Mark-recapture estimates suggest declines in abundance of common bottlenose dolphin stocks in the main Hawaiian Islands

Abstract

Species conservation relies on understanding population demographics, yet this information is lacking for many species and populations. Four stocks of common bottlenose dolphins Tursiops truncatus inhabiting 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.

Priorities for ecological research on cetaceans in the Galápagos Islands

“…Given the logistical and funding challenges of conducting cetacean research in Galaṕagos, we suggest that the development of a collaborative research agenda should include specific actions aimed at establishing and strengthening meaningful partnerships between foreign and local researchers, and include the involvement of (Palacios and Salazar, 2002;Denkinger et al, 2013;Denkinger et al, 2020), or the population assessment surveys for cetaceans conducted through a collaboration between U.S. academics and the Ecuadorian Navy through INOCAR, its institute for oceanographic and Antarctic research (Biggs et al, 2017;O'Hern et al, 2017). Continued partnership with the local tourism industry, in particular, would provide additional opportunities for participation by trained naturalists and citizen scientists in opportunistic but valuable data collection that would enhance and enrich research goals in a relatively inexpensive manner (e.g., Van Cise et al, 2021).…”

Long‐term associations of common bottlenose dolphins with a fish farm in Hawaiʻi and impacts on other protected species

“…Makako Bay is known as a traditional resting area for spinner dolphins (Tyne et al, 2018), and pantropical spotted dolphins sometimes join spinner dolphins in these resting bays. In contrast to spinner dolphins, a recent study showed that the abundance of the Hawaiʻi Island bottlenose dolphin stock appears to be relatively stable and in the low 100 s (the 2018 abundance estimate was 136 individuals, 95% CI [61, 303]; Van Cise et al, 2021). Bottlenose dolphins off Hawaiʻi Island have high levels of site fidelity and genetic differentiation from other populations in the Hawaiian Islands (Baird et al, 2009; Martien et al, 2011).…”

“…We identified 36 individual bottlenose dolphins at the farm, about one-quarter of the estimated Hawaiʻi Island resident population (Van Cise et al, 2021). Twenty-two of the dolphins seen at the farm were repeatedly documented there.…”

Population parameters and conservation implications for one of the world's rarest marine fishes, the red handfish (Thymichthys politus)

“…Mark‐recapture is a common approach used to track individuals through time and can be used to obtain estimates of abundance. The approach considers encounter histories of marked individuals over multiple capture events and has been widely used for marine animals (e.g., Claassens & Harasti, 2020; Grossman et al, 2019; Harasti, 2016; Harasti et al, 2012; Martin‐Smith, 2011; Shine et al, 2021; Van Cise et al, 2021). Depending on the model used, assessments can be made of population parameters, such as survival and probability of re‐capture (e.g., Cormack, 1964; Jolly, 1965; Seber, 1965), probability of entry to a population (e.g., Schwarz & Arnason, 1996), recruitment, and population growth (e.g., Link & Barker, 2005; Pradel, 1996).…”