The decline of the Nile crocodile population in Loskop Dam, Olifants River, South Africa

Abstract

The apparent decline in the number of Nile crocodiles present in the Loskop Dam prompted a study to determine the num-ber, size and distribution of Nile crocodiles now present in the reservoir. The number of crocodiles in the Loskop Dam was surveyed using aerial counts and spotlight counts. Surveys revealed the presence of a very low total number of crocodiles and also a poor distribution of crocodiles in the different size classes over almost 30 years since 1981. Eight surveys carried out between 2001 and 2010 revealed that the distribution pattern of crocodiles in the Loskop Dam did not vary between win-ter and summer. These distribution patterns indicate that crocodiles occur most frequently in the eastern and western inlets and not in the main basin of the dam. Thirteen crocodiles were re-introduced into the dam during March 2007; however the August 2009 spotlight survey results indicated that none of these animals had survived.

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* To whom all correspondence should be add ressed.
 +2713 262 4844; fax: +2713 262 4858;
e-mail: nilecrocs@mweb.co.za
Received 18 May 2010; accepted in revised form 3 November 2010.
The decline of the Nile crocodile population in Loskop Dam,
Olifants River, South Africa
Hannes Botha1, 2*, Wouter van Hoven1 and Louis J Guillette Jr3, 4
1 Centre for Wildlife Management, University of Pretoria, Pretoria, 0002, South Africa
2 Mpumalanga Tourism and Parks Agency, Scientic Ser vices, PO Box 1250, Groblersdal, 0470, South Africa
3 Department of Biology, University of Florida, Gainesville, Florida, USA
4 Department of Paraclinical Sciences, Faculty of Veterinary Science, University of Pretoria,
Private Bag X04, Onderstepoor t, 0110, South Africa
Abstract
The apparent decli ne in the nu mber of Nile crocodiles present i n the Loskop Dam prompted a study to deter mine the num-
ber, size and d istr ibution of Nile crocodiles now present in the reservoir. The number of crocodiles in the Loskop Dam was
surveyed using aerial cou nts and spotlight counts. Surveys revealed the presence of a very low total number of crocodiles
and also a poor distribution of crocodiles in the different size classes over almost 30 years since 1981. Eight surveys carried
out between 2001 and 2010 revealed that the distr ibution patter n of crocodiles in t he Loskop Dam did not var y between win-
ter and su mmer. These distribution patterns indicate that crocodiles occur most f requently in the eastern and western inlets
and not in the main basin of the dam. Thirteen crocodiles were re -introduced into the dam duri ng March 20 07; however the
August 20 09 spotlight sur vey results indicated that none of these an imals had sur vived.
Keywords: Crocodylus niloticus, numbers, size classes, density, distribution
Introduction
The Loskop Dam is situated on the Olifants River, approxi-
mately 32 km south (upstream) of the town of Groblersdal
in the Mpumalanga province of South Africa (Fig. 1).
Constr uction work on the Loskop Dam was completed in
1938 by the Department of Water Affairs and in 1979 the wall
was raised to its current height of 54 m above the foundation
(Loskop Ir rigation Board, 2009).
Jacobsen (1984) stated that the substantial decline in
the population of Nile crocodiles in the Loskop Dam was a
cause for concer n. Two main reasons for this decline had
already been suggested in the early 1980s. The rst hypo
thesized that pollution emanating from the upper reaches of
the Olifants River catchment could have a detrimental effect
on the reproductive potential of the crocodiles in the dam;
the second suggested that the raising of the Loskop Dam
resulted in ooding of basking and nesting areas making
these unusable by crocodiles and thus reducing recruitment
into the population (Jacobsen, 1984). The cause of peri-
odic deaths of large numbers of crocodiles in the Loskop
Dam has not been resolved over the last 26 years. Jacobsen
(1984) warned that should the decline in crocodile numbers
in the Loskop Dam be a result of pollution, then recovery
was unlikely and re-introduction of crocodiles into the
system would be pointless.
A growing body of literature clearly demonstrates that a
wide array of contaminants, including pesticides, metals and
nutrients, inuences the growth and reproduction of aquatic
organisms, including top predators such as crocodilians
(Guillette and Crain, 2000; Milnes et al., 2006; Milnes and
Guillette, 2008). Although at this time there is very limited
data on contaminant effects in Nile crocodiles, the data
on exposure levels (Phelps et al., 1986; Skaare et al., 1991)
and physiological-molecular processes (Katsu et al., 2006)
indicate that all the data published to date demonstrating
adverse effects on the American alligator, and other croco-
dilians, are directly applicable. Thus, we have to consider
the hypothesis that contaminants are, in large part, a threat
to the maintenance of crocodilian populations in Africa, and
further studies are needed to begin to test this hypothesis
in greater detail. Further, in-depth observations of avail-
able nesting and basking sites must be catalogued at varying
water levels so that a true representation of available recr uit-
ment can be made. The study described below represents
the begin ning of such research.
