TANZANIAN CROCODILE SURVEY, OCTOBER, 1999estimate of the river is obtained and the coefficient of variation calculated. However, the lengths of river involved in the Selous and other

APPENDIX 1

TANZANIAN CROCODILE SURVEY, OCTOBER, 1999

A REPORT TO THE DIRECTOR OF WILDLIFE

I. GAMES and E.L.M. SEVERRE

0 200km

Protected AreasLakesRiversSurvey Sections

Ugalla

Mara

GrumetiRubondoIsland

Lake Tan ganyika

Lake Malawi

LakeVictoria

Dar es SalaamRungwa

Grea

t Rua

ha

Nyumba yaMungu Dam

RufijiKilombero

#Msombe

LakeRukwa

Ruvuma

PanganiMalagarasi

SerengetiNP

SelousGR

Ruaha NP

Aerial Survey for Crocodiles in Tanzania, 1999Contents

- Page i -

CONTENTS

EXECUTIVE SUMMARY....................................................................................... iii

INTRODUCTION .................................................................................................. 1

METHODS .......................................................................................................... 2Data Collection.......................................................................................... 2Data Analysis............................................................................................ 2

RESULTS............................................................................................................ 4The Selous Game Reserve .......................................................................... 5Other Rivers and Lakes............................................................................. 13

DISCUSSION .................................................................................................... 18The Selous Game Reserve ........................................................................ 18Other Rivers and Lakes............................................................................. 21

ACKNOWLEDGEMENTS ..................................................................................... 23

REFERENCES .................................................................................................... 23

APPENDIX 1 – CROCODILE ESTIMATES FROM SAMPLE COUNTS

APPENDIX 2 – SAMPLE WAYPOINTS

Aerial Survey for Crocodiles in Tanzania, 1999Contents

- Page ii -

TABLES

A Summary of crocodile densities (crocodiles/km) in some Tanzanian rivers andlakes as estimated by aerial survey in 1999. ......................................................... iii

1 Summary of aerial surveys for crocodiles in Tanzania 1989 to1999........................... 12 Aerial Surveys for crocodiles in the Selous Game Reserve, October, 1999.................... 53 Summary of Selous crocodile surveys, 1999........................................................... 54 Summary of crocodile surveys in the Selous Game Reserve, October, 1999 ................ 65 Aerial surveys for crocodiles outside the Selous Game Reserve................................. 136 Surveys for crocodiles in Tanzania, October, 1999................................................. 137 Night count – Lake Nzerakera to Rufiji River Camp ................................................. 178 Night count correction – Lower Rufiji River – Selous Game Reserve, 1999................. 179 Summary of crocodile densities in the Selous Game Reserve as estimated

by aerial survey................................................................................................ 1810 Comparison of night counts in the Selous Game Reserve since 1990 ........................ 2011 Aerial surveys of the Rufiji lakes.......................................................................... 2012 Summary and comparison of crocodile densities in some Tanzanian rivers and lakes

as estimated by aerial survey.............................................................................. 2113 Night count correction – Lower Rufiji River – Selous Game Reserve .......................... 22

FIGURES

A: Density trends for crocodiles in the major rivers in the Selous Game Reserveas estimated from aerial surveys. ....................................................................... iii

B: Density trends for crocodiles in Rufiji river as estimated from by spotlightcounts at night. ............................................................................................... iv

1 Aerial surveys for crocodiles in Tanzania, 1999 ....................................................... 42 The "Selous Triangle".......................................................................................... 53 Lower Rufiji - Crocodile densities October 2nd ......................................................... 74 Lower Rufiji - Crocodile densities October 3rd and 4th ............................................... 85 Lower Rufiji - Crocodile densities October 5th .......................................................... 96 Ulanga and Luwego – Crocodile densities October 3rd .............................................. 97 Ulanga – Crocodile densities October 2nd and 3rd ................................................... 108 Ruaha - Crocodile densities October 2nd and 3rd.................................................... 119 Kilombero - Crocodile densities October 2nd and 4th ................................................ 1210 Great Ruaha and Msombe - Crocodile densities, October 6th ................................... 1411 Rungwa, Kavu Lukima and Lake Rukwa - Crocodile densities, October 7th ................. 1512 Ugalla – Crocodile densities October 7th ............................................................... 1513 Mara and Serengeti – Crocodile Densities October 9th ............................................ 1614 Ruvuma – Crocodile densities October 9th ............................................................ 1615 Density of crocodiles recorded by aerial survey in the RUFIJI river since 1989 ........... 1816 Density of crocodiles recorded by aerial survey in the ULANGA river since 1989 ........ 1917 Density of crocodiles recorded by aerial survey in the KILOMBERO river since 1989 .... 1918 Density of crocodiles recorded by aerial survey in the RUAHA river since 1989 .......... 1919 Densities of crocodiles recorded from night counts in the Selous Game Reserve........ 20

Aerial Survey for Crocodiles in Tanzania, 1999Executive Summary

- Page iii -

EXECUTIVE SUMMARY

An aerial survey was carried out on selected rivers and lakes in Tanzania in October, 1999. Thesurvey was initiated to continue a series of surveys started in 1988 and to collect further datafor presentation at the 11 CITES Conference of the Parties in April, 2000 in Nairobi.

