Eradication of ship rats from Goat Island April-October 2005

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					                   Eradication of ship rats from Goat Island
                             April-October 2005
                  Report prepared for the Department of Conservation
                                    November 2005

                           Jamie MacKay and James Russell
                        Universities of East Anglia and Auckland


Goat Island (Hawere) is a 9.3ha island off the town of Leigh, north of Auckland, New
Zealand. The island is situated in the middle of the Goat Island marine reserve and
public access is discouraged although swimmers and snorkellers frequently use the large
flat rock platform on the south-western side of the island. At low tide the channel
between the island and the mainland is only 50m across, well within the swimming
range of ship rats and stoats. Rats have been present on the is land since at least 1970
(Esler, 1975), probably longer. Craig (1977) recorded densities of around 12-20 rats/ha
(Table 5) and reports that there were ‘a lot’ of rats (J. Craig, pers. comm.). In 1994 rats
were eradicated from the island by Department of Conservation staff but were re-
detected in 1996 (Russell & Clout in press). Students from Auckland University
undertook ecological surveys of the island in 1999 and 2000 and in both years rats were
caught on the island leading to the conclusion that the population was large and
reproductively viable (Ussher, 1999 & 2000). The vegetation of the island was
described by Esler in 1975 and in 1996 an Auckland Botanical Society group visited the
island and provided a creative report on their day's activities (Benham, 1996).

Grey- faced petrels breed in burrows on the southern side of the island and little blue
penguins nest across the island. 42 burrows (from approximately 50) were blocked with
small twigs on the night of April 31st 2005 to estimate burrow activity. The following
morning 35 burrows (~80%) had been visited, either by penguins or petrels. A visiting
seabird ecologist estimated around 50 pairs of petrels using the island based on the
number circling the island at dusk that night (B. Keitt, pers. comm.).

The island has been chosen as a study site in a wider investigation into the invasion
ecology of rats on islands. Before this investigation could begin it was necessary to
eradicate rats from the island and so trapping and poisoning was initiated in late April
2005 as part of a University of East Anglia three month MSc project (MacKay, 2005).
The eradication campaign coincided with the petrel breeding season (June-September)
so this should result in increased breeding success for the birds in 2005.

Motu Haweri (Goat Island) was occupied by Ngaitahuhu following the landfall by the
Whakatuwhenua canoe, captained by Tahuhunuiarangi. He was a great grandson of the
first Manaia and claimed dominion of the district from Te Arai to Tawharanui, prior to
moving to Otahuhu. A small midden is visible on the south-eastern cliff-top of the
island directly adjacent to the marine laboratory.



Goat Island was trapped on two separate occasions using an approximately 50m by 50m
trapping grid. The southern half of the island was trapped in Phase I between 26th April
2005 and 1st May 2005, before the rest of the grid was established on 19th June 2005
with Phase II traps set between 20th June 2005 and 24th June 2005. Victor Professional
traps were used and covered with either chicken wire or stiff plastic covers and pinned
to prevent traps being removed. All rats caught were weighed, sexed, assigned as adult
or juvenile depending on weight and sexual characteristics, and measured for head-body
length (HBL) and tail length (TL) (sensu Cunningham & Moors, 1996). The stomachs
were removed from all rats caught and preserved in 70% ethanol for diet analysis.

Diet analysis and condition index

Stomach contents were examined at the Leigh Marine Laboratory. The stomachs were
opened and their contents washed into a 250µm mesh sieve with a 500µm mesh sieve
beneath. A jet of water was then used to rinse the stomach contents to remove any finely
chewed matter. The contents of the 250µm sieve were then examined under a dissecting
microscope and the contents assigned to groups and classified according to volume
(trace (<10%); medium (10-50%), large (>50%); sensu Moors, 1985). The 500µm only
contained small fragments of the same contents found in the larger sieve. All nematodes
present were collected and counted and any easily recognisable items (e.g. insect pupae)
were collected and counted. Stomach contents were divided into inland and coastal
areas depending on where the rat was caught. A body condition index (Moors, 1985)
was calculated for all rats caught using the formula:

