The Translocation and Establishment of Spotted Skink (Oligosoma Lineoocellatum) from Matiu-Somes Island to Mana Island

Abstract

Mana Island and Matiu-Somes Island are two of the nine islands in the Wellington Conservancy managed by the Department of Conservation, and are classified as Scientific Reserves. Fifty spotted skinks (Oligosoma lineoocellatum) were transferred from Matiu-Somes to Mana on 25 February 1998 as part of the ecological restoration of Mana Island. As the spotted skink is rare or absent from former mainland sites in the southern North Island, establishing another population is also important for securing the future distribution of this native lizard species in the Wellington region. The transferred population was monitored in order to record how individuals dispersed, any weight changes occurring, and if their introduction had any effects on the lizard populations present. The composition, abundance, size distribution and movements of the lizard populations on each island were described. Excluding the transferred population, the lizard species recorded in this study on Mana Island were Oligosoma nigriplantare polychroma and Hoplodactylus maculatus. On Matiu-Somes Island the lizard species recorded were O. lineoocellatum, O. n. polychroma and Cyclodina aenea. On Mana, the most abundant species captured was O. n. polychroma, while on Matiu-Somes, the most abundant species overall was O. lineoocellatum. A density estimate of 800/ha was generated for O. n. polychroma on the shore platform of Mana Island. The mean weight and snout-vent length (SVL) of O. n. polychroma was significantly less for the Mana Island population than the Matiu-Somes population. The level of tail loss was significantly greater for O. n. polychroma captured on Mana Island than on Matiu-Somes Island. At the time of transfer, the mean weight and SVL of the 30 female O. lineoocellatum transferred to Mana were significantly greater than the parent population on Matiu-Somes. However, their weights were within the range expected for their SVL. The mean weight and SVL of the 20 male O. lineoocellatum transferred to Mana Island were not significantly differently to the parent population on Matiu-Somes Island. The capture rate of O. n. polychroma did not decline on Mana following the transfer. The capture rate of all three species recorded on Matiu-Somes Island declined following the removal of the transferred population, but may have been the result of cooler weather conditions. The level of site attachment, and mean distances moved between captures did not significantly differ for any of the species on either island following the removal and transfer of O. lineoocellatum. A weak correlation was found between the total number of lizards captured on Mana Island and maximum/minimum temperatures. However, there were no correlations when the species were analysed separately. There were no significant correlations found between the weather variables and the catch rates on Matiu-Somes Island. Following the release on Mana Island, there were 32 O. lineoocellatum recaptures from 14 individuals. Two juveniles were also captured. Movements of up to 48.4 m were recorded after the skinks were released. Eight of the ten females recaptured lost between 0.5 g and 3g, while three of the four males recaptured had weight increases of between 0.5 g and 1.5 g. During a post-winter survey in November, there were three O. lineoocellatum recaptured on Mana Island. The weights of one male and one female were unchanged since their first capture on Matiu-Somes Island. The third individual (male) had lost 5.5 g. This male had recently lost a large portion of his tail, and the tips of two toes. The female recaptured in November was originally thought to be gravid because of her heavy weight and large appearance. However, as her weight was unchanged, and several individuals of similar weights were also captured on Matiu-Somes Island in November, it appears that gravid females cannot be identified with certainty by their weight and appearance alone. A study on Mana Island investigating the relative effectiveness of tin and plastic pitfall traps in terms of capture rates found that tin traps caught significantly more O. n. polychroma. There was no difference in the capture rate of H maculatus although this species is known to climb out of plastic traps. There were differences in the way the two traps were used that may have created a bias against plastic traps. A different method of lid attachment may reduce this bias. Despite the difference, this study demonstrated that plastic traps are efficient in capturing a high number of lizards. Plastic traps have several advantages over tin traps that may outweigh any possible decreases in capture rates.