Foraging ecology of shy albatrosses Thalassarche cauta breeding in Australia : implications for interactions with longline fisheries
Creators: name: Hedd, A
Publisher: name: University Of Tasmania
Year: 2023

description: Globally, bycatch associated with longline fisheries poses the most serious threat to albatross populations. Almost half of the populations for which adequate data exist are either currently, or in the recent past they have been in decline, and there is widespread acknowledgment that bycatch in longline fisheries is responsible for these declines. Within the Australian Fishing Zone (AFZ), large numbers of seabirds have been killed in both the Japanese and Australian domestic pelagic longline fisheries targeting Southern bluefin tuna Thunnus maccoyii. Attracted to the vessels by baits and offal discharge, seabirds, mainly albatrosses and petrels, ingest baited hooks, become entangled and subsequently drown. Conservative estimates of seabird mortality for the Japanese portion of the fishery indicated that at least 1,000-3,500 birds were killed annually within the AFZ. The Australian domestic longline fishery also has a serious seabird by-catch problem, but the overall mortality rates have yet to be quantified. Shy albatrosses constitute a significant component of the seabird by-catch in longline fisheries within the AFZ, where an estimated 5,000 individuals have been killed in the Japanese portion of the fishery in the past decade. Shy albatrosses are also the most frequently caught species in the Australian domestic longline fishery. Shy albatrosses Thalassarche cauta are endemic to Australia, and they breed in three colonies off the coast of Tasmania; Albatross Island to the north, and Pedra Branca and Mewstone off the south coast. The breeding population size approximates 12,300 pairs annually. Despite the prevalence of this species as bycatch in longline fisheries, their ecology was poorly known. This study, which has examined integrated aspects of both the breeding biology on land and the foraging behaviour at sea, was undertaken to redress the lack of ecological information for the species, and to quantify their degree of overlap with longline fisheries operating within the AFZ. The foraging ecology was investigated in three main ways; using satellite telemetry to identify the foraging zones at sea; using archival recorders to determine both the diving behaviour and at-sea activity patterns; and by collecting diet samples throughout the chickrearing period. Parental breeding and provisioning strategies were also investigated using a combination of VHF telemetry and artificial nests which continually recorded chick mass. Patterns of attendance by adults at the colony were also quantified during the breeding and non-breeding periods. All aspects of the foraging ecology and parental provisioning strategies were examined across three breeding seasons to quantify the magnitude of interannual variation. Finally, using the full suite of satellite tracking data, an assessment was made of degree of overlap between the three Shy albatross populations and longline fisheries operating within the AFZ. Adult Shy albatrosses from all breeding sites in Tasmania foraged locally during the breeding season, being distributed at-sea within 200-300 km of their colonies. Foraging occurred exclusively in neritic waters over the southeast Australian continental shelf, and, at the population level, the foraging locations were highly consistent between years. Despite the spatial proximity of the three colonies, the birds used mutually exclusive foraging zones during the breeding season. While the birds were active at sea during both the day and night, much , of their travelling 80%) was undertaken during daylight hours. Diving formed a significant component of the foraging repertoire of Shy albatrosses. With birds diving to depths of almost 8 m, they substantially exceeded their designation as surface-feeders. Diving occurred exclusively during daylight hours, and as the deepest depths were attained at or near midday, prey capture appeared to be largely visually cued. The diet of Shy albatrosses was dominated by fish, and to a lesser extent by cephalopods, with tunicates and crustaceans also taken. Jack mackerel Trachurus declivis, Redbait Emmelichthys nitidus, and Gould's squid Nototodarus gouldi were the three most important prey species in all sampling years. These species are common in the shelf region of Tasmania, where they are known to surface school during the day. Information on the behaviour of the birds at sea, coupled with behavioural information on the main prey species combine to indicate that Shy albatrosses have largely predatory foraging habits. Given that the main prey species occur in surface waters during the day, Shy albatrosses could obtain the majority of their food live from surface waters. Almost year-round attendance at the colony by adults from Albatross Island indicated that the birds likely remained within southeast Australian waters year-round. Unlike any other albatross species studied, Shy albatrosses spend a significant portion of their non-breeding period attending the colony, suggesting that prey remain locally available throughout the year. The provisioning pattern of adults was consistent with short foraging trip durations and neritic foraging habits. Chicks were provisioned at 400 g/day from the end of the brooding period until fledging, and they were fed at the highest frequency recorded for any albatross species. The flexibility offered by locally abundant prey resources was exemplified during the chick-rearing period. Parents were able to increase provisioning rates when their chicks were hungry, and even faced with nest abandonment by their mates, some single parents were able to successfully raise their chick. The segregated at-sea distribution of the three Shy albatross populations results in different likely impacts from fisheries. Given the distribution of effort in longline fisheries within the AFZ and the bird's consistent foraging locations between years, it is likely that birds breeding at Pedra Branca and Mewstone suffer substantially from overlap with longline fisheries. However, seemingly during both the breeding and non-breeding periods adult birds from Albatross Island remain relative remote from fishing operations. The unknown status of the populations at Pedra Branca and Mewstone, along with the little known distribution of juveniles, is therefore of considerable concern. Future research on Shy albatrosses must endeavor to identify the foraging grounds of non-breeding birds (both adults and juveniles), and determine the demographic parameters and status for the three populations. This study has examined a number of aspects of the foraging ecology of the Shy albatrosses, elevating the species from one of the least known (Croxall 1998) to one of the better understood. It is clear that only armed with a through understanding of albatross ecology coupled with widespread adoption of - appropriate and effective mitigation measures, that we can begin securing a future for albatrosses and succeed in ensuring their long-term survival.