J. P. Croxall

EFFECTS OF SATELLITE TRANSMITTERS ON ALBATROSSES AND PETRELS

Abstract Effects of deployment of miniaturized transmitters and loggers have been well studied in penguins, but much less so in flying seabirds. We examined the effects of satellite tag (platform terminal transmitter, PTT) deployment in Black-browed (Thalassarche melanophris) and Gray-headed (T. chrysostoma) albatrosses at South Georgia and reviewed the recent literature for other albatrosses and petrels. In our study, although a few individuals may have slightly extended their foraging trips, overall there was no significant difference in trip duration, meal mass, breeding success, or rate of return in the next season between birds with PTTs and controls. By comparison, most other studies of albatrosses and petrels recorded extended trip durations and, in some cases, high rates of nest desertion following PTT attachment. That occurred particularly where transmitter loads exceeded 3% of adult mass. Extended trip durations may result from reduced flight efficiency, as well as the effect of capture and temporary restraint, but affected birds seem nonetheless to commute to representative foraging areas. To minimize device effects, we suggest that transmitter loads be reduced to a minimum, use of harnesses be avoided (particularly for breeding season deployments when tape attachment to feathers is an effective alternative), and careful attention be given to limiting handling times during incubation when some species are particularly sensitive to disturbance.

Foraging behaviour of Antarctic fur seals during periods of contrasting prey abundance

1. Foraging behaviour of Antarctic fur seals rearing pups at Bird Island, South Georgia, was assessed using at-sea activity patterns measured by electronic time-depth recorders. Information was obtained for a total of 75 individuals and 191 foraging trips to sea over five reproductive seasons from 1988/89 to 1992/93; this included one season (1990/91) of low prey abundance. A method was developed to divide the diving record up into logical units or bouts which differed from past methods used for defining bouts of behaviour. 2. Foraging trips were significantly longer in 1990/91 than in the other years. There were significant differences between years in the proportion of time spent foraging when at sea and in the distribution of foraging through the day and night. These differences probably represent behavioural responses to changes in prey distribution and abundance and were reflected in the frequency of occurrence of different types of foraging behaviour. 3. Four types of foraging bout were recognized using a cluster analysis. Type I (short) bouts were of short duration (17 min) and occurred mainly during daytime and at dusk. They probably represented exploratory behaviour. Type II (long) bouts occurred mainly at night and were of long duration (80 min). They increased in frequency in 1990/91 when food was scarce and 61-73% of time spent foraging was in these bouts. Type III (shallow) bouts occurred mainly at night, were of short duration (12 min) and represented feeding close to the surface, possibly in association with other, surface-feeding krill predators. Shallow bouts accounted for 8-14% of time spent foraging. Type IV (deep) bouts were of medium duration (19 min) and represented feeding at greater depth (40-50 m) than other bout types. They were most abundant around dawn. 4. Mean dive duration during bouts exceeded the theoretical aerobic dive limit on > 30% of occasions for short, long and deep bouts. There were positive correlations between mean dive duration and surface interval duration for most of these bout types in most years. This suggested that long dives incurred a cost in terms of the amount of time spent at the surface between dives. 5. The study demonstrated that female fur seals invest a significantly greater effort in foraging during periods of low prey abundance by both increasing the time spent foraging and by increasing activity during foraging. This could represent a 30-50% increase in the costs of foraging during years of low food abundance.

Foraging Energetics of Antartic Fur Seals in Relation to Changes in Prey Availability

This research examines the energy budget of breeding female Antarctic fur seals, both when food was plentiful and when it was scarce. The energy expenditure and change in body mass of lactating female Antarctic fur seals, Arctocephalus gazella, foraging at sea was measured in two years using doubly labeled water at South Georgia Island. There was no difference between years in mass gain, water influx, mass-specific field metabolic rate (FMR), or absolute FMR. Mean at-sea FMR over both years was 9.52 ? 0.55 W/kg (n = 22), a value that is 6.7 times the predicted basal rate but only 1.9 times the FMR measured onshore. Comparable results have been reported for similar-sized northern fur seals. Krill, the nearly exclusive prey of breeding females, were very scarce in 1984 at South Georgia. Fur seal foraging trips were twice as long in 1984 as in 1985 and total mass- specific energy expended by females during these trips was significantly greater. In addition, females were significantly lighter at parturition in 1984, and both pup mortality and the proportion of pups that died from starvation were double the 1985 values. Female condition at parturition and average foraging-trip duration (i.e., offspring-pro- visioning rate) appear to reflect prey availability. The similarity between years in mass increase suggests that females do not return to feed their pups until they replenish their own reserves. Antarctic fur seal females may have a limited ability to increase the relative time spent foraging because even in normal years only 5% of their time at sea is spent resting. This contrasts with northern fur seals, Callorhinus ursinus, which typically spend 17% of their time at sea inactive. Apparently these northern seals can increase their foraging effort by increasing the proportion of time spent foraging. This would account for the observed between-year difference in at-sea FMR of C. ursinus while foraging-trip duration remained fairly constant.

