The spatial ecology and population structure of humpback whales (Megaptera novaeangliae) in mainland Mexico; Implications for management and conservation

Nicola Ransome

Effective management and conservation of large mysticete whales is challenged by their extensive migrations and use of multiple seasonal habitats that often cross many national jurisdictional boundaries. These challenges mean demographic data needs to be collected over large spatial scales covering diverse environments and that international collaboration is essential for research, management and effective conservation of populations. The humpback whale (Megaptera novaeangliae) is a large mysticete with a global distribution that migrates annually between high-latitude summer feeding areas and low-latitude winter breeding areas. This study investigated the spatial ecology and population structure of North Pacific humpback whales that winter in the tropical breeding area of mainland Mexico, a region of the central Pacific coast of Mexico. The earliest recorded occurrence of humpback whales in the region dates back over 200 years. However, there was little published information on the species available, for example of the density and distribution of humpback whales throughout the area, the residency and site fidelity of individuals, population structure and migratory connections to other nearby regional habitats, and threats to the species regionally. These subjects were investigated as individual chapters in the thesis as follows. In Chapter 2, evidence of vessel strike with large whales throughout the broader region of the Eastern Tropical Pacific was collated from historical newspapers, scientific journals and reports, and online articles. Humpback whales were identified as the most commonly hit species, and Mexico as the location where most large whale strike events had occurred. In Chapter 3, systematically designed aerial surveys covering the whole mainland Mexico region were undertaken during two wintering seasons (2018 and 2019) using distance sampling methodology. Density surface models of humpback whale groups were developed, which revealed a predominantly coastal distribution of the species with highest densities found in a previously unsurveyed area. This is the first available broadscale information on species distribution throughout the region. In Chapter 4, the seasonal presence of 213 individually identified humpback whales of known sex and long sighting histories (>8 years) was investigated using photo-identification data. This revealed high site fidelity of whales to mainland Mexico, with some individuals documented returning to the region over four decades and seasonal residencies of up to 2.5 months. Strong migratory connections to the neighbouring Central America breeding area were discovered and evidence that Central America whales use mainland Mexico for mating, calving and nursing. In Chapter 5, atypical mass feeding was documented in the region during five wintering seasons over an eight-year period, and its drivers investigated. Binomial general linear models revealed a significant relationship between the probability of whales feeding and sea surface temperature (SST). Feeding predominantly occurred when SST dropped below 25°C, with sustained feeding in lower temperatures (~23°C) and associated with La Nina years of the El Nino Southern Oscillation (ENSO). Feeding whales were predominantly juvenile animals from the more northern high-latitude feeding areas of Russia to Northern British Columbia, Canada, with more prolific feeding episodes following marine heatwaves of the North Pacific that occurred during the study period. In Chapter 6, investigation of trans-Pacific movements of humpback whales demonstrated migratory interchange between Mexico and Russia, the principle feeding destination of the endangered Western North Pacific population. An unprecedented number, 117 individuals, were identified in both countries, and this subsect of whales appears to regularly migrate across the North Pacific to breed, with one Russia whale recaptured in mainland Mexico in ten different years. Chapter 7 is an overview of the most significant results of the combined work of the thesis and future research directions are considered to address the remaining knowledge gaps. The products of this thesis greatly enhance our understanding of migratory interchange in the North Pacific and population structure of the Mexico breeding assemblage, particularly the seasonal occurrence of whales throughout mainland Mexico. The findings from my research have already contributed to the assessment of humpback whale habitat use nationally and internationally, and for management of the species throughout the North Pacific, and it is my hope that in the future it will continue to be used to inform and aid the conservation of the species.

The spatial ecology and population structure of humpback whales (Megaptera novaeangliae) in mainland Mexico; Implications for management and conservation

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