As in the previous years the survey concentrated on the Selous Game Reserve as this area musthave one of the most impressive Nile crocodile populations in Africa. The survey also addedseveral new rivers to the series (Table A) which included sections outside protected areas.

Table A: Summary of crocodile densities (crocodiles/km) in some Tanzanian rivers and lakesas estimated by aerial survey in 1999.

RIVER DENSITY RIVER/LAKE DENSITY

Selous Game Reserve Other Rivers and Lakes

Ulanga 3.55 Mara 0.33

Rufiji 16.90 Grumeti 1.24

Ruaha 2.27 Ruaha (in Ruaha N.P.) 2.42

Kilombero 6.08 Rubondo Island 0.60

Luwego 0.46 Ugalla 1.50

Luhombero 0.35 Msombe 0.15

Rungwa 0.31

Ruvuma 0.05

It appears that crocodile densities have increased in the rivers and lakes surveyed in the SelousGame Reserve with that of the Rufiji river having the highest rate of increase (Figure A). This isalso apparent from spotlight counts for crocodiles in the Rufiji river (Figure B).

Figure A: Density trends for crocodiles in the major rivers in the Selous Game Reserveas estimated from aerial surveys.

0

5

10

15

20

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999

Year

Den

sity

cro

cod

iles

Rufiji

KilomberoUlangaRuaha

Aerial Survey for Crocodiles in Tanzania, 1999Executive Summary

- Page iv -

Figure B: Density trends for crocodiles in Rufiji riveras estimated from by spotlight counts at night.

Data from other rivers and lakes is more difficult to interpret but and some rivers show increaseswhile other areas indicate a decrease. Problems with equipment also meant that some of theresults were not directly comparable with previous years.

Density increases were recorded from the Ruaha and Ugalla rivers – quite substantial in the caseof the Ugalla. Decreases were noted in the Mara, Grumeti and the Rubondo island shoreline. Inthe latter case all of these are in, or adjacent to protected areas.

Several new sections of river were added to the survey this year which included the Msombe,Upper Ruvuma and the Lukima rivers. Densities were low in all.

y = 2 . 3 3 4 5 x + 8 . 6 4 3 9

R 2 = 0 . 9 5 6 1

0

1 0

2 0

3 0

4 0

5 0

1 9 9 0 1 9 9 1 1 9 9 2 1 9 9 3 1 9 9 4 1 9 9 5 1 9 9 6 1 9 9 7 1 9 9 8 1 9 9 9 2 0 0 0

Y e a r

Den

sity

(cr

ocs

/km

)

N z e r a k e r a

K i d a i Y = 1 . 2 1 6 7 x + 3 2 . 5 0 3 r 2 = 1

Aerial Survey for Crocodiles in Tanzania, 1999Introduction

- Page 1 -

INTRODUCTION

Tanzania has carried out aerial surveys for crocodiles since 1988 (see references) as part of itsdata collection activities for proposals to CITES. Much of this work has been concentrated in theSelous Game Reserve which has the largest crocodile population in the country. However,surveys have also been carried out in many other rivers and lakes (Table 1).

Table 1: Summary of aerial surveys for crocodiles in Tanzania 1989 to1999.

Area 1989 1990 1993 1995 1996 1999Selous Rufiji

UlangaLuwego

RuahaKilombero

RufijiLakes

LuwegoUlangaRuaha

Kilombero

RufijiLakes

UlangaRuaha

Kilombero

RufijiLakes

UlangaRuaha

Kilombero

RufijiLakes

UlangaRuaha

Kilombero

RufijiLakes

LuwegoUlangaRuaha

KilomberoLuhombero

West RuahaRungwa

Lake RukwaKavuu

Lake ChadaUgalla

Malagarasi

RuahaRungwa

Lake RukwaKavuu

Lake ChadaUgalla

Malagarasi

RuahaRungwa

Lake RukwaUgalla

Malagarasi

Ruaha RuahaMsombeRungwa

Lake RukwaKavuuUgalla

East PangaiNyumba

WamiNyumba

Nyumba Nyumba

North RubondoMara

Grumeti

RubondoGrumeti

Rubondo RubondoMara

Grumeti

RubondoMara

GrumetiSouth Ruvuma Ruvuma

This survey was initiated by the Wildlife Division to continue the monitoring effort and to gatherinformation for a downlisting of its Nile crocodile population to Appendix II at the 2000 COP inNairobi.

The current survey was country-wide on selected rivers and lakes but, as in previous years, itconcentrated on the Selous Game Reserve.

While reading this report it should be appreciated that it is just that – a report. Time and deadlineconstraints meant that the analysis of some of the results was not as exhaustive as if it wasbeing written up for publication.

Aerial Survey for Crocodiles in Tanzania, 1999Methods

- Page 2 -

METHODS

DATA COLLECTION

Aerial Surveys

Two aircraft were used on this survey; a 206 (5H-SGR) with a six hour endurance and a 182(5H-MPZ) with a four hour endurance. The majority of the crews had been on crocodile surveysin previous years as had both the pilots. Both aircraft started the survey in the Selous GameReserve but then split to allow greater ground coverage in the time available. The 206 remainedin the south to cover the Ruaha, Rukwa and Ruvuma rivers while the 182 continued to cover thenorthern lakes and rivers (Nyumba ya Mungu, Victoria, Mara, Grumeti and Ugalla).