SUMMER DISTRIBUTION AND MIGRATION OF NONBREEDING ALBATROSSES: INDIVIDUAL CONSISTENCIES AND IMPLICATIONS FOR CONSERVATION

Many birds show a surprising degree of intraspecific variability in migratory tendency and choice of wintering site. In this study, we tracked the seasonal movements of 35 nonbreeding Black-browed Albatrosses Thalassarche melanophrys from South Georgia, including 24 birds followed in two consecutive years. This revealed consistent patterns of status-related, sex-specific, and individual variation in wintering strategies, and provided the first description of the summer distribution of failed/deferring breeders. Individuals exhibited a striking degree of site fidelity, returning to the same region (southwest Africa or Australia) and showing correlated centers of distribution, as well as remarkable consistency in the chronology of their movements, in consecutive years. Nonetheless, a degree of behavioral flexibility remained, and particularly on the return migration, birds moved between, or bypassed, alternative intermediate staging sites depending on local circumstances. Initiation of the outward migration varied according to breeding status, timing of failure, and sex: deferring breeders and those that failed early departed two months before successful birds, and successful females departed 1–2 weeks earlier than males. Sex-related latitudinal variation in distribution was also apparent, with females wintering farther north within the Benguela system. Moreover, the only migrant to Australia was a male, supporting an apparent tendency for male-biased breeding dispersal inferred from genetic analyses. Distribution and timing of movements appeared in general to relate to avoidance of competition from congeners and conspecifics from other populations. From a conservation perspective, the study indicated that, for the declining Black-browed Albatross population at South Georgia, the primary focus should be toward improving the management (especially reducing bycatch levels) of fisheries in the central and eastern South Atlantic.

Seasonal sexual segregation in two Thalassarche albatross species: competitive exclusion, reproductive role specialization or foraging niche divergence?

Sexual segregation by micro– or macrohabitat is common in birds, and usually attributed to size–mediated dominance and exclusion of females by larger males, trophic niche divergence or reproductive role specialization. Our study of black–browed albatrosses, Thalassarche melanophrys, and grey–headed albatrosses, T. chrysostoma, revealed an exceptional degree of sexual segregation during incubation, with largely mutually exclusive core foraging ranges for each sex in both species. Spatial segregation was not apparent during brood–guard or post–guard chick rearing, when adults are constrained to feed close to colonies, providing no evidence for dominance–related competitive exclusion at the macrohabitat level. A comprehensive morphometric comparison indicated considerable species and sexual dimorphism in wing area and wing loading that corresponded, both within and between species, to broad–scale habitat preferences relating to wind strength. We suggest that seasonal sexual segregation in these two species is attributable to niche divergence mediated by differences in flight performance. Such sexual segregation may also have implications for conservation in relation to sex–specific overlap with commercial fisheries.

Satellite tracking of wandering albatrosses ( Diomedea exulans ) in the South Atlantic

The movements of two wandering albatrosses, one of each sex, breeding at South Georgia, were tracked using satellite telemetry, particularly to assess whether such birds could be at risk from longline fishing operations in the subtropics. Full details of the performance (number and quality of uplinks) of the Toyocom transmitters are provided, together with data on flight speeds and night and daytime travel by the albatrosses. The female, tracked for seventeen days—covering three foraging trips totalling 13951 km - had a much more northerly distribution than the male, which made two trips to sea during the same period and travelled a minimum distance of 9280 km. On one trip the female frequented the area off Brazil known to be used for longline fisheries. The distributional differences between the sexes support earlier suggestions, based on at-sea observations, that the observed high mortality rates of South Georgian females could be due to a greater likelihood of incidental mortality in longline fishing. These results also show that the presence of females off Brazil can include birds still rearing chicks, rather than simply representing post-breeding dispersal.

