Abstract
As climate change intensifies, warming ocean temperatures, extreme weather events, and perturbations to the marine biome are becoming increasingly prevalent. A consequence of these changes are shifts in prey availability which can offset marine food webs and alter the foraging behaviours of marine predators. Seabirds are especially affected by alterations to the marine environment during the breeding season, when they are spatially constrained by the need to return to their colony and provision chicks. Thus, it is essential that their foraging habitats are productive and provide suitable conditions and prey stocks for successful foraging while minimising the energy expenditure required to locate and consume prey. To conserve seabirds in the face of significant changes to their ecosystem, it is necessary to develop a strong knowledge base of their habitat use and the degree of plasticity in their foraging behaviours.
The Fiordland crested penguin (Eudyptes pachyrhynchus), also known by the Maori name of tawaki, are distributed throughout New Zealand’s southwestern coast and southern islands. However, little is known about the marine ecology of tawaki breeding within fiords even though fiordic systems cover a significant proportion of their range. Recent research has suggested that Milford Sound is able to buffer fiord-based tawaki from the usually detrimental effects of an El Niño year and provide them with favourable foraging conditions during their breeding season. This study presents the first comparison of guard stage foraging behaviour between two tawaki colonies from different locations within Milford Sound. Through the attachment of GPS/dive data loggers, dive parameters and spatial use were examined to determine differences in the foraging strategies of the inner-fiord colony (Harrison Cove) and the outer-fiord colony (Moraine) over the years of 2019 and 2020.
Outer-fiord birds from Moraine foraged almost exclusively outside the fiord in both years while inner-fiord birds from Harrison Cove primarily foraged within the fiord in 2020 but not in 2019. Tawaki from Harrison Cove and Moraine also displayed contrasting dive behaviour across years, either adopting a strategy of deeper, longer dives with fast velocities or shallower, shorter dives with slower velocities. Foraging efficiencies for both colonies appeared to be greater in the year 2020 than 2019 although birds foraged differently to achieve this - Harrison Cove birds dived primarily to depths of 0-20m while Moraine birds switched between shallower dives at this depth and longer dives in the deeper 60-120m of the water column. Moraine birds adopted alternative foraging strategies across the years, with 2019 birds expending more energy on longer trips but diving shallowly while 2020 birds made shorter trips closer to their colony but exhibited higher foraging effort by diving deeply. Changes in dive behaviour across years were likely due to contrasting oceanographic factors affecting prey availability while differences between colonies reflect their differential foraging habitats.
Environmental models revealed that ocean-foraging tawaki engaged in higher foraging activity and made deeper dives with faster descent velocities over warmer water and their dive behaviour was also influenced by salinity, concentrations of chlorophyll-a and seafloor bathymetry. Although the Oceanic Niño Index proved too coarse of a measure to assess changes in foraging patterns across years, local conditions in the fiord such as the interactive effects of salinity, rainfall and wind speed, were found to impact the dive behaviour of fiord-foraging tawaki. Rainfall, salinity and wind speed are all factors that contribute to the thickness and degree of mixing in the low salinity layer (LSL). A thicker LSL, as indicated by low salinity in the upper fiord waters, increased foraging activity and caused tawaki to dive deeper, exhibiting how the lower light environment caused by a thicker LSL does not necessarily compromise the foraging of tawaki.
These findings highlight the considerable plasticity that tawaki possess in their foraging behaviours – they can forage in a range of environments and adapt their dive behaviour to fit the conditions. This bodes well for their future survival in an uncertain climate. Furthermore, this study emphasises the importance of not assuming that the foraging strategy of one colony is consistent with that of another nearby colony. Being situated in the inner fiord and within a marine reserve, has clearly influenced the foraging preferences of the Harrison Cove colony and it remains to be seen whether birds from the Moraine colony are capable of modifying their behaviour to make use of the fiord foraging habitat in years of harsh oceanic conditions and low productivity. The behaviour of tawaki across the rest of New Zealand’s fiord systems should be investigated in future to further understand the foraging opportunities fiords can provide and the significance of fiords in the conservation of this species.