As on previous surveys the rivers were divided into arbitrary sections based on time or characterof the river. Sections were called out by an observer/recorder and these were marked on allrecorder sheets.

In both aircraft two observers sat on the right hand side of the aircraft and one on the left handside. The pilot was left to concentrate on flying and was not used as an observer. On occasionsthe rear right hand seat observer acted as a recorder and did not actively search for crocodiles. Thehighest count of the right observers was used for the calculations.

The pilot positioned the aircraft for maximum visibility of the river. Height and speed were dictatedto by safety considerations and pilot experience. On most rivers counting was carried out fromboth sides of the aircraft. There was no cross counting (i.e. those on the "far side" did not count orpoint out crocodiles to the "near side" observers).

The advent and development of GPS technology and its related software has made studies suchas this easier and less prone to error. On this survey tracks were recorded at 15 second intervalson short flights and 30 second intervals on longer flights. Waypoints were recorded to mark thestart and finish of each survey as well as section breaks during the survey. After each surveyboth the track and waypoints were downloaded to a computer using Fugawi navigationalsoftware and converted for use with mapping software (Arcview). However, having been on thereceiving end of technology lapses in the bush on several occasions all waypoints were alsotranscribed on paper prior to being deleted from the GPS.

Night Counts

A single count for crocodiles was done at night on a section of the Rufiji river inside the SelousGame Reserve (Lake Nzerakera to Rufiji River Camp). Standard spotlighting techniques were used(Games, 1990).

DATA ANALYSIS

"Traditional" sample counts require that the river is stratified prior to survey and that selectedrepresentative samples are surveyed from each of the strata (Graham, 1988). In this way an overallestimate of the river is obtained and the coefficient of variation calculated. However, the lengths ofriver involved in the Selous and other areas meant that, in many cases, the entire river wassurveyed. This led to the possibility of stratification "after the event", as some sections of therivers showed higher concentrations of crocodiles than others. It was hoped to relate the higher (orlower) concentrations to some biological (e.g. many fishing villages) or physical feature (e.g. rockyareas) of the river.

Aerial Survey for Crocodiles in Tanzania, 1999Methods

- Page 3 -

Analysis of the sample counts followed the method outlined by Graham (1988) which was basedon that of Jolly (1969). The coefficient of variance or CV (the standard error as a percentage ofthe estimate) is a measure of the precision of a count. It was estimated by first calculating Sd2

with:

Sd2 = (∑d2 - (∑ d)2) /(n)

(n-1)

where:d = density of the samplesn = number of samples

The variance of the count (V) was then calculated by:

Var N = (Z2 /n)Sd2

where:Z = total length surveyedn = number of samples

and the CV calculated by

CV = (√ V / N)100

where:V = variance of the countN = number of crocodiles

As these were not sample counts in the true sense (i.e. the whole length of the river was counted)a population estimate was not directly derived from them. Densities, and fluctuation thereof overthe years was considered more important.

Aerial Survey for Crocodiles in Tanzania, 1999

- Page 4 -

RESULTS

AERIAL SURVEYS

The surveys concentrated on the Selous Game Reserve as this has the largest population of crocodilesin Tanzania. Sections of the Ruaha, Msombe, Rungwa, Kavuu, Lukima, Ugalla, Mara and Grumeti riverswere surveyed. In addition parts of Lakes Nyumba ya Mungu, Mtera, Rukwa and Victoria (RubondoIsland) were also surveyed (Figure 1).

Figure 1: Surveys for crocodiles in Tanzania in October, 1999

0 200km

Protected AreasLakesRiversSurvey Sections

Ugalla

Mara

GrumetiRubondoIsland

Lake Tan ganyika

Lake Malawi

LakeVictoria

Dar es SalaamRungwa

Grea

t Rua

ha

Nyumba yaMungu Dam

RufijiKilombero

#Msombe

LakeRukwa

Ruvuma

PanganiMalagarasi

SerengetiNP

SelousGR

Ruaha NP


- Page 5 -

Selous Game Reserve

Surveys were carried out between the 2nd and 5th October, 1999 in the Selous Game Reserve(Table 2). Both aircraft were used.

Table 2: Aerial surveys for crocodiles in the Selous Game Reserve, October, 1999

Date Sections Surveyed Comments02 Rufiji, Ulanga, Kilombero, Ruaha03 Rufiji, Ulanga, Ruaha, Luwego, Luhombero04 Rufiji, Kilombero Rain05 Rufiji, Incl. outside Reserve

In summary the Rofiji (downstream of Stiegler’s Gorge) was surveyed five times, the Ulanga (Stiegler’sGorge to Shughuli Falls) three times, the Kilombero and the Ruaha two times and the Luwego andLuhombero once each (Table 3; Figure 2).

Table 3: Summary of Selous aerial surveys, 1999

SummaryLower Rufiji = 5 surveysUlanga = 3 surveys

Kilombero = 2 surveysRuaha = 2 surveys

Figure 2: The “ Selous Triangle”

Crocodiles were concentrated in the Rufiji river and its associated lakes (Tagalala, Manze, Nzerakera,Siwandu and Mzizimia) but only inside the Selous Game Reserve. Once outside the Reserve crocodiledensities dropped dramatically (Table 4).


