Influence of ecosystem variability on the demography and reproductive performance of two Eudyptes penguins, Macaroni and Eastern Rockhopper Penguins, at sub-Antarctic Marion Island, 1994–2019

Dakwa, Farisayi E

Influence of ecosystem variability on the demography and reproductive performance of two Eudyptes penguins, Macaroni and Eastern Rockhopper Penguins, at sub-Antarctic Marion Island, 1994–2019

Master Thesis

2022

Abstract

Penguins are among the most important avian predators in the Southern Ocean, consuming large amounts of prey. Macaroni Penguins Eudyptes chrysolophus and Eastern Rockhopper Penguins E. filholi are listed as Vulnerable due to large-scale population declines across their distributions. The aim of my thesis is to update trends in the breeding populations of Macaroni and Eastern Rockhopper Penguins at Marion Island from 1994–2019. The breeding population of Macaroni Penguins has decreased more or less consistently at an average rate of 1.9% per year since 1994, whereas Eastern Rockhopper Penguins showed a rapid initial decline of 13% per year from 1994–2001, then stabilised to fluctuate between 55 000 and 80 000 pairs. I assess the long-term trends in pre-breeding condition (arrival dates and mass on arrival for breeding) and breeding performance (breeding success and fledgling mass) of both Macaroni and Eastern Rockhopper Penguins. I also compare within and between these variables to explain trends in the breeding population of both penguins for the last two decades. There was inter-annual variation in pre-breeding variables (arrival dates and mass on arrival) and breeding variables (breeding success and fledging mass) in both penguins which could indicate variability in prey availability and climatic conditions across the breeding seasons from 1994–2019. Over the study period, Macaroni Penguins arrived to breed 3–5 weeks earlier than Eastern Rockhopper Penguins on Marion Island. There was no consistent trend in mass on arrival among male or female Macaroni Penguins but Eastern Rockhopper Penguin mass on arrival decreased from 1994 to 2007 and then had an increasing trend post 2007 for both sexes. Breeding success of Macaroni Penguins fluctuated annually with no obvious trend, whereas the breeding success of Eastern Rockhopper Penguins increased at all three study locations. This increase in body condition and breeding success could explain the stabilising population of Eastern Rockhopper Penguins at Marion Island. Breeding variables (breeding success, fledgling mass) of both penguins were related to pre-breeding variables (arrival dates and mass at arrival). The breeding performance of both penguins is not only affected by localised prey availability and climatic conditions during breeding, but the effect might potentially be at a broader scale, before the onset of breeding. I used diet samples collected from these two species during breeding to determine interannual variation in prey composition to assess whether long-term changes in diet might explain trends in their breeding populations. The population size of the two penguins at Marion Island is more evenly matched compared to any other location where the two Eudyptes penguins breed sympatrically. They are expected to exhibit a level of segregation in their biology to allow their co-existence, from selection of breeding grounds, foraging strategies, diet and response to changing oceanographic conditions. I also tested for differences in preferred prey species. Both penguins mainly fed on euphausiids Thysanoessa vicina and Euphausia vallentini. Myctophid fishes (mainly Krefftichthys anderssoni and Protomyctophum tenisoni) were more important in the diet of Macaroni than Eastern Rockhopper Penguins. There was no long-term change in their diets that could explain their decreasing population trends at Marion Island. However, the more diverse diet of Macaroni Penguins could potentially allow them to compensate for years of low euphausiid availability compared to Eastern Rockhopper Penguins. The considerable overlap in diet between the two penguins suggests potential competition of resources between the two penguins, during breeding. Variation in breeding performance of both Macaroni and Eastern Rockhopper Penguins was related to breeding diet composition. Macaroni Penguins had higher breeding success when they fed more on myctophids, whereas Eastern Rockhopper Penguins fledged lighter chicks when they preyed on myctophids. Macaroni Penguins are more efficient at foraging on myctophids compared to Eastern Rockhopper Penguins and hence, foraging on myctophids could have opposing effects on duration of foraging, provision rate and parental care between the two penguin species. Variation in the diet composition when breeding was related to the latitudinal position of the oceanic fronts. Macaroni and Eastern Rockhopper Penguins fed on more fish Protomyctophum tenisoni when the Sub-Antarctic Front (SAF) and Antarctic Polar Front (APF) were further away from Marion Island and fed on more crustaceans Thysanoessa vicina when the fronts were closer to the island. The variable latitudinal position of the fronts is known to influence species composition and oceanographic conditions around the Island. Variation in breeding performance was not readily explained by large-scale oceanographic conditions or local scale oceanographic conditions except for sea surface height (SSH). Macaroni Penguins tended to fledge heavier chicks in years of low SSH, associated with more upwelling and mesoscale eddies, whereas Eastern Rockhopper Penguins tended to fledge heavier chicks in years of higher SSH associated with less upwelling and fewer mesoscale eddies. This indicates the difference in habitat preference and response to oceanographic conditions between the two penguin species: Macaroni Penguins forage in cooler waters compared to Eastern Rockhopper Penguins. In summary, this study reports trends in the population size, breeding performance and diet of two sympatric penguin species and how they respond to their environment. Both penguins' populations have been decreasing since 1994 but for the Eastern Rockhopper Penguin most of the decline happened before 2001 and their population has since stabilised. The two-closely related penguins manage to segregate some aspects of their ecology to facilitate co-existence on the same island while foraging in broadly the same area and feeding on the same prey. Macaroni Penguins arrive on the Island 3–5 weeks earlier than Eastern Rockhopper Penguins, which reduces competition for resources during breeding. Macaroni Penguins have a more diverse diet and feed on more fish compared to Eastern Rockhopper Penguins. The breeding performance of Macaroni Penguins responded positively to fish in their diet while Eastern Rockhopper Penguin chick condition was negatively affected by fish in their diet. The breeding biology and diet of the two penguins responded to changes in oceanographic settings around Marion Island, showing the possible effect of climate change on these two species.

Keywords

Biological Sciences

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