                     Condition =     W   x 105

Poisoning and monitoring

Towards the end of Phase II (June 23rd 2005) 49 poison bait stations were established
across the island. At each location five 50g blue chocolate- lured brodifacoum Pestoff
rodent blocks (Animal Control Products, Whanganui) were wired to trees under a
plastic corflute cover (Day 0). These were checked the following day (Day 1). Pellets of
green brodifacoum Pestoff 20R pellets (Animal Control Products, Whanganui) were
spread on cliffs and other inaccessible areas at approximately 10kg/ha on June 28th 2005
to ensure complete coverage of the island. Bait-take was again assessed (Day 5). All
methods followed the consent application by Towns (2005). 47 traps were left out in
order to catch any remaining rats. On July 9th 2005 (Day 16) bait-take was assessed and
10 rodent motel-style ‘Protecta’ bait stations were placed across the island baited with
six 50g rodent blocks and a handful of sawdust and 20R pellets. On September 15th
2005 the island was checked by the DOC predator control programme rodent dog (Fin
Buchanan and Jak) and all motels except one were removed along with dissolved poison
stations (approximately 12 weeks). 15 tracking tunnels and 15 waxtags were left on the
island. These were checked on October 5th 2005.

Mainland trapping

Trapping on the mainland adjacent to the island was carried out simultaneously with
island trapping as part of a wider genetics project looking at rodent migration between
sites and across barriers and to provide a buffer to prevent rapid reinvasion of the island.
Traps were placed in the forested stream valley east of the marine reserve access road
and along the forested coastal path east of the marine laboratory, both adjacent to the
coast and likely source populations for invasion of Goat Island.


Tracking cards and waxtags deployed the night before Phase 1 trapping began showed
low levels of interference and therefore suggested a small rat population.

Trapping results

A total of 20 rats were caught in Phase I and eight in Phase II (Table 1). One fur ther
female rat was caught after the end of Phase II trapping and was found on the first
monitoring visit on July 9th 2005. The rat had been dead for around 7 days and was too
decomposed to determine whether it had ingested poison or not. Phase I trapped the
southern two-thirds of the island to zero density. When traps were laid out across the
island for Phase II six rats were caught in areas that had not previously been trapped and
two were caught in coastal areas on the east of the island already covered by Phase I.
Catch success (captures per 100 corrected trap nights) declined through each trapping
period as areas became trapped out (Table 1). Figure 1 shows the locations where rats
were caught during the eradication.

                       Date           Traps         Rats caught TNC BG Rats per 100 CTN
                   26/04/2005            36               6         1      0            18.5
                   27/04/2005            36               4         3      0            12.3
                Phase I

                   28/04/2005            51               6         1      1            12.6
                   29/04/2005            51               3         1      0            6.1
                   30/04/2005            51               1         1      0            2.0
                   01/05/2005            51               0         1      0            0.0
                      Overall           n/a              20         8      1            7.6
                   20/06/2005            35               6         0      0            18.8
                Phase II

                   23/06/2005            33               1         3      1            3.2
                   24/06/2005            45               1         1      0            2.2
                      Overall           n/a               8         4      1            7.8
                       Total            n/a             28         12      2            7.5
  Table 1: Summary of daily trap status and daily trapping success per 100 corrected trap nights (CTN)

The overall catch success of 7.5 rats per 100 CTN is low compared t the values
recorded in other studies, both on Goat Island and elsewhere. This is largely due to the
apparently small population of rats present on the island. Two nights trapping on Goat
Island in May 1977 had an overall catch success of 20 rats per 100 CTN (Craig, 1977).
Surveys by Auckland University students in June 1999 and 2000 had overall trapping
successes of 65 and 40 rats per 100 CTN respectively (Ussher, 1999; 2000), though
using very few traps. Studies of rat populations on two other recently reinvaded islands
(Motutapere and Tawhitinui) found catch successes of 18.5 and 34.5 rats per 100 CTN
respectively (MacKay & Russell, 2005; Russell & MacKay, 2005). Miller & Miller

(1995) recorded a maximum trapping success of 12 rats per 100 CTN on Rangitoto
Island and Harper et al. (2005) recorded a success of 3.26 per 100 CTN. The three
earlier Goat Island studies were only run for two nights yet they indicate a much higher
rat population than this study found at the same time of year five years later. It is
unknown why this might be the case, but anecdotal rumours of recent poisoning on the
island were mentioned, but never substantiated.

                                 Figure 1: Locations of rat captures

Population demographics

                         Sex      Age        Phase 1      Phase 2      Total
                         M       Adult          6            0            6
                                Juvenile        4            2            6
                         F       Adult          5            5           10
                                Juvenile        5            1            6
    Table 2: Numbers of adult and juvenile rats caught in each phase of trapping, classified by sex

In Phase I approximately equal numbers of adult and juvenile males and females were
caught. In Phase II no adult male s were caught, possibly because adult males range
more widely than females and came into contact with Phase I traps more often than
females. It was not possible to classify all rats as adults or juveniles simply based on
sexual characteristics so an arbitrary weight of 130g was used to distinguish between
adult and juvenile rats. Three pregnant female rats were caught during Phase I, no other
pregnant or lactating females were caught.

