Baker_whole_thesis.pdf (3.11 MB)
Demography of shy and white-capped albatrosses : conservation implications
thesisposted on 2023-05-27, 10:28 authored by Baker, GB
Shy and white-capped albatrosses, Thalassarche cauta and T. steadi respectively, are closely related and phenotypically similar seabird species. Shy albatrosses breed in Australia on three islands around Tasmania, whereas white-capped albatrosses breed on islands in the Auckland and Antipodes Islands groups in New Zealand's subantarctic. Humans have impacted shy albatrosses for over a century, with at least one population devastated by feather and egg collectors in the early 1900s. Both species are also caught and killed as bycatch in fisheries across a wide range. The impact of this threat alone on these species may well be unsustainable. Here I have adopted two approaches to prepare a current conservation assessment of the both shy and white-capped albatrosses. Both approaches have been used independently in studies to assess the impacts of fisheries related mortality on other seabird species, but rarely have both been implemented simultaneously. First, I reviewed the levels of effort in fisheries known to kill both species and developed an estimate of the global bycatch level. Second, I developed and fitted population models for both species to evaluate the impact of bycatch on population growth. I also undertook annual population censuses of white-capped albatrosses at three sites in the Auckland Islands (where 99% of the population breed), from 2006 to 2013, to estimate population size and track population trends. I complemented these analyses with at-sea experiments to test the efficacy of a mitigation method known as the Smart Tuna Hook (STH). This method employs a specially designed shield that disarms the hook once it has been baited, making it difficult for any seabird to be hooked. The shield is released within 15 minutes of the hook being immersed in salt water. The experimental work was conducted on tuna longline vessels fishing out of Cape Town, South Africa, and involved a direct comparison of the Smart Tuna Hook and conventional pelagic hooks in tuna fishing operations. The analyses of global fishing effort and fisheries bycatch rates indicate that over 8 500 shy and white-capped albatrosses may be killed annually. Trawl fisheries were responsible for 75% of all estimated mortality, with longline fisheries accounting for 25%. Most birds were killed in South African, Namibian and New Zealand fisheries. As most adult shy albatrosses are comparatively sedentary and rarely found outside Australian waters, it is primarily juvenile shy albatrosses that regularly encounter fishing fleets known to kill large numbers of albatrosses. In contrast, throughout most of their range both juvenile and adult white-capped albatrosses are exposed to fisheries that collectively kill many thousands of these birds each year. The Auckland Island censuses estimated the mean number of annual breeding pairs to be 90 141, with annual estimates ranging from 73 838 to 116 025 pairs. Trend analysis using regression splines showed no clear evidence for monotonic decline, providing insufficient evidence to reject the null hypothesis of no trend in the total population. Trend analysis using Program TRIM, currently used by the Agreement on the Conservation of Albatrosses and Petrels to assess albatross population trends, indicated an average growth rate of -3.16% per year, assessed by TRIM as moderate decline. However, a simple linear trend analysis as performed by TRIM is not well suited to a data set with high inter-annual variability. I therefore concluded that the population trend is uncertain. Population models developed for both shy and white-capped albatrosses indicated that the levels of estimated global fisheries bycatch is unsustainable for both species, and particularly for white-capped albatrosses. However, as the observed population trend for both species over the last 10 years has not shown the rate of decline predicted by modelling, it is likely that the bycatch estimates for both species have been over-estimated. The Potential Biological Removal level calculated for white-capped albatross and used in current risk prioritisation is also likely to be unsustainable. Application of a PBR based on a low recovery factor (FR = 0.1 or FR = 0.2) would be appropriate for both species. While considerable progress has been made in mitigating bycatch in trawl and demersal longline fisheries, proven seabird avoidance measures in pelagic fisheries require substantial improvement. My tests of the Smart Tuna Hook showed that bycatch could be reduced by between 81.8% ‚Äö- 91.4%. Importantly, there was no detectable detrimental effect on fish catch for any commercial species. The Smart Tuna Hook provided a significant deterrent to seabirds attacking baits, and offers a feasible option for pelagic fishers to significantly reduce seabird bycatch. The bycatch of shy and white-capped albatrosses occurs over the entire range of both species and at levels that are impacting population growth. Reducing bycatch in fisheries poses significant challenges for gear technologists and fisheries managers. Finding solutions requires a mix of legislative and political measures to facilitate industry engagement and provide incentives for action, combined with sound science to define problems and provide robust assessments of the impact of bycatch at a species and population level, and to ensure development and implementation of effective mitigation measures.
Rights statementCopyright 2016 the Author Chapter 1 appears to be the equivalent of a post-print version of an article published as: Baker, G. B., Double, M. C., Gales, R., Tuck, G. N., Abbott, C. L., Ryan, P. G., Petersen, S. L., Robertson, C. J. R., Alderman, R., 2007. A global assessment of the impact of fisheries-related mortality on shy and white-capped albatrosses: conservation implications, Biological conservation, 137(3), 319‚ÄövÑvÆ333.