Methods
Surveys to determine the number, size and spatial dist ribution
of the crocodiles in the Loskop Dam were performed using 2
methods: aer ial counts from aircraft and spotlight counts from
boats. In both types of survey, the total length (TL) of individ-
ual crocodiles encountered was estimated to the nearest metre
and animals assigned to the following broad size classes:
• Class 1: Small-sized crocodiles (TL <1.4 m)
• Class 2: Medium-sized crocodiles (TL 1.4-2.1 m)
• Class 3: Large-sized crocodiles (TL 2.1-4.0 m)
• Class 4: Very large-sized crocodiles (TL >4.0 m)
The position of each crocodile counted was marked with a
handheld Global Positioning System (GPS) and the TL noted
with the waypoint number on a datasheet or palm computer and
downloaded later to a notebook computer.

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104
It is known that reproductive maturity is more closely
related to size (approximately 2 m) than age in crocodilians
(Cott, 1961; Graham, 1968; Whitworth, 1971; Joanen and
McNease, 1975; Hutton, 1984; Games, 1990; Magnusson et
al., 1990). Thus crocodiles in the small and medium size
classes (less than 2.1 m TL) were grouped together as they
are likely to be non-reproductive. Similarly, all crocodiles in
the large size class (between 2.1 and 4.0 m TL) were grouped
together, as they are likely to be the reproductive animals,
whereas those over 4.0 m TL were a third distinct group as
they are likely the large dominating males in the population.
The size (total length) of completely submerged crocodiles
was estimated using specic environmental and behavioural
characteristics. These included factors such as habitat type,
water depth, water swirl, mud trails and wakes (Jacobsen,
1984; Woodward and Moore, 1993). According to Jacobsen
(1984), the tendency to underestimate the size of crocodiles
spotted from the air is regarded as a constant factor and can
therefore be ignored. Economic reasons eventually necessi-
tated that the crocodilian population surveys be conducted by
spotlight counts rather than aerial counts. However, spotlight
counts are regarded as a suitable and reliable method for esti-
mating crocodilian population size (Webb and Messel, 1979;
Bayliss et al., 1986; Hutton and Woolhouse, 1989; Games,
1990; Woodward and Moore, 1993). To account for the num-
ber of crocodiles missed by observers during surveys, correc-
tion factors were calculated and applied to the data according
to methods described by Magnusson et al. (1978), Bayliss,
et al. (1986), Hutton and Woolhouse (1989) and Stirrat et al.
(2001).
Results
The sur vey data demonstrated a very low total number of
crocodiles as well as a very poor distribution of crocodiles
over the size classes, compared to that expected to be present
in healthy populations. During 1979 a total of 21 crocodiles
were counted in the Loskop Dam (Jacobsen, 1984). Taking
the expected undercount into consideration (Bayliss 1987;
Swanepoel, 2001; Botha, 2005), this translates to an estimated
32 animals that could have been present in the dam at the time
of the survey. The February and August 2010 spotlight sur-
vey results (Table 1) indicate that the population is currently
at an extremely low level with only 4 individual crocodiles
found in the dam. Accounting for the expected undercount,
we estimate the total Nile crocodile population in the Loskop
Dam to be no more than 4 to 6 animals at this time. A total
of 8 crocodiles were found in the whole of the Loskop Dam
during the 2006 spotlight survey (Table 1). Previous surveys
in 2001 and 2005 produced similar low results, of 10 and 6
animals, respectively. The brief increase registered during
the July 2006 and Januar y 2007 spotlight sur veys is likely to
be a function of the observers gaining experience rather than
of a successful population increase.
Also of interest is that no crocodiles in the large (2.1-4.0
m) size class were found during the July 2006, January 2007,
August 2007, August 2009, February 2010 or August 2010
surveys, whereas no crocodiles in the very large (>4.0 m)
size class were found in any of the surveys from 2001 to 2010
(Ta ble 1).
