Crocodile Count 04 of 2010 – St Lucia

Jun 21, 2010

MISSION 22 of 2010

Name of Mission: Crocodile Count 04 of 2010 – St Lucia
Date of Mission: 21st to 23rd June 2010
Aircraft used: Cobra – Light Sport Aircraft
Beneficiary: Xander Combrink

Pilot: Peter Vosloo

Report from the beneficiary: Xander Combrink

Objective of the Flights:

The best time to count crocodiles from the air is during the cold winter months when they leave the cool water to bask on sandbanks and other suitable areas along the shoreline. When basking on land, they are much more visible from the air than in water or within the shoreline vegetation. Every year, Ezemvelo KZN Wildlife conducts a single crocodile count at Lake St Lucia during the winter months as part of its crocodile monitoring programme. We know that bias/error (e.g. visibility bias, observer bias, weather conditions etc.) result in an undercount of the true (unknown) population. We can increase the accuracy of the 2010 count result by increasing the number of surveys in quick succession.

03_Croc-04_LSL_Crocodile_aggregation_at_Lake_St_Lucia
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The surveys will also be part of the monthly distribution monitoring of the Lake St Lucia crocodile population and to see how changes in the broad distribution pattern relates to environmental or other changes (e.g. courtship, mating, salinity etc.) in the lake. The survey is also part of the mark-resight and spatial use programme to determine population dynamics in the lake through the re-sighting of colour coded tags on individual crocodiles. This will be an important component of these surveys, as crocodiles are much more visible basking in winter. Also the relative slow speed of the microlights will increase our chance to record the colour tag code.

08_Croc-04_LSL_Tagging_a_crocodile_in_Lake_St_Lucia
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We conducted three aerial surveys at Lake St Lucia as part of an Ezemvelo KZN Wildlife and University of KZN study investigating the distribution and movements of crocodiles at Lake St Lucia. Unfortunately, two of the three surveys were abandoned prematurely. The first survey was cancelled at the Mphathe River due to strong wind and the second survey was abandoned as a result of electrical problems (erroneous readings from the water temperature gauge).

Although we could not complete three total surveys, we were able to survey the entire shoreline at least once, except for the Hluhluwe River (Table 1). We counted the largest crocodile aggregation in the Narrows, just south of the Mphathe River, three times, and the other areas of high crocodile density (between Mphathe River and Makakatan Bay), twice.

The total count for Lake St Lucia was 646 crocodiles, based on the 22nd of June survey as well as the Eastern Shores survey the following day (Mamba Stream & Tewati Bay).

It seems as though the majority of crocodiles in Lake St Lucia are distributed at the moment between the (closed) estuary mouth and the northern tip of Potter’s Channel (northeastern tip of Makakatana Peninsula), including the Mphathe River, see Fig 1. Low density areas with at least five crocodiles counted, include Tewati Bay (25), Lake Bhangazi South (38), Catalina Bay (16) and Nkazana Stream (5).

22_FREP_Croc_Count_UKZN_04_St_Lucia_by_XC_-_ED_html_m360f1a55
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22_FREP_Croc_Count_UKZN_04_St_Lucia_by_XC_-_ED_html_4a4149f4
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The aggregation of 30 crocodiles observed at the mouth of the Mkhuze River and Lake St Lucia on 10 May 2010, are no longer there. This possibly is related to decreased water flow and depth of the Mkhuze River, which would have resulted in a vulnerable position, especially with nearby human activity.

It seems as though the majority (74%) of crocodiles in the lake are currently concentrated in a few areas of high density (see Fig. 2). This includes an aggregation in the Narrows, just south of the Mphathe River, where 129 crocodiles were counted from a series of aerial photographs. This congregation equals an encounter rate of 251 crocodiles per kilometre, based on the number of crocodiles counted in relation with the length of the adjacent shoreline (see Table 2). Other areas with high encounter rates include the southeastern tip of Makakatana Peninsula with 68.8 crocs/km and Potter’s Channel with 45 crocs/km, and a total of 123 crocodiles.

Although the total count (646) is down from last year’s survey (817, see Figure 3), it does not necessarily represents an actual decrease in the population, as the proportion of crocodiles present in the lake, but missed during the survey, is unknown. This figure should be regarded as an index of relative abundance. We are optimistic that the ongoing mark re-sight study will provide insight into the abundance of crocodiles in St Lucia.

Table 1

Area

21-Jun ’10

22-Jun ’10

23-Jun ’10

Msunduzi River

*

1

*

Mfolozi River

4

2

1

Narrows: Estuary mouth to bridge

6

9

7

Narrows: Bridge to Mphathe River

186

193

207

Mphathe River

15

24

*

Mphathe River to Forks

*

65

48

Western Fork

*

11

16

Eastern Fork

*

88

25

eSingeni wetland

*

1

*

Makakatana Point

*

40

28

Potters Channel

*

105

75

Potters Channel mouth to Makakatana Bay

*

18

20

Makakatana Bay

*

3

4

Catalina Bay

*

16

8

Nkazana Stream

*

5

***

Lake Bhangazi South

*

38

***

Mamba Stream

*

**

2

Tewati Bay

*

**

25

Mkhuze River

*

0

0

False Bay

*

0

***

Hluhluwe River

*

*

***

 

* Not surveyed due to strong wind

** Not surveyed due to mechanical problems

*** Not surveyed due to different survey route flown

Table 2

Area

Crocodiles counted

Length (km)

Encounter rate (crocs/km)

Narrows/Mphathe River confluence

129

0.51

251.0

Makakatana Peninsula SE tip

40

0.58

68.8

Potter’s Channel

123

2.74

45.0

Mphathe River to start of Forks

65

2.05

31.8

Eastern Fork

88

3.93

22.4

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