Abstract:
Studies of avian physiology provide insights into sub-lethal impacts of stress and diet, indicating potential underlying causes of decline and adaptive capacity. This knowledge can be used to improve conservation management of at-risk populations. Hoiho (Megadyptes antipodes) breeding on the south-eastern coast of the South Island, New Zealand, exhibit poor reproductive success and have small populations compared to subantarctic populations. This offers a unique opportunity to map broad physiological traits indicative of sub-lethal (chronic) stress and diet changes in a Sphenisciformes species. Diving is essential to seabird foraging and survival. This is the first study to estimate oxygen stores and dive capability (maximal dive depth), maximal dive duration and calculated aerobic dive limit in hoiho. Calculated maximum dive depth estimates exceeded reported depths, suggesting high adaptive capability in the event that hoiho prey shift to cooler, deeper waters, and that hoiho continue to employ a benthic foraging strategy. However, current maximum theoretical dive durations often exceed calculated aerobic dive limit (cADL), which suggests that performing deeper dives, should key prey shift to greater depths, will be more energetically expensive with longer wait times between them to recover oxygen debt. Nutritional stable isotopes and corticosteroid hormones can identify chronic stress or shifts in diet. As such, these were quantified to understand how hoiho populations vary spatially and among breeding seasons. Compared to subantarctic populations, mainland hoiho had higher carbon and nitrogen values that were indicative of inshore/benthic feeding on higher-trophic-level prey. This suggests that mainland birds have lower prey abundance and diversity, leading to lower diet quality. Comparisons across several seasons indicated a trophic increase, suggestive of a prey shift. Chick stress increased and adults fed further offshore during La Niña seasons, suggesting nutritional stress in hoiho chicks results from prey distribution shifts induced by warming sea temperatures. This work provides first insights into the diving and stress physiology of hoiho which may augment existing management plans for the species. Spatio-temporal differences in physiology and foraging ecology reflected contrasting environmental conditions experienced by each population which may finesse species distribution modelling methods to predict future locations. This validated the integrated use of feather corticosterone and stable isotopes of hoiho populations to monitor changes in marine ecosystem health. I recommend further studies should undertake compound-specific stable isotope analyses to determine if differences in stable isotopes are due to genuine differences in foraging niche or differing environmental baselines.