Abstract
INVESTIGATION OVERVIEW:
Many mainland breeding colonies of Sooty Shearwaters are thought to have become extinct, although there is no concern for the abundant offshore island populations of these burrow-nesting seabirds. This preliminary study set out to survey some of the remaining mainland and near-shore islet colonies in the Otago region, standardise methodologies and begin gathering baseline demographic information for a long-term monitoring programme on Sooty Shearwaters. The long-term programme aims to monitor: the status of mainland colonies of Sooty Shearwaters, the impacts from threats on colonies, and marine food-chain fluctuations. Research into the sustainable harvesting of muttonbirds (Sooty Shearwater chicks) on offshore islands, and the harvesting potential of mainland colonies, will also be part of the long-term programme.[…]
RESULTS AND CONCLUSIONS:
1) It appears the number of mainland colonies has declined. However, earlier measures for burrow occupancy at colonies are inaccurate so there is no evidence that burrow occupancy or productivity at remaining colonies is declining.
2) A number of problems were encountered when determining burrow occupancy as many burrows were long (>2 m). The best method for accurately estimating burrow occupancy was the fibre-optic "burrowscope".
3) Eighty-five Sooty Shearwater fledglings were washed up during one high-tide on Dunedin beaches in early May 1993 (this is called a "beach-wreck"). All were under-sized compared with chicks banded at the end of the 1991/92 breeding season, and had little subcutaneous fat.
4) Along with predation, starvation may have been a key factor contributing to fledgling mortality in 1992/93, evidenced from the undersized beach-wreck fledglings and the poor muttonbirding season (the chicks were unusually small).
5) Discriminant function analysis on morphometric data from beach-wreck fledglings generated formulae which correctly sexed up to 83% of individuals.
6) Fledging success at three of the Nugget Point colonies ranged from O - 41 % and was mainly affected by predation from introduced mammals (eg. cats, ferrets). Chick predation ranged from 43 - 69%.
7) At Taiaroa Head and Tuhawaiki Island, fledging success ranged from 64- 100%. Low predator abundance may have been an important contributing factor in the high fledging success at these colonies .
8) One colony at Nugget Point suffered high adult mortality in November and December during the nest preparation and egg laying period. All mortality was attributable to predation, possibly by ferrets.
9) The mainland colonies studied were relatively small ( < 60 occupied burrows) with the exception of Taiaroa Head (Private) (> 236 occupied burrows).
10) The PV A modelling showed that:
• if predation levels measured in 1992/93 are typical, mainland populations are declining.
• MVP size with no predation was 44 occupied burrows . With predator control (modelled at removal of 80% of predators) the MVP was 101 occupied burrows.
• predator control could make the difference for the survival of small mainland colonies.
• predation of adults had a larger impact on population growth rates than chick predation, therefore predator control should concentrate on protecting adults.
• until long-term predator control has allowed the size of populations to increase, sustainable harvesting of mainland colonies is unlikely.
• mortality in the first year, "marine failure catastrophes", adult predation, and age at first breeding, were key parameters affecting population growth rates.
• MVPs were greatly affected by varying immigration rates.
11) As the preliminary PV A model was based on fragmentary Sooty Shearwater data and information on congeneric species, conclusions should be treated with caution until more reliable reproductive and survival parameters are measured in the longterm study. […]
[Extract from Executive Summary]