- Page 6 -

Other rivers had moderate to high densities of crocodiles with some clumping of the populationevident, especially in the Kilombero river. Again crocodile densities dropped outside the reserve on theKilombero.

Table 4: Summary of crocodile surveys in the Selous Game Reserve – October, 1999

River Density Comments11.77 02; Start 09:0014.78 02; Start 15:3014.75 03: Start 09:0016.90 04: Start 09:1513.74 05: Start 09:30; Only inside Selous used for density

Lower Rufiji

0.078 Outside Selous only2.49 02: Start 09:203.55 02: Start 16:30

Ulanga(Upper Rufiji)

3.34 03: Start 10:553.67 02: Start 09:456.08 04: Start 10:150.38 02: Outside Selous

Kilombero

0.14 04: Outside Selous2.27 02: Start 16:15Ruaha1.57 03: Start 09:15

Luwego 0.46 03: Start 16:30Luhombero 0.35 03: Some outside SelousLakes are total counts and not densitiesTagalala 339Manze 87Nzerakera 145Siwandu 93Mzizima 25

Poor surveys

The densities of the rivers surveyed and also in the individual sections for all the surveys are shown inFigures 3 to 9.

Summary of Figures on following pages

Figure 3 Lower Rufiji – 2nd OctoberFigure 4 Lower Rufiji – 3rd and 4th OctoberFigure 5 Lower Rufiji 5th OctoberFigure 6 Ulanga and Luwego rivers – 3rd OctoberFigure 7 Ulanga River – 2nd and 3rd OctoberFigure 8 Ruaha River – 2nd and 3rd OctoberFigure 9 Kilombero River – 2nd and 4th October


