017)
Summary
Modern strategies for the conservation and management of species rely primarily on the knowledge of population demographics, such as abundance, recruitment, survival, migration, and trends in these over time. Bradford et al 2017) These surveys are decreasing in frequency, and do not always capture nearshore populations sufficiently to permit abundance estimation for the entire population. In these cases, mark−recapture methods that use natural scars and coloration patterns to identify individuals can be an effective way to estimate trends in population abundance, recruitment, and apparent survival, including both death and emigration Mark− recapture surveys can be conducted from small vessels nearshore, and are often an effective highresolution tool to assess the demographics of neritic populations
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