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Summary1. Several albatross species, including the wandering albatross Diomedea exulans , have shown marked declines in abundance throughout their range. These seabirds are frequently taken as by-catch in longline fisheries and this mortality has been implicated in the population declines. 2. We developed a deterministic, density-dependent, age-structured model for assessing the effects of longlining on wandering albatross populations. We used demographic data from field studies at South Georgia and the Crozet Islands, data on albatross abundance from 1960 to 1995, and reported effort data from the tuna longline fisheries south of 30 ° S, to model estimated by-catch levels and other population parameters in the model. 3. The model used two alternative assumptions about patterns of at-sea distribution of wandering albatross (uniform between 30 ° S-60 ° S; proportional to the distribution of longline fishing effort between these latitudes). 4. Our model was able to predict reasonably closely the observed data from the Crozet Islands wandering albatross population, but the fit to the South Georgia population was substantially poorer. This probably reflects: (i) greater overlap in the Indian Ocean than in the Atlantic Ocean between the main areas of tuna longline fishing and the foraging ranges of wandering albatrosses from the Crozet Islands and South Georgia, respectively; and (ii) greater impact of poorly documented longline fisheries, especially the tuna fisheries in the south Atlantic and the Patagonian toothfish Dissostichus eleginoides fishery, within the foraging range of wandering albatrosses from South Georgia. 5. The model results suggest that the marked decline in both populations, and subsequent recovery of the Crozet Islands population (but not the continued decline of the South Georgia population), can be explained by the tuna longline by-catch. They further indicate that populations may be able to sustain some level of incidental take. However, the likely under-reporting of fishing effort (especially in non-tuna longline fisheries) and the delicate balance between a sustainable and unsustainable level of by-catch for these long-lived populations suggest great caution in any application of such findings.

1. Incidental mortality (bycatch) in fisheries remains the greatest threat to many large marine vertebrates and is a major barrier to fisheries sustainability. Robust assessments of bycatch risk are crucial for informing effective mitigation strategies, but are hampered by missing information on the distributions of key life-history stages (adult breeders and non-breeders, immatures and juveniles).2. Using a comprehensive biologging dataset (1,692 tracks, 788 individuals) spanning all major life-history stages, we assessed spatial overlap of four threatened seabird populations from South Georgia, with longline and trawl fisheries in the Southern Ocean. We generated monthly population-level distributions, weighting each lifehistory stage according to population age structure based on demographic models. Specifically, we determined where and when birds were at greatest potential bycatch risk, and from which fleets.3. Overlap with both pelagic and demersal longline fisheries was highest for blackbrowed albatrosses, then white-chinned petrels, wandering and grey-headed albatrosses, whereas overlap with trawl fisheries was highest for white-chinned petrels.4. Hotspots of fisheries overlap occurred in all major ocean basins, but particularly the south-east and south-west Atlantic Ocean (longline and trawl) and south-west Indian Ocean (pelagic longline). Overlap was greatest with pelagic longline fleets in May-September, when fishing effort south of 25°S is highest, and with demersal and trawl fisheries in January-June. Overlap scores were dominated by particular fleets: pelagic longline-Japan, Taiwan; demersal longline and trawl-Argentina, Namibia, Falklands, South Africa; demersal longline-Convention for Conservation of Antarctic Marine Living Resources (CCAMLR) waters, Chile, New Zealand. Synthesis and applications.We provide a framework for calculating appropriately weighted population-level distributions from biologging data, which we | 1883Journal of Applied Ecology CLAY et AL.

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