- Page 7 -

Figure 3: Lower Rufiji – Crocodile densities October 2nd 1999

0 5km

Old Rufiji Bed

02 October 1999Start 09:00Average = 11.8 crocs/kmCV = 16%

Mzizimia

Siwandu

NzerakeraManze

Tagalala

Stiegler'sGorge

6.9

6.7

6.8

7.514.014.8

15.0

12.510.9

21.6

20.0

24.4

0.0

15.5

26.015.9

19.5

15.9

6.2 5.1 10.0

6.5

Stiegler'sGorge

Tagalala

Manze Nzerakera

Siwandu

Mzizimia


Old Rufiji Bed

0 5km

MORNINGVia "MAIN" CHANNEL

AFTERNOONVia MBUYU CHANNEL

Lakes =

Lakes =


- Page 8 -

Figure 4: Lower Rufiji – Crocodile densities October 3rd and 4th 1999

MORNINGVia "MAIN" CHANNEL

MORNINGVia MBUYU CHANNEL

0 5km

Old Rufiji Bed


Mzizimia

Siwandu

NzerakeraManze

Tagalala

Stiegler'sGorge

Stiegler'sGorge

Tagalala

Manze Nzerakera

Siwandu

Mzizimia


Old Rufiji Bed

0 5km

0.0 - Mzizimia Channel

3.2

10.910.9

13.8

24.533.3

8.5

10.35.7

10.013.1

23.126.8

28.824.0

Lakes =

Lakes =


- Page 9 -

Figure 5: Lower Rufiji – Crocodile densities October 5th 1999

Figure 6: Ulanga and Luwego – Crocodile densities on 3rd October, 1999

0 10km

Old Rufiji Bed


Mzizimia

Siwandu

NzerakeraManze

Tagalala

MORNINGVia MBUYU CHANNEL

Lakes =Mbuyu SabaVillage

0.0

0.3

0.0

4.711.5

10.55.919.9

18.8

11.123.3

20.2

25.16.9

#

Selo

us B

ound

ary13.8

0 20km

ShughuliFalls

3.8

3.4

3.4

3.5

03 October, 1999Start 16:30Average = 3.6 crocs/kmCV = 14%

03 October, 1999Start 16:30Average = 0.5 crocs/kmCV = Negative #

LUWEGO

ULANGA

0.5

0.4

0.3

0.8

MaweraRiver


- Page 10 -

Figure 7: Ulanga – Crocodile densities 2nd and 3rd October, 1999

0 20km

#

#

#

#

#

##

#

#

#

#

#

#

##


ShughuliFalls

Stiegler's Gorge

2.91.9

0.41.4

2.34.0

1.41.9

3.3

3.6

4.62.62.5

5.1

ULANGA

ULANGA

Stiegler's Gorge

ShughuliFalls


0 20km

#

#

#

#

#

#

#

#

#

#

#

5.9

4.44.5

3.44.3

2.1

2.5

2.5

0.0

RuahaRiver

MaweraRiver

MaweraRiver


- Page 11 -

Figure 8: Ruaha – Crocodile densities 2nd and 3rd October, 1999

02 October, 1999Start: 16:15Average = 2.3 crocs/kmCV = 58%

Tazara Railway

0 20km2.8

11.60.8 1.7

2.82.0

0.61.0 16.2

2.8 4.60.8

0 20km

Tazara Railway

1.3

4.71.91.82.00.7

0.4

03 October, 1999Start: 09:15Average = 1.6 crocs/kmCV = 20%


- Page 12 -

Figure 9: Kilombero – Crocodile densities October 2nd and 4th , 1999

0 20km

#

#

#

#

#

#

#

#

#

#

#

#

#

#

0.77.2

4.4

5.0

2.70.53.3

3.2

4.01.6

8.7

5.40.4

02 October, 1999Start 09:45Average = 2.9 crocs/kmAverage inside Selous = 3.7 crocs/kmCV = 21%

Ifakara

Luho

mbe

ro

ShughuliFalls

Sel

ous

Bou

ndar

yS

elou

s B

ound

ary

ShughuliFalls

Luho

mbe

ro

Ifakara

04 October, 1999Start 10:10Average = 4.8crocs/kmAverage inside Selous = 6.1 crocs/kmCV = 43%

0 20km

0.1 7.626.4

1.75.4

5.4

3.1

3.32.6

2.2

1.3

KILOMBERORIVER

KILOMBERORIVER


- Page 13 -

OTHER RIVERS AND LAKES

A number of other rivers and lakes were surveyed between the 5th and 10th of October using two aircraft(Table 5). These were Lakes Nyumba ya Mungu, Mtera, Rukwa and Victoria and the Great (or upper)Ruaha, Msombe, Mara, Grumeti, Rungwa, Kavuu, Lukima, Ugall and Ruvuma rivers.

Table 5: Aerial Surveys for Crocodiles Outside the Selous Game Reserve

DATE SECTIONS SURVEYED COMMENTS

05th Lake Nyumba ya Mungu Afternoon

06th Upper Ruaha, Mtera Dam, Msombe Morning, Afternoon

06th Mara, Grumeti Afternoon

07th Rungwa, Lake Rukwa, Kavuu, Lukima Morning

08th Ruvuma Morning

09th Rubondo Afternoon

10th Ugalla Morning

The results of these surveys are summarised below (Table 6). Figures 10 to 14 show the sections ofriver/shoreline surveyed and their relevant densities.

Table 6: Surveys for crocodiles in Tanzania, October 1999

RIVER DENSITY

COMMENTS

Msombe 0.15 Morning

Ruaha 2.42 Late afternoon

Rungwa 0.31 Morning

Mtera Dam 0.03 35 kilometres shoreline surveyed

Kavuu 0,04 Few pools left

Lake Rukwa 0.61 Only northern shore surveyed

Lukima 0.00 No crocodiles seen

Ruvuma 0.05 Shallow river upstream of Lugenda junction

Lake Nyumba ya Mungu 0.10 Entire shoreline covered

Mara 0.33 Outside Serengeti National Park

Grumeti 1.24 Most in Kirawira section

Rubondo 0.60 No GPS readings

Ugalla 1.50 No GPS readings


- Page 14 -

Figure 10: Great Ruaha and Msombe rivers – Crocodile densities 6th October, 1999

0 25km

RuahaNational Park

Rungwa Game Reserve

MteraDam0.03 crocs/km

2.6 0.30.8 9.7

15.00.5

0.30.4

0.5Msembe Pools

0.00.1

0.2

0.2

0.3

0.1

Msom

be

Rive

r

Gre

at R

uaha

Riv

er

Dry river bed

RUAHA RIVER06 October, 1999Start: 16:30Average = 2.42 crocs/km

MSOMBE RIVER06 October, 1999Start: 09:50Average = 0.15 crocs/km


- Page 15 -

Figure 11: Rungwa, Kavuu and Lukima and Lake Rukwa – Crocodile densities 7th October

Figure 12: Ugalla – Crocodile densities 10th October, 1999

0 25km

#

LAKE RUKWA44 kilometres surveyed0.6 crocs/km (27 crocs seen)24 seen at mouth of Rungwa

Rungwa Falls

Lukima Falls0.4

0.40.40.2

0.2

0.4

Rungwa River

Kavuu River

LukimaRiver

Lake Rukwa

0.0

0.60.04

0.04

RUNGWA RIVER07 October, 1999Start 08:50Average = 0.31 crocs/km

0 25km

#

# Kigoma Railway

Mpan

da R

ailwa

y

UGALLAGAME

RESERVE

Ugalla

Ugalla R

iver

MalagarasiRiver

SurveyStart

SurveyStop

Lake Sagara

Ugalla Floodplain

Swamp Only start and stop recorded150 km of river surveyed219 crocodiles seenDensity of 1.5 crocs/km


- Page 16 -

Figure 13: Mara and Serengeti – Crocodile densities 6th October, 1999

Figure 14: Ruvuma - Crocodile densities 8th October, 1999

0 km 50

KENYA

SerengetiNational Park

LakeVictoria

Mara River

Grumeti River

MasiroriSwamp

0.3

0.40.3

1.84.10.4

Mara RiverDensity = 0.33 crocs/km

Grumeti RiverDensity = 1.24 croc/kmMost in Kirarwira section

KirawiraLakeVictoria

0 50km

# Masasi

Lugenda River

MOZAMBIQUE

Mas

ugur

u

#

Lumesule-LukwikaGame

Reserve

0.03

0.0

0.070.0 0.13

0.00.11

0.04

RUVUMA RIVERAverage = 0.05 crocs/km


- Page 17 -

Night Counts

The night count revealed 442 crocodiles in 13.2 kilometres of river giving a density of 33.5 crocodilesper river kilometre (Table 7).