Global relationships amongst black-browed and grey-headed albatrosses: analysis of population structure using mitochondrial DNA and microsatellites.

The population structure of black‐browed (Thalassarche melanophris and T. impavida) and grey‐headed (T. chrysostoma) albatrosses was examined using both mitochondrial DNA (mtDNA) and microsatellite analyses. mtDNA sequences from 73 black‐browed and 50 grey‐headed albatrosses were obtained from five island groups in the Southern Ocean. High levels of sequence divergence were found in both taxa (0.55–7.20% in black‐browed albatrosses and 2.10–3.90% in grey‐headed albatrosses). Black‐browed albatrosses form three distinct groups: Falklands, Diego Ramirez/South Georgia/Kerguelen, and Campbell Island (T. impavida). T. melanophris from Campbell Island contain birds from each of the three groups, indicating high levels of mixture and hybridization. In contrast, grey‐headed albatrosses form one globally panmictic population. Microsatellite analyses on a larger number of samples using seven highly variable markers found similar population structure to the mtDNA analyses in both black‐browed and grey‐headed albatrosses. Differences in population structure between these two very similar and closely related species could be the result of differences in foraging and dispersal patterns. Breeding black‐browed albatrosses forage mainly over continental shelves and migrate to similar areas when not breeding. Grey‐headed albatrosses forage mainly at frontal systems, travelling widely across oceanic habitats outside the breeding season. Genetic analyses support the current classification of T. impavida as being distinct from T. melanophris, but would also suggest splitting T. melanophris into two groups: Falkland Islands, and Diego Ramirez/South Georgia/Kerguelen.

Large-Scale Fluctuations in Distribution and Abundance of Krill — A Discussion of Possible Causes

Unusually low abundance of krill may last for periods of several months in the Scotia Sea near South Georgia and in Bransfield Strait. Two longer data sets on krill predators suggest that such events may occur two or three times in a decade, and that the situation normally returns to normal in the following season. It seems most unlikely that these events can be ascribed to features of krill biology. Simple models of recruitment failure or mortality cannot explain the observed changes, and alteration in small-scale distribution is not indicated by the available data. More probable mechanisms must involve large-scale changes in distribution of krill brought about by ocean-atmosphere processes. Whilst natural variation in mesoscale features has an appropriate spatial scale, the likely duration is too short. However, a breakdown of hydrographic structure in the surface water over a large area would drastically decrease the residence time of krill and it would take a longer time to reestablish high krill biomass. A prolonged period of southwards airflow over the Scotia Sea is identified as the likely driving force in this model. Such an airflow has been identified from atmospheric pressure distribution in the winters of 1983 and 1986, and was associated with southwards displacement of both warm surface water and of pack ice in the northern Weddell Sea.

Antarctic Seabird and Seal Monitoring Studies

The need to investigate and understand the nature of changes in abundance of economically important marine living resources has been widely recognized. They may be naturally occurring fluctuations, cycles or undirectional processes, or changes in response to artificial influences such as pollution or commercial harvesting. As direct investigation of status and population structure of some of these resources can often prove difficult, or incompatible with continuing exploitation, attention has also focussed on the identification and study of more convenient species which may be used as indices of environmental change.

Marine debris surveys at Bird Island, South Georgia 1990-1995

The Antarctic marine environment has relatively few direct sources of man-made marine debris; however, there is concern over the dangers posed to wildlife by increasing amounts of such debris. Between 1990 and 1995 beached debris was monitored at Bird Island, South Georgia. This was part of a programme developed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) to monitor compliance with waste disposal under MARPOL and the effectiveness of additional regulations to reduce entanglement of marine mammals and birds. Overall, and in all but one year, the highest incidence of debris occurred during the winter months when 75% of all items were collected. The most numerous category overall (76%), and in all samples since 1991, was pieces of synthetic line as used in the long-line fishery for the Patagonian toothfish Dissostichus eleginoides around South Georgia. Packaging bands (6%) and polythene bags (6%) were the next commonest items. There was a substantial increase in the number of items found ashore in 1995 which coincided with an apparent increase in the long-line fishing effort in the area. The increase in the incidence of synthetic line found ashore corresponds to the increase in the proportion of Antarctic fur seal Arctocephalus gazella entangled in this material at South Georgia in a parallel study. An increasing use of environmentally-aware scientific observers on all fishing vessels, leading to an increased awareness of existing legislation, should result in less man-made debris entering the marine environment in the area around South Georgia.