Morphometric measurements

Table 3 summarises the morphometric measurements of all rats caught on Goat Island
in Phases I and II. Both adults and juvenile rats are included and the range of values
falls within those previously described for ship rats (Innes, 2005).

                            Weight (g)              HBL (mm)                 TL (mm)
                        Mean        Range       Mean        Range       Mean        Range
             Male        128        40-233       163       119-206       191       143-238
            Female        123       47-124       160       124-178       192        141-234
               Table 3: Summary of morphometric measurements of all rats caught by sex
 (because all animals were captured except one, error on mean estimates is effectively reduced to zero)

Colour morphs

Ship rats in New Zealand are found in three different colour morphs (Innes, 1990). Rats
of the ‘frugivorus’ (brown upper body, white lower) and ‘alexandrinus’ (brown upper,
grey lower) colour morphs were caught during this study. No ‘rattus’ (black upper, grey
lower) individuals were found. Around 70% of rats were ‘frugivo rous’ morph, the rest
were ‘alexandrinus’. Craig (1977) recorded 90% ‘alexandrinus ’ and 10% ‘frugivorus’.
The same colour morphs were caught by Ussher (1999; 2000) but the numbers of each
morph were not recorded. Exact distinction between these two morphs can be difficult
and subjective. Eleven out of the 29 rats caught on the island had distinct white tail tips
(~5mm). Ten of these were ‘frugivorus’, one was ‘alexandrinus’. White tail tips have
not been recorded on the island before and are likely to be a result of a founder effect or
some level of inbreeding, which subsequent genetic studies may reveal. Only three rats
out of 60 caught on the mainland had these white tail tips. 5% of “alexandrinus” morph
rats caught on Stewart Island had white blazes under their chin which may be due to
similar inbreeding or founder effects (G. Harper, pers. comm.). The change in ratio of
colour morphs on the island f om Craig (1977) is probably a direct consequence of
founder effect from the reinvaders in 1996. Colour morph is apparently a complex
genetically inherited trait.

Stomach contents

Fruit and seeds dominated the stomach contents of rats from the island (Table 4, Fig. 2)
but a large number of stomachs were almost completely empty. These individuals could
have been caught at the beginning of the night before they had a chance to forage or
they may indicate food shortages on the island. All seed material found in the stomachs
had been well chewed and no whole seeds were found. Figure 2 illustrates the
proportion of rats in each habitat and the proportion of males and females with each
item in their stomach. There were no differences found in stomach contents between
male and female rats. One rat had digested meat in its stomach and another had a single
feather but no other trace of animal or bird material was found. Insect remains were
found in 46% of stomachs but only one stomach had a large (>50%) volume of insect

                                   Trace                  Medium               Large
                             Coastal     Inland      Coastal    Inland   Coastal     Inland
              Seeds             3           0           7          8        3           0
               Fruit            1           1           4          3        2           4
            Nematodes           5           5           1          0        4           0
              Insect            2           2           3          4        1           0
               Hair             3           2           5          1        0           0
           Unidentified         0           0           0          1        3           0
            Vertebrate          2           0           0          0        0           0
               Plant            0           0           0          1        0           0
 Table 4: Items found in rat stomachs classified by volume (Trace <10%; Medium 10-50%; large >50%)

The lack of invertebrate material found in stomach contents is unusual compared to
other studies. In almost all other studies carried out at similar times of year ship rats
living in native forests have diets dominated by insects (Innes, 1979; Miller & Miller,
1995; Innes, 2001; Innes et al., 2005). A study in a pine plantation in winter found that
insects dominated the die t in this season as well (Clout, 1980) but in native forest rats
tend to eat more plant food in winter (Innes, 2005). Invertebrates were not sampled as
part of this study but turning over logs and raking through leaf litter at various times
turned up very low numbers of insects (pers. obs.). Pan-trapping studies carried out by
Auckland University in 1999 and 2000 found invertebrate communities dominated by
amphipods with low numbers of other invertebrates (Ussher, 1999; 2000). The island is
being studied as part of a large research project into the interactions between rats and
seabirds on New Zealand islands (RASP, Landcare Research). Preliminary data
suggests that the island has the lowest soil fertility and insect diversity out of 20 islands
being studied (D. Towns, pers. comm.).

