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Prohibited immigrants: the rat threat to island conservation

Moors P.J., Atkins Iae and Sherley G.

Islands : Strangers in Paradise

Strangers in Paradise takes us on a discovery journey to identify and learn about various species living on the islands of Hawaii, Solomon Islands, Rarotonga, New Zealand, and Easter Island. In contrast, we also take a look at the impact of human development on such species. The introduction of rats and new predators had devastating effects on these, which prompted research into control measures to address this problem.

Seabird on islands : threats, case studies and action plans / Edited by D.N. Nettleship, J. Burger and M. Cochfeld.

Environmental effects of rodent Talon baiting Part I : Monitoring for toxic residues : Part II Impacts on invertebrate populations

Available Online

Morgan, D. R.

Spurr, E. B.

Wright, G. R.

Although Talon® baits containing brodifacoum have been used successfully in eradicating rats from some of New Zealands offshore islands, little is known about any environmental effects of this toxin. We sampled invertebrates, blackbirds, soil, and water at intervals of 2 days to 9 months to determine whether brodifacoum residues were present after aerial distribution of Talon® 20P cereal pellets on Red Mercury Island and after bait-station use of Talon® 50WB wax-coated cereal blocks on Coppermine Island. No brodifacoum residues were found in soil, water, or most (99%) invertebrate samples. Low to moderate residues were found in one sample of slugs collected 2 days after aerial sowing. Tissues from all birds (n=4) and rats (n=3) found dead and livers from all six birds collected alive 8 months after aerial baiting also contained low to moderate residues. These preliminary results suggest that few invertebrates are likely to be contaminated as a result of Talon® baiting. Tentatively, we suggest that although some invertebrates may eat Talon® baits, it appears that the brodifacoum is either metabolised and/or excreted within a few days. The dead blackbirds found, therefore, were more likely to have been killed by primary than by secondary poisoning. Further monitoring for brodifacoum residues after Talon® operations should be undertaken to confirm that contamination of invertebrates, soil, and water is unlikely. Some bird species may be at risk from Talon® baiting. Likely effects on population levels of such species should be considered to help assess the risk and benefits of Talon® use in rodent eradication.

Monitoring possum numbers following 1080 poison control at Mapara reserse

Stephens, Theo

1080 poisoned baits (Wanganui No. 7) were aerially spread throughout the three blocks of Mapara reserve in September 1990 and again in October 1991 to control possums. Changes in possum abundance were monitored by trapping, using a modified version of Seber's "removal method". A trapping method was chosen in preference to spotlight counts, bait take and pellet counts because a suitable control area, critical to these methods, was not available; traps were already in place on permanent sets; and trapping contributes to the control objective. The removal index was considered more suitable for monitoring than the simpler catch per unit effort index (number of possums caught per 3 trap-nights) because the latter is affected by variation in possum catchability, as well as possum abundance. The removal method depends on obtaining declining catches on successive nights. There is a significant risk that the method will not give a meaningful result if trapping intensity is inadequate or if nightly variation in catchability is excessive. The catch per unit effort index is still available if an abundance index based on the removal method is unobtainable. The 1990 poison operation reduced possum numbers to 21% of pre-poison abundance (95% Confidence Limits (CL) were 13.8% and 28.2%). During the following year, possum numbers built up to 39.5% (95% CL 28.7% and 50.3%) of pre-poison abundance and 1991 poison operation caused a non-significant reduction to 32.2% (95% CL 21.8% and 42.6%). Thus the September 1991 operation probably did not reduce possum densities to the level attained in October 1990. The reason for the poor kill following the 1991 poison drop is unknown, although several possibilities are identified.

Control of a stoat population irruption to enhance yellowhead breeding success / by ColinF.J. O'Donnell, Peter J.Dilks and Graeme P. Elliot

O'Donnell, Colin F.J.

The mohua or yellowhead (Mohoua ochrocephala) is an endangered, hole-nesting forest bird endemic to New Zealand. Mohua suffer periodic population crashes due to severe predation by the introduced stoat (Mustela erminea). In 1990, a stoat population irruption provided an opportunity to reassess the impact of stoat predation on mohua and to test two linked hypotheses: that adaquate control of stoats by trapping is possible, and that it is a viable management option to assist mohua recovery. The primary experiment (summer 1990/91) was repeated in the summers of 1991/92 and 1992/93 when stoat numbers were lower. Mohua productivity and adult female mortality were compared in two study areas, one trapped and one untrapped, in the Eglinton Valley, Fiordland. Sixty-two stoats were caught in the 50 ha trapped area during summer 1990/91. The fledging of many first clutches, and the laying of second clutches, coincided closely with the period when high numbers of stoats were being caught in traps. Eighty percent of the nests in the trapped area fledged young, compared with only 36% in the untrapped area. Pairs produced nearly twice as many young in the trapped area. A higher proportion of breeding females disappeared from the untrapped area. In the two subsequent summers, 29 and 14 stoats were caught, and breeding success was higher than previously recorded in both trapped and untrapped areas. We suggest that trapping in the year following a stoat irruption may also be warranted.

Assessment of the effectiveness of Transonic ESP and Yard Guard ultrasonic devices for repelling stoats (Mustela erminea)

Available Online

Manaaki Whenua - Landcare Research

Spurr, E. B.

The Department of Conservation, West Coast, requested Manaaki Whenua - Landcare Research, Lincoln, to test the effectiveness of two ultrasonic devices, Transonic ESP and Yard Gard, for repelling captive stoats (Mustela erminea) from a food source. If they repelled stoats, ultrasonic devices could be used to protect nesting birds such as kiwi (Apterygiformes), penguins (Sphenisciformes), and petrels (Procellariiformes) and their eggs and chicks from predation.

Questions on Brodifacoum

Available Online

Eason, CT.

Questions on brodifacoum bait and impacts on the environment after decomposition, or leakage to water supply, etc.

Susceptibilty of terrestrial amphipods, especially Tara taranaki, to possum control operations

Available Online

Duncan, K. W.

The role and nature of Tara taranaki are described. Its links to other species are considered; and the possible effects of possums on the species is discussed. It is likely that 1080 poison is highly toxic to amphipods, but no matter how poisonous it is to Tara taranaki, its use is likely to be far less damaging in the long term to this species than are the effects of an uncontrolled possum population.

Eradication of kiore double island, mercury group in Northern New Zealand / by Ian McFadden

McFadden, Ian

Kiore were eradicated on Double Island using two different methods. The larger islet was treated with one manual application of STORM 4 g wax block rat baits containing 0.005% by weight Flocoumafen, in October 1989. The smaller islet received one application of toxic kibbled maize (0.005% bromodialone by weight) after 4 nights prefeeding in November 1989. Kibbled maize was dispensed in silos. No sign of kiore has since been found during four visits to both islands. The cost per hectare for both methods were quite comparable. The nett cost for eradication on the larger islet (19 ha) was $310/ha, the smaller islet cost $432/ha. Both methods are considerably cheaper than the technique used to eradicate Norway rats (R. norvegicus) from Hawea Island. For future rodent eradication campaigns it is suggested that aerial applications of poisons may be made on all islands greater than 50 ha, and that size should be no barrier to rodent eradication. However, substantial non-target have been recorded when 1080 baits were broadcast aerially to control possum. Non-target kills of a similar magnitude may be experienced on larger and more sensitive islands.

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