Table 7: Night count – L. Nzerakera to Rufiji River Camp

Section Distance (km) Crocodiles Density1 2.15 63 29.32 0.30 10 33.33 0.38 13 34.14 0.60 16 26.75 0.48 17 35.46 1.04 54 51.97 1.43 78 54.58 0.68 30 44.19 0.66 20 30.310 0.92 35 38.011 1.32 48 36.412 1.20 27 22.513 0.96 18 18.814 1.09 13 11.9

TOTALS 13.21 442 33.5

A correction factor the aerial surveys was also calculated from the night counts in the lower Rufiji. Thenight count revealed 442 crocodiles between Lake Nzerakera while two separate aerial counts alongthe same stretch of river counted 89 and 114 crocodiles. This gives a “correction factor” of 4.9 and3.9 respectively (Table 8).

Table 8: Night count correction – Lower Rufiji River - Selous Game Reserve, 1999

NUMBERS DENSITY CORRECTION FACTOR

15:30 89 6.7 4.9

09:15 114 8.6 3.9

Night – Boat 442 33.5

Aerial Survey for Crocodiles in Tanzania, 1999Discussion

- Page 18 -

DISCUSSION

Selous Game Reserve

Densities of crocodiles as recorded by aerial survey appear to be increasing or remaining stable in themajor rivers in the Selous Game Reserve. Densities for these rivers are summarised below (Table 9).Some fluctuations are evident but these are believed to be due to observer bias and pilot experience.The Rufiji, Ulanga and Kilombero populations show an overall increase since 1989 while the Ruahaseems to be stable.

Table 9: Summary of crocodile densities in the Selous Game Reserveas estimated by aerial survey

Parker/Graham

Hutton/Katalihwa

Games and Severre

1963 1988 1989 1990 1993 1995 1996 1999

Ulanga 1.95 - 3.51 0.98 3.15 2.89 2.26 2.6 5.55 3.55

Lower Rufiji 6.75 11.83 10.49 10.1 19.98 16.90

Lake Tagallala 18.07 23.38 28.2 46.07 26.08

Ruaha 1.56 1.77 1.57 1.68 1.6 1.67 2.27

Kilombero 0.28 (7.74) 2.86 3.54 3.2 5.6 6.08

Upper Luwego 2.74

Lower Luwego 0.33 1.64 0.46

Luhombero 0.35Notes: • Kilombero and Lower Rufiji estimates for inside the Selous Game reserve.

• 1988 survey carried out during high flood conditions so not really comparable except for the Ruaha. This isbecause Ruaha is controlled by a dam so levels do not vary much.

• Lower Rufiji estimates are the best count of the day. i.e. 16:30 in 1990; 18:00 in 1993, 17:30 in 1995, 16:30in 1996 and 09:15 in 1999.

• The 1989 estimate for the Kilombero is so far in excess of the others that the originaldata was accessed and reworked and the implications of this discussed below.

Figure 15: Densities of crocodiles recorded by aerial survey in the RUFIJI river since 1989

y = 0.9582x - 1897.6

R2 = 0.553

0

5

10

15

20

25

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Year

Den

siti

es

RUFIJI


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Figure 16: Densities of crocodiles recorded by aerial survey in ULANGA the river since 1989

Figure 17: Densities of crocodiles recorded by aerial survey in KILOMBERO the river since 1989

y = 0.1198x - 235.53

R2 = 0.14880

1

2

3

4

5

6

7

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Year

Den

siti

es

ULANGA

y = 0.3773x - 748.22

R2 = 0.73860

1

2

3

4

5

6

7

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Year

Den

siti

es

KILOMBERO


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Figure 18: Densities of crocodiles recorded by aerial survey in RUAHA the river since 1989

These results were comparable to surveys in 1990, 1993 and 1995 (Table 10). Results were alsoavailable for the Kidai area (just south of Lake Tagalala) for 1990 and 1995.

Table 10: Comparison of night counts in the Selous Game Reserve since 1990

1990 1993 1995 1996 1999Lower Rufiji 12.7 16.7 20.7 33.5Kidai area 33.8 41.1

These surveys show a definite increase in crocodile densities in both areas, with that for the lowerRufiji being almost three times what it was in 1990 (Figure 15)

Figure 19: Density of crocodiles as estimated from spotlight counts at night in the RUFIJI river

y = 0.0421x - 82.086

R2 = 0.3751

0

1

2

3

4

5

6

7

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Year

Den

siti

es

RUAHA

y = 2.3345x + 8.6439R2 = 0.9561

0

10

20

30

40

50

1 9 9 0 1 9 9 1 1 9 9 2 1 9 9 3 1 9 9 4 1 9 9 5 1 9 9 6 1 9 9 7 1 9 9 8 1 9 9 9 2 0 0 0

Year

Den

sity

(cr

ocs

/km

)

Nzerakera

K idai Y=1.2167x + 32.503 r2 = 1


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Data from the Rufiji lakes was disappointing in 1999 (Table 11). However, as stated in previous years,these are very difficult to survey accurately. The aircraft used was fully loaded with 400 litres of fueland hence flying was more “sedate” than in 1996. We believe that this is one of the reasons for thepoor counts. The ability of the pilot is also crucial.