58% of rat stomachs that were examined contained nematodes (Fig. 2) and a higher
proportion of rats in inland areas had nematode infections than those in coastal areas
(Fig. 2). However, individual rats in coastal areas had higher parasite loads with a
quarter of infected coastal rats having “large” quantities of nematodes (>50%) (Table
4). Some inland rats had stomachs containing up to 90% nematodes. Three female rats
had diseased livers; a biopsy carried out at Massey University revealed this was most
likely due to parasite migration. All these females were caught on the coast and all had
nematode infections in their stomach.

Two different worm morphologies were noted but identification to species level has not
been done. The two main nematode species reported from rat stomachs in New Zealand
are Physaloptera getula and Mastophorus muris (Charleston & Innes, 1980, Innes,
2005). Reported levels of infection include 24% in a Pinus radiata plantation (Clout,
1980); 59% on Rangitoto Island (Miller & Miller, 1995) and 67% in some North Island
bush areas (Charleston & Innes, 1980). The intermediate hosts for these nematode
species in New Zealand are not known but weta are a possible intermediate host for P.
getula (Charleston & Innes, 1980).

Figure 2: Comparing frequency of items found in rat stomachs. a) comparison between Coastal (open
bars) and Inland (closed bars). b) comparison between Females (open bars) and Males (closed bars). 1 =
seeds; 2 = fruit; 3 = nematodes; 4 = insect; 5 = hair; 6 = unidentified; 7 = vertebrate; 8 = plant.

Condition index

There was no change in the average condition index between rats caught in Phase I and
those caught in Phase II (Fig. 3) although some individual rats caught in Phase II were
in much better condition than those caught in Phase I. Overall there was a lot of
variation in body condition but no trends were found between adults and juveniles or
males and females. Nematode infections probably have some effect on rat fitness but
did not appear to have an effect on the rats body cond ition (Fig. 3).


 Condition Index




                                             I                                         II

                          Figure 3: condition index values for all rats caught in Phases I and II

Poisoning and monitoring

Following the trapping of 29 rats the only poison-take was of two and a half 50g blocks
from halfway along the summit track (a lethal dose), in the middle of the most
intensively trapped area of the island (Phase I & II). Although rain dissolved some of
the bait, interference is still clearly discernible after over two weeks. After 12 weeks no
further bait-take was visible amongst the dissolved baits. It seems most likely that a rat
survived the trapping campaign and was subsequently killed by a change- in-methods to
poisoning. Overall it was surprising to find the majority of rats (29/30) could be
trapped, as in other trapping programmes (e.g. Russell & Abdelkrim, unpubl.) it is often
difficult to catch most of the population due to variation in individual neophobia. This
suggests ship rats may be eradicated on small islands by trapping alone given enough
effort, tho ugh it probably helped that the island habitat was of such low quality so as not
to present alternative food sources. Nonetheless it is strange that only one rat might so
stubbornly avoid trapping. Rodent dog monitoring found ‘heavy’ sign around the flat
area west of the summit (amongst RASP Landcare plots) which was never resolved.
This has retrospectively been attributed to the possible last survivor eating poison and
dying a week later, and its decaying corpse leaving sign two months later (although this
is hardly a likely scenario).

On October 27th 2005 rat gnaw was found on the waxtag on the rocky-platform
peninsula, considered the most likely location for rodent reinvasion. This occurred
around two weeks after all devices were checked on the island and no rat sign was
found. Subsequent trapping and detection efforts revealed no further sign. It is possible
a reinvader has arrived after only three months and may be detected in ongoing summer
trapping, or the invader may have returned to the mainland as the distance is well within
the swimming distance of ship rats (Russell & Clout, 2005).

Density estimate

29 rats were caught in the trapping programme and one suspected further rat may have
died after consuming two and a half 50g poison blocks. This suggests there were a total
of 30 rats living on a 9ha island; a density of 3.3 rats ha -1 . This value is generally similar
to mainland populations, but low compared to other islands, including previous
estimates from Goat Island (Table 5).

Location                                   Season                   Density         Source
Orongorongo Valley, nr. Wellington         Mean over 29 months      1.7             Daniel, 1972

Rotoehu Forest, Bay of Plenty              Summer                   6.2             Hooker & Innes,
Puketi Forest, Northland                   Spring                   2.9             Dowding & Murphy,
Kaharoa Forest, Bay of Plenty              Summer (January)         4.8             Brown et al., 1996

Haulashore Island, Nelson                  Not recorded             25-50           R.H. Taylor, unpubl.

