Previous LP Capture Data

In most years since 1996 groups of undergraduate or postgraduate students have counted LP at Tiritiri Matangi. The counts undertaken were conducted over the same area of beach, although not usually the full 0.68km. The methodology employed was not mark-recapture but a simple visual count. Results are summarised in Table 6.3.

Table 6.3: Previous LP count data by students on Tiritiri Matangi Island.

Date

Location

No.

Students

Nights Count

April '96 Hobbs - wharf to cliff <12 2 39

April '97 Hobbs - wharf to cliff <10 3 118

April '98 Hobbs - beach only <12 3 47

April '99 Hobbs - wharf to cliff <10 1 56

March '00 Hobbs - wharf to cliff <10 3 53

Sept '02 Hobbs - wharf to cliff <12 2 42

Sept '03 Hobbs - wharf to cliff <12 2 20

June '08 Hobbs - wharf to cliff <12 2 28

Capture numbers ranged from 5 to 97, with a mean of 24 LP per night (data not shown). The first five counts were undertaken in autumn and the next three in spring. The 2008 count was undertaken in winter. During autumn the average catch was 26 per night, in spring it was 24 per night and in winter it was 14 birds per night.

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6.4.3.1 Alternative Population Estimate

An alternative method to estimating the LP population is from the number of active breeding burrows (Ryan & Moloney, 2000; Challies & Burleigh, 2004) i.e. no. of burrows x pair of LP. By this method, the total number of active nests from the four sites on Tiritiri Matangi is 61. This should equate to 122 breeding adults. Challies & Burleigh (2004) also allow a 0.39 : 1.0 ratio of juveniles to adults adding 48 birds, giving a total estimate of 170.

6.5 Discussion

The mark-recapture survey using a closed population method was selected because a) the penguins had to be counted on land rather than at sea, b) many of the burrows are inaccessible and c) the opportunity arose to utilise volunteer help over a short period, satisfying one of the assumptions for a closed population. The timing of the survey at the beginning of the breeding season (late winter) was also favourable to ensure sampling the full breeding population. The result indicated an estimated adult breeding population of 147 LP within the survey site. When extrapolated to include other regions of Tiritiri Matangi Island, the population was estimated at approximately 598. As it stands, this figure would have to be considered a minimum for Tiritiri Matangi Island, as not the whole island was included. However, it cannot be assumed that the ratio of 9.8 penguins per active nest site is consistent across the entire island as there are more nests on the west coast of the island, than on the east coast (50 versus 11). The east coast is more exposed to the open ocean whereas the west coast faces the inner Hauraki Gulf harbour and in many ways is more sheltered. The east coast is steeper and there is less favourable nesting habitat, therefore it is highly likely that the number of LP resident on the east coast is fewer than on the west.

It is less than satisfactory that the re-capture number was < 10. Such a low number would have a large variance (Seber, 1982), resulting in the population size estimate being unreliable. However, this first attempt at standardising a population estimate methodology does provide a baseline from which to

127 improve the technique and from which to begin to monitor the resident breeding population on Tiritiri Matangi on an ongoing basis.

A further, unpredictable factor associated with this particular survey, was the severe storm conditions experienced in the Hauraki Gulf in the two days prior to the survey commencing. It is unknown how this may have affected the number of penguins coming ashore at that time. The storm may have forced more penguins ashore than usual or stranded many at sea. Alternatively, they may have been completely unaffected. Only future repetitions of this study, at the same time of year, in the same site, will provide the answer. Moon phase, tide and survey times were recorded for future reference.

6.5.1 Previous LP Capture Data

Because none of the previous counts used the mark-recapture methodology, a direct comparison of population size cannot be made, with this study. What the previous data do show is high variability in catch rates per night over the 10 year period, from a low of 5 to a high of 97. From the individual catch night data (not shown) counts did not necessarily decrease on successive nights, which may indicate that the penguins do not experience ‘trap shyness’. Alternatively, without the penguins being individually marked, this could also indicate a larger population with individuals coming ashore on different nights. In this study, the count was undertaken in winter with an average catch per night was 7 birds. In the previous counts, the only other winter count also had a low capture rate. It is possible this even lower catch rate is reflective of the severe storm event.

6.5.1.1 Alternative Population Estimate

The alternative population estimate is far lower than the mark-recapture Peterson estimate. Although the count itself may be accurate to a degree, i.e. it is fairly certain that each active nest would have two breeding LP, the method is likely to underestimate the population because many active nests would go undetected in the vegetation on Tiritiri Matangi. It is known, from

128 observation of LP coming ashore at night that they go in to the regenerated bush, but thereafter finding their nests is somewhat more problematic.