Table 11: Aerial surveys of the Rufiji lakes

Lake 1991 1993 1995 1996 1999

Tagallala 181 304 319 599 339

Manze 71 366 120 87

Nzerakera 79 139 187 145

Siwando 43 250 127 93

Mzizima 14 113 59 25

Conditions Poor Good Variable1 Poor

1 = The ability of the pilot is crucial on the lake surveys. Tight turns close to the ground are the order of the day andthe counts increase the better the pilot is.

Other Rivers and Lakes

Trends from other rivers and lakes are more difficult to assess (Table 12). There appears to be in increasesince the 1990 surveys in the Ruaha river adjacent to the Ruaha National Park. The Rungwa river appearsto have been fairly constant over the years. Both rivers are seasonal and shrink up to large pools duringthe dry season. Crocodiles are found in some of these pools.

There also appears to be a substantial increase in the numbers of crocodiles seen in 1999 in the Ugallariver. Unfortunately the data has only a start and end point so it is difficult to draw conclusions from this.However, we believe that some of the increase can be explained by and actual increase in crocodilenumbers while some of it could be explained by pilot expertise or the flight path, time of day or othervariables (water level etc.).

Similarly there is an apparent decrease along the Rubondo island shoreline. Rubondo is an isolated islandin Lake Victoria and is a National Park. We believe that a major decline would have been noted by theParks staff. Again without GPS data it is difficult to draw any precise conclusions about this survey.

Several new river sections were covered on this survey. These include the Ruvuma (a different section tothe 1993 survey), the Lukima and the Msombe rivers. The Ruvuma and Lukima both flow (albiet slowly)throughout the year. Both were chosen as they featured in the problem animal reports. The Ruvuma hada low density of visible crocodiles in the main river. Again, it seems likely that the crocodiles would lie upin the deeper pools rather than in the main river – which is only a few centimetres deep over much of itslength.

The Msombe river is a seasonal river and again crocodiles are found in pools along its length. It is anunusual river in Tanzania in that almost its entire catchment falls into protected areas (Ruaha NationalPark and Rungwa Game Reserve).

Table 12: Summary and comparison of crocodile densities in some Tanzanian rivers and lakesas estimated by aerial survey.

RIVER 1990 1993 1995 1996 1999

Mara 0.88 0.62 0.33

Grumeti 0.83 2.04 1.24


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Ruaha (in Ruaha N.P.) 0.86 1.78 2.66 2.42

Rubondo Island 0.62 0.82 0.70 2.66 0.60

Ugalla 0.67 0.61 0.21 1.50

Malagarasi 0.5 0.19

Msombe 0.15

Rungwa 0.46 0.20 0.40 0.31

Ruvuma Upper 0.05

Ruvuma Lower 0.32


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Correction Factors

Although not the primary purpose of these surveys correction factors were calculated for a shortsection of the Rufiji river in 1993, 1995, 1996 and 1999 (Table 13).

Table 13: Night count correction – Lower Rufiji River - Selous Game Reserve

NUMBERS DENSITY CORRECTION FACTOR

1993

09:30 137 7.2 2.3

12:30 52 2.7 6.1


1995

16:30 50 7.1 2.9


1996

09:00 183 19.6 2.1


1999

15:30 89 6.7 4.9

09:15 114 8.6 3.9


The results varied considerably with the time of the aerial count. For example, a midday count in 1993saw only 38% of the crocodiles that were seen at 9:30 am in the same year (1993). An average ofthese “correction factors” is 3.7 indicating that (on average) aerial survey estimates should bemultiplied by this factor to give a more precise estimate of the total numbers of crocodiles in the river.

However, other factors must also be taken into account. If the 1999 estimates are excluded a briefglance at the figures shows that (apart from the 12:30 count in 1993) the “correction factor” isapproximately 2.5. The 1999 figures are considerably higher and this is probably related to an increaseof crocodiles in the river rather than to other variables such as observer ability etc. Also thesecorrection factors are for the lower Rufiji. Other rivers in the area have different characteristics and theRufiji correction factors may not be applicable.

Therefore, if one was attempting an estimate of the total population, it would be better to use anaverage of the 1999 figures (i.e. 4.4) and all sections should have the density multiplied by 4.4 toallow a more precise population estimate.

Recent work by the GTZ Selous Conservation Project has resulted in a detailed map of the Rufiji areabased on satellite imagery. It is now possible to estimate channel lengths in the area with some degreeof accuracy. BUT it should also be noted that the area was subject to severe flooding AFTER the dateof the satellite image and the main channel has changed. The main water now flows a previously smallshallow channel. These calculations will be carried out at a later stage.


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ACKNOWLEDGEMENTS

We would like to thank Captain Masota and Captain Shayo for flying us during this survey.Observer duties were ably handled by Charles Mdoe, Festo Msoffe, Julius Kibebe, ObedMbangwa and Willness Minja. Mr M. Madehele was responsible for logistics. The support of FelixLyimo (Assistant Director of Anti-Poaching) and Juma Kyera (Assistant Director of WildlifeUtilisation) is greatly appreciated.

The hospitality of Luigi Busione at Rufiji River Camp was greatly appreciated. The camp alsosupplied a boat for the night count on the Rufiji River. Bernard was able to guide us withoutmishap.

