A decade of humpback whale abundance estimates at Bermuda, an oceanic migratory stopover site

Abstract

We constructed annual abundance of a migratory baleen whale at an oceanic stopover site to elucidate temporal changes in Bermuda, an area with increasing anthropogenic activity. The annual abundance of North Atlantic humpback whales visiting Bermuda between 2011 and 2020 was estimated using photo-identification capture-recapture data for 1,204 whales, collected between December 2009 and May 2020. Owing to a sparse data set, we combined a Cormack-Jolly-Seber (CJS) model, fit through maximum likelihood estimation, with a Horvitz-Thompson estimator to calculate abundance and used stratified bootstrap resampling to derive 95% confidence intervals (CI). We accounted for temporal heterogeneity in detection and sighting rates via a catch-effort model and, guided by goodness-of-fit testing, considered models that accounted for transience. A model incorporating modified sighting effort and time-varying transience was selected using (corrected) Akaike’s Information Criterion (AICc). The survival probability of non-transient animals was 0.97 (95% CI 0.91-0.98), which is comparable with other studies. The rate of transience increased gradually from 2011 to 2018, before a large drop in 2019. Abundance varied from 786 individuals (95% CI 593-964) in 2016 to 1,434 (95% CI 924-1,908) in 2020, with a non-significant linear increase across the period and interannual fluctuations. These abundance estimates confirm the importance of Bermuda for migrating North Atlantic humpback whales and should encourage a review of cetacean conservation measures in Bermudian waters, including area-based management tools. Moreover, in line with the time series presented here, regional abundance estimates should be updated across the North Atlantic to facilitate population monitoring over the entire migratory range.

Humpback whales inflate throat pleats to rub against the seafloor for skin maintenance

“…The study presented here focused on waters of Bermuda (Figure 1), an archipelago of roughly 180 islands used as a mid‐migration resting or stopover site in the spring (starting in mid‐February and peaking in mid‐March through April, when water temperatures range from 20°C to 22°C) for North Atlantic humpback whales (Grove et al, 2023; Homfeldt et al, 2022) returning from winter breeding grounds in the Caribbean. After staying briefly (typically 1–2 weeks, although with great variation, including some humpbacks potentially spending most of the winter near Bermuda), humpbacks head north to widespread summer feeding grounds in the Western or Eastern North Atlantic, in regions ranging from the Gulf of Maine, Eastern Canada, Greenland, Iceland, and Norway (Grove et al, 2023). Typically, humpbacks, like other whales, generally forage entirely or almost exclusively seasonally on summer grounds (Nichols et al, 2022; Savoca et al, 2021), but they feed opportunistically at lower latitudes (Findlay et al, 2017; Pirotta et al, 2021).…”