Although these sur veys failed to locate any crocodiles
in the over 4.0 m TL categor y, they did at least confir m
the presence of 3 crocodiles in the 2.1-4.0 m size class
during 2005. By July 2006, these large crocodiles also
disappeared from the dam (Table 1). The current croco-
dile population density in the Loskop Dam is very low at
0.06 crocodiles/km of shoreline (Table 1). The standard
deviation of the population density is 0.06 (Table 1), indi-
cating that over the time period of this study, population
numbers did not deviate widely from the mean density
and constantly remain at a low level. During the 2001
and 2005 aerial sur veys, as well as the 2006, 2007, 2009
and 2010 spotlight surveys, crocodiles were found in the
Olifants River around the western inlets of the dam and
also in the area of the eastern inlets of the dam at the
Kranspoortspruit and Scheepersloop areas (Fig. 1).
Tab l e 1
Summar y of the numbers, size distribution and density (crocs/km) of Nile crocodiles in the Loskop Dam, Olifants River,
Mpumalanga province
Survey year Type of survey Size class Tot a l
number
Estimated
number
Number
re-
introduced
Density
(crocodiles/
km)
< 1.4 m 1. 4 -2 .1
m
2.1 - 4 .0
m
>4.0 m Unsure
1981#Aerial survey 0 2 3 1 0 6 9 0 0.09
20 01 Aerial survey 10900 10 15 00.14
December 2005 Aerial survey 2 1 3 0 0 6 9 0 0.09
July 2006 Spotlight survey 7 1 0 0 0 8 12 00.11
January 2007 Spotlight survey 6 4 0 0 2 12 18 00.17
August 2007 Spotlight survey 7 7 0 0 2 16 25 13 0.23
August 2009 Spotlight survey 2 7 0 0 1 10 15 00.14
Feb r uar y 2010 Spotlight survey 1 3 0 0 0 4 6 0 0.06
Augu st 2 010 Spotlight survey 0 2 0 0 2 4 6 0 0.06
Total of all surveys 26 27 15 1 7 76 115 13 -
Mean of all sur veys 3 3 2 0 1 8 13 -0.12
Standard deviation 2.93 2. 55 3.04 0.33 0.97 3.97 6.20 -0.06
# = Jacobsen (1984)

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The 8 sur veys conducted since 2001 revealed that the
distribution pattern of crocodiles in the Loskop Dam did
not vary between winter and summer. Thirteen crocodiles,
all between 1.09 and 1.86 m TL, were re-introduced during
March 2007. This caused an articial increase in the num-
ber of crocodiles present in the dam during the August 2007
spotlight survey, but the results of the August 2009, February
2010 and August 2010 spotlight surveys clearly show that
none of these reint roduced animals survived longer than 2
ye a rs ( Ta ble 1).
The 1981 population structure reported by Jacobsen
(1984) indicates that the segment of the population consisting
of small- and medium-sized crocodiles (all crocodiles less
than 2.1 m TL) was smaller in number than the large size class
which consists of crocodiles between 2.1 and 4.0 m TL (Table
1). Our data indicate that in 2001 and 2005 no crocodiles in
the over 4.0 m TL size class were present in the Loskop Dam
population and this size class was still absent during the 2006,
2007, 2009 and 2010 surveys (Table 1). However, the results
of the 2006, 2007, 2009 and 2010 sur veys also indicated that
all crocodiles in the 2.1-4.0 m TL size class were also absent
from this population, leaving only the small and medium size
classes intact and suggesting that either some recruitment
(e.g. reproduction had occurred in the previous few years) had
taken place or that younger animals had immigrated into the
dam from upstream. This has created a highly skewed popu-
lation str ucture which differs substantially from that expected
in a healthy crocodilian population.
Discussion
Crocodilian populations have been studied worldwide, as
most wild populations today are under serious threat due
to habitat loss, degradation and pollution (Pooley, 1969;
Jacobsen, 1988; Ross, 1998; Campbell, 2006; MacGregor,
2006). Although these large predators are perceived by
many as non-essential members of healthy aquatic eco-
systems, they are act ually keystone and sentinel species
that can provide information on the long-term health of an
ecosystem. The data reported here clearly indicate that the
Nile crocodile population in the Loskop Dam is not healthy,
nor has it been for decades. The underlying reasons for
this are still not fully understood and require fur ther inves-
tigation, but our data have established that the population
structure and distribution of these animals is not normal. It
is important to recognize that, for these long-lived organ-
isms, 1 or 2 surveys of a population cannot easily predict the
health of that population, but the combination of this survey
data with that of earlier studies can be conclusive and dem-
onstrates that the population at Loskop Dam is not healthy.
In turn, these data suggest that the Olifants R iver system
in general, of which this dam is but a part, also has major
problems that must be addressed. Finally, our data suggest
that if the underlying problem with the crocodile population
at Loskop Dam is more than just the loss of nesting sites,
and this seems reasonable given our data, then other aquatic
species in this ecosystem are also likely impacted, as croco-
dilians do serve as an effective sentinel species for other
vertebrates.