The assistance of GTZ and Andrew Cauldwell with GPS technology and computer software isappreciated.

REFERENCES

Cott, H.B. 1961. Scientific results of an enquiry into the ecology and economic status of the Nilecrocodile (Crocodilus niloticus) in Uganda and Northern Rhodesia. Transactions of the ZoologicalSociety of London 29:211-356.

Games, I. 1990. The feeding ecology of two Nile crocodile populations in the Zambezi valley.Unpublished D.Phil. manuscript. University of Zimbabwe.

Games, I. 1994. Aerial Surveys for Monitoring Trends and Estimating Population Size ofCrocodylus niloticus or The theory and practise of aerial survey in Africa. Proceedings of the 12thWorking meeting of the IUCN SSC Crocodile Specialist Group, Pattaya, Thailand. pp 245-255

Games, I and Severre, E.L.M. 1989. A survey of crocodile densities in the Selous Game Reserveand adjacent Game Controlled Areas, Tanzania. September 1989. Report to the Director ofWildlife, Tanzania and the CITES Nile crocodile project.

Games, I and Severre, E.L.M. 1990. A survey of crocodile densities in Tanzania. October, 1990.Report to the Director of Wildlife, Tanzania.

Games, I. and Severre, E.L.M. 1993. The status and distribution of crocodiles in Tanzania.Proceedings of the 11th Working meeting of the IUCN SSC Crocodile Specialist Group, VictoriaFalls, Zimbabwe

Games, I. and Severre, E. 1993. Tanzanian Crocodile Survey, November, 1993. A PreliminaryReport to the Director of Wildlife, Tanzania. Report to the Director of Wildlife, Tanzania.

Games, I. and Severre, E. 1995. Tanzanian Crocodile Survey, November, 1995. Report to theDirector of Wildlife, Tanzania. Report to the Director of Wildlife, Tanzania.

Games, I. and Severre, E. 1996. Tanzanian Crocodile Survey, October, 1996. Report to theDirector of Wildlife, Tanzania. Report to the Director of Wildlife, Tanzania.

Games, I. and Severre, E. 1996. Crocodiles in Tanzania, A Review and Assessment. Report to theDirector of Wildlife, Tanzania.

Games, I., Zohlo, R. and Chande, B. 1989. Utilization of the crocodile resource on Lake CahoraBassa, Mozambique, during 1987 and 1988. Report to the co-ordinator, CITES Nile crocodileproject.

Graham, A.D. and Parker, I.S.C. 1964. Unpublished data on aerial survey of the Rufiji river.


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Graham, A.D. 1988. Methods of surveying and monitoring crocodiles. Proceedings of the SADCCWorkshop on Management and Utilization of Crocodiles in the SADCC Region of Africa. pp. 74-101. Eds. J.M. Hutton, J.N.B. Mpande, A.D. Graham and H.H. Roth.

Hetz, P. 1993. Grumeti river crocodile and hippopotamus survey. October 4/5, 1993. Typescriptfrom Serengeti National Park files.

Hirji, K.N. A preliminary assessment of the crocodile population in Lake Rukwa. Report to SerengetiWildlife Research Institute, Arusha, Tanzania.

Hutton, J.M. and Katalihwa, M. 1988. The status and distribution of crocodiles in the region of theSelous Game Reserve, Tanzania, in 1988. Report to the Director of Wildlife, Tanzania.10 pp.

Hutton, J.M. and Woolhouse, M.E.J. 1989. Mark-recapture to assess factors affecting theproportion of a Nile crocodile population seen during spotlight counts at Ngezi, Zimbabwe, and theuse of spotlight counts to monitor crocodile abundance. Journal of Applied Ecology 26:381-395.

Katalihwa, M. and Lema, R. 1988. The status and management of the Nile crocodile in Tanzania.Proceedings of the SADCC Workshop on Management and Utilization of Crocodiles in the SADCCRegion of Africa. pp. 33-38. Eds. J.M. Hutton, J.N.B. Mpande, A.D. Graham and H.H. Roth.

Jolly, G.M. 1969. Sampling methods for aerial censuses of wildlife populations. East AfricanAgricultural and Forestry Journal 34 (Special Issue) 46-49

Magnusson, W.E. 1982. Techniques of surveying for crocodiles. Proceedings of the 5th Workingmeeting of the IUCN SSC Crocodile Specialist Group, pp 389-403.

Magnusson, W.E., Caughley, G.J. and Grigg, G.C., 1978. A double-survey estimate of populationsize from incomplete counts. Journal of Wildlife Management 42(1):174-176

Jelden, D., Games, I. and Rosser, A. 1994. Crocodile Management in Tanzania. Summary report ofan IUCN/SSC Crocodile Specialist Group mission to Tanzania. Report to the CSG and the WildlifeDivision, Tanzania.

Tello, J.L. 1985. CITES Nile Crocodile Status Survey. In CITES-working documents andAppendices 1987, pp. 67-83

Euroconsult (1980). Identification Study on the Ecological Impacts of the Steigler's Gorge Powerand Flood Control Project. Arnhem.

Documents

TANZANIAN CROCODILE SURVEY, OCTOBER, 1999estimate of the river is obtained and the coefficient of variation calculated. However, the lengths of river involved in the Selous and other