Abstract
Dietary and foraging specialisation can put seabird species at risk when the resources they depend on for survival overlap with industrial activities in their marine environment. Behavioural plasticity allows individuals and populations to adapt to short-term change, but the consequences of such change can have population-level effects if sustained in the long-term. Yellow-eyed penguins/hoiho (Megadyptes antipodes) breeding on the mainland of Aotearoa New Zealand are non-migratory, inshore foragers, with specialised diets and benthic foraging strategies. Contemporaneous industrial activities, including bottom-set gillnetting, bottom contact trawling, and deposition of dredged sediments, occur throughout the foraging range of yellow-eyed penguins on the Otago coast, and pose direct and indirect threats to mainland yellow-eyed penguin population viability. Population decline in the last two decades, combined with observations of chronic and acute nutritional stress, have suggested evidence for declining diet quality affecting adult and juvenile survival. I investigated evidence for behavioural flexibility in prey choice, and to determine data deficient dispersal and foraging locations by undertaking diet analyses, comparing dietary change to historical observations, and by tracking the movements of juvenile and adult penguins during non-breeding periods when they are at most risk of mortality. My results indicate that there has been a dietary shift favouring a higher proportion of blue cod/rāwaru (Parapercis colias), a large-bodied, fast-moving fish species that presents as a challenge for capture and ingestion by smaller yellow-eyed penguins, including juveniles and females, and for neonate chicks when partially digested blue cod is too large for feeding transfers. There was little evidence for declining diet quality over time, however, as the macronutrient content and energy density of meals were similar across the 1980s, 1990s, and 2010s, in spite of the higher proportion of blue cod and other miscellaneous prey in the diet. More research is required to determine how the energy budget of yellow-eyed penguins is affected by this dietary change, as the energy gained from ingesting a higher proportion of blue cod might not outweigh the energy expenditure required for pursuit and capture. For chicks that survive the nestling stage, post-fledging dispersal presents a significant period of mortality, potentially exacerbated by entanglement in bottom-set gillnets. Tracking of juvenile yellow-eyed penguins post-fledging revealed consistent patterns of dispersal to the Canterbury Bight, an area associated with juvenile yellow-eyed penguin gillnet bycatch. While local gillnetting prohibitions have recently increased in this area, they protect c. 28.9% of the main marine habitat of juvenile yellow-eyed penguins (95% k-LoCoH isopleth), with a 51.6% overlap of commercial gillnetting activities with the core foraging area (50% k-LoCoH isopleth). For adults foraging at pre-moult and over winter periods, sex-specific differences in foraging and diving were apparent, with females moving more slowly than males in horizontal and vertical planes, with less bottom time for prey capture than males, and performing up to 32% more dives per hour. Pre-moult and winter foraging movements remained inshore, but were much further from the coast than during the breeding season, with yellow-eyed penguins of both sexes attaining depths up to 156.2m on benthic dives. Across pre-moult and winter periods, there was individual consistency in daily benthic home ranges, with individuals repeatedly visiting the same small foraging areas, indicating that yellow-eyed penguins maintain consistent foraging grounds as a means of reliably foraging for suitable prey. However, generalised additive models indicated a concerning spatial overlap with winter foraging preferences of female yellow-eyed penguins and commercial bottom contact trawling locations, which might degrade benthic habitats, reducing the assembly and quality of prey found in these disturbed environments. Management actions to prevent the localised extinction of yellow-eyed penguins in their mainland range include appropriate resourcing of marine research and monitoring to inform the phasing out of commercial gillnetting, and improving the data deficiencies associated with anthropogenic activities in the marine environment to ensure that mitigation actions are robust, supported, and realistic, while balancing the socioeconomic impact of protecting yellow-eyed penguins from manageable threats in their marine environment.