White-capped albatrosses Thalassarche steadi are endemic to New Zealand, breeding on Disappointment Island, Adams Island and Auckland Island in the Auckland Island group, and Bollons Island (50-100 pairs) in the Antipodes Island Group. Population estimates suggest most (95%) of the global population breeds on Disappointment Island, an area where access is restricted to maintain environmental values at the site. Virtually all aspects of the biology and ecology of white-capped albatrosses are poorly known and although approximate population sizes have developed there have been no well-documented population estimates for any of the colonies.
Between 2006/07 and 2010/11 (hereinafter 2006 and 2010, respectively) we undertook repeated population censuses of the white-capped albatrosses breeding in the Auckland Islands using aerial photography. These population censuses were carried out in December each year to estimate population size and track population trends. We have now undertaken two additional counts at the Auckland Islands: on 11 January 2012 and 14 January 2013 (2011 and 2012 breeding seasons, respectively).
In 2011 we estimated that there were 93,752 (95%CI 93,140 — 94,364), 5,846 (5,604 — 5,999) and 178 (151— 205) annual breeding pairs at Disappointment Island, South West Cape and Adams Island, respectively, giving a total for these sites of 99,776 (99,144—100,408) breeding pairs.
In 2012 we estimated that there were 111,312 (95%CI 110,645 — 111,979), 6,571 (6,409 — 6,733) and 215 (186 — 244) annual breeding pairs at Disappointment Island, South West Cape and Adams Island, respectively, in 2012, giving a total for these sites of 118,098 (117,411 — 118,785) breeding pairs.
To assess population trend in total counts we used an appropriate Generalised Linear Model where the response was specified as an over dispersed Poisson distribution and the link was logarithmic. To allow for possible non-linear trend effects we used smoothing splines (with smoothing parameter set at 3) for the variable ‘Year’. We also assessed trend using software program TRIM (TRends and Indices for Monitoring Data), the standard tool used by the Agreement for the Conservation of Albatrosses and Petrels (ACAP).
Evidence from a series of ‘close-up’ photographs taken each year (2007-2012) indicates that the number of non-breeding birds present in the colonies differed somewhat between December and January. The proportion was very low in December counts (1-2% of birds present) to 7 and 15% for the January counts taken in 2011 and 2012, respectively. Estimated annual counts for all three breeding sites in the Auckland Islands were adjusted to account for the presence of non-breeding birds, giving adjusted total estimates of annual breeding pairs of 116 025, 90 036, 96 118, 73 838, 76 119, 92 692 and 100 501 for each year from 2006 to 2012 inclusive. These adjusted figures were used as inputs into models used for assessment of population trend.
The population size estimates computed from the TRIM model indicate an average growth rate of
- 2.19% per year (λ = 0.9781 ± 0.001); assessed by TRIM as moderate decline. We note, however, that a simple linear trend analysis, as performed by TRIM is not well suited to a data set with high inter-annual variability. Trend analysis using smoothing splines is more appropriate to such data sets, and showed no evidence for systematic monotonic decline over the 7 years of the study, therefore providing support to the null hypotheses of no trend (stability) in the total population.
In a global review of fisheries-related mortality of shy and white-capped albatrosses it was estimated that 8,000 white-capped albatrosses were killed each year as a result of interactions with trawl and longline fisheries in the Southern Ocean. This level of mortality highlights the need to continue to acquire accurate population estimates and trends for white-capped albatross populations to assess the impact of fisheries operations on this species. Although annual counts over the last seven years indicate the population is stable, ongoing population monitoring is recommended to clarify if current levels of fishing mortality are sustainable.
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