Motutapere Island, Coromandel              Summer (April)           4.7             MacKay, 2005

Tawhitinui Island, Marlborough Sounds      Autumn (May)             2.6             MacKay, 2005

Goat Island, Hauraki Gulf                  Autumn (May)             12-20           Craig, 1977

Shiant Islands, Outer Hebrides, Scotland   Spring (May)             12              McDonald et al.,
Ponui Island, Hauraki Gulf                 Summer (December)        6               Shapiro, 2005
Ponui Island, Hauraki Gulf                 Summer (February)        10              Shapiro, 2005
Halfmoon Bay, Stewart Island               Winter (August-          2.0-2.5         Hickson et al., 1986
           Table 5: Ship rat density estimates from other studies on mainland and island sites

Mainland trapping

21 rats were caught in the valley adjacent to the access road, and 39 in the forest along
the coastal walkway. These along with the island population will be used to study gene
flow across barriers such as pasture, the marine laboratory (where poisoning is regularly
undertaken) and across the water gap to the island.


Two blackbirds were caught in traps on the island during Phase II. Two further
blackbirds, two hedgehogs, 28 mice and a weasel were also caught in mainland traps.
Blackbirds were sent to the University of Auckland for genetic analysis. The weasel
was lodged at Auckland Museum. Various specimens of ship rats from the island and
mainland, with and without white-tail tips have been lodged at Te Papa, Auckland
Museum and the University of Auckland.


Ship rats were found across the whole of Goat Island in all the different habitats on the
island (Fig. 1). Unusually, they were only found at very low densities and the
population was almost completely eradicated by snap trapping. Data from the Rats and
Seabirds Project (D. Towns, pers. comm.), stomach content analysis and personal
observations of invertebrates suggest that food on the island is scarce and the trapping
programme coincided with the period when naturally-available food is harder to find
making rats more susceptible to trapping. This suggests that trapping could be a suitable
method for rat eradication programmes on small islands with low productivity and
seasonal food availability, therefore avoiding some of the public resistance and possible
ecological effects associated with poisoning, howeve r trapping requires many more man
hours than a poisoning programme. High parasite loads on the island combined with
scarce food are likely to contribute to the low population numbers.

The proximity of Goat Island to the mainland means that reinvasion is highly likely. Rat
sign was detected on the island less than three months after the end of the poisoning
programme. The timing of the rodent detection dogs suggest this rat was not a survivor
but at the time of writing it is unclear whether the rat remains on the island. The risk of
reinvasion could be reduced by increasing the poisoning regime on mainland areas
adjacent to the island and by maintaining and regularly monitoring motel-style bait
station on the island itself, which additionally provide shelter to arriving rodents. The
long-term efficacy of these methods is questionable however (Russell & MacKay,
2005). Both these steps are relatively easily achieved as most of the land adjacent to the
island is owned by DOC or the University of Auckland. Monitoring and bait
replacement shouldn’t be a problem due to the ease of access to the island. Some large
woolly nightshades in canopy gaps were removed. The position of Goat Island in the
centre of New Zealand’s first marine reserve should make it a flagship island for the use
of inshore islands for conservation.


Thanks to the volunteers Laura Bullas, Julia Latham, Jane Dudley, Tadashi Fukami, Fin
Buchanan (& Jak) & the Moko team (Jared, Josh, Jack). Also to Thelma Wilson (DOC
Warkworth), Dave Towns (DOC Auckland Conservancy), Leigh Marine Laboratory
and Jake Tahitahi (Ngati Manuhiri). DOC research permit No. AK/14285/RES.


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Appendix 1 – bird list

Tui                                       Kaka
Kereru                                    Fantail
Silver eye                                Grey warbler
Morepork                                  Kingfisher
Welcome swallow                           Goldfinch
Blackbird                                 Starling
Spur-winged plover                        Eastern rosella (group)

Paradise shelduck (pair)
Pied shag                                 Reef heron
Little blue penguin                       Red-billed gull
South Island pied oystercatcher (pair)    Variable oystercatcher
Gannet (coast)                            White- fronted tern (coast)
Little shearwater (dead on island)        Grey- faced petrel (~100)

Colony of 50 black-backed gulls breeding on Northern points (September)


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Description: Eradication of ship rats from Goat Island April-October 2005