The low total number of individuals encountered dur-
ing the 2006 and 2009 surveys cor responds closely with
the numbers recorded during the 2001 and 2005 surveys.
More importantly, the low number of animals in general,
and in par ticular the numbers from 2006 which are virt ually
the same as the result of the 1981 survey (Jacobsen, 1984),
and the very low numbers from the 2010 surveys, indicate
that recr uitment via reproduction or im migration from the
Olifants River system into the dam has been almost nonex-
istent for decades. That is, population size has not changed
noticeably upward over the preceding 25 years and the
observed decline in population numbers over the long-term
is not a function of a naturally uctuating population or poor
census techniques.
Given the reint roduction of 3 crocodiles during 1983/1984
(Transvaal Nat ure Conservation Division, 1985), 6 crocodiles
during 1984/1985 (Transvaal Nature Conservation Division,
Figure 1
Distribution of Nile
crocodiles in the
Loskop Dam during
aerial and spotlight
surveys completed
between 20 01 and
2010. Inset shows
the location of the
map area within
South Africa.

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106
1986) and 13 crocodiles in 2007, the population should
have shown at least some increase over the 25 year period.
Moreover, given that these were adults introduced into the
population, one would expect some population increase in
the small size class animals if recruitment via reproduction
was occurring. However, the survey results indicate that the
population did not increase, even though a few smaller ani-
mals are now seen. In short, recruitment of this population is
not overcoming the death or emigration rate.
The mean population density of the cur rent crocodile
population in Loskop Dam equates to 0.12 crocodiles/km
of shoreline. However, the entire shoreline is not good
habitat for basking, nesting and other behaviours. In con-
trast, a much greater proportion of the shore provided good
habitat in earlier years, prior to the dam wall being raised
in 1979. Historical records show that human settlement
of the area where the dam is located started as long ago
as 1886 (Loskop Irrigation Board, 2009), when crocodiles
were abundant along the entire Olifants River. In spite of
the experimental re-introduction of 13 adult animals into
the population during 2007, the overall density trend seems
to indicate that the population has remained at a very low
level since 1981. The standard deviation for the population
density from all surveys is 0.06 indicating that the annual
population density gures do not deviate much from the
mean value. Therefore, although the total counts uctuate
from year to year, the population density remains at a ver y
low level. The low density of crocodiles in Loskop Dam has
been an ongoing issue for over 25 years. When compared to
other crocodilian populations in similar habitats (i.e. liv-
ing in water storage reservoirs or lakes), the low population
density of the Loskop Dam stands out as unique. The Flag
Boshielo Dam, situated 85 km downstream from the Loskop
Dam, on the Olifants River, has a density of 3.25 crocodiles/
km of shoreline (Botha, 2005). The Olifants River in the
Kruger National Park has a density of 3.98 crocodiles/km
of shoreline whereas the Olifants River Gorge in the lower
reaches of the Olifants River within the Kruger National
Park has a very high density of 30 crocodiles/km of shore-
line (Botha, 2005).
In addition to the stationary nature of the population,
no hatchling crocodiles (animals less than 1 year old) were
found during the 2006, 2007, 2009 or 2010 spotlight surveys
(no survey was performed in 2008). Recruitment into the
Loskop Dam crocodile population is therefore seriously
compromised. Only 1 animal in the >4.0 m size class was
reported during the 1981 survey and no animals in this
size class have been recorded during any of the subsequent
surveys. These very large animals are the dominant animals
necessary for normal competition, behaviour and success-
ful nesting in any population (Lang, 1987; Lang, 1990) and
it is possible that the absence of this size class in a wild
population will hamper that population’s chances of expand-
ing normally. The disappearance of the large animals from
the population is unlikely to be linked to the lack of nesting
areas or even food, which is plentif ul, because one would
then expect the small crocodiles to disappear rst because
no recruitment had taken place or because they could not
compete for food. The survey data (Table 1) reveal that
this did not happen; that is, the largest animals disappeared
from the population rst. A further concern is the lack of
animals observed in the 2.1-4.0 m TL size class during the
2006, 2007, 2009 and 2010 surveys. If, as we have noted,
the animals in the >4.0m TL size class are absent from
the population, then it is expected that the next size class,
those animals that are sexually mature and in the 2.1-4.0 m
TL class, will ‘stand in’ for the absent dominant animals.
However, if they too are now absent from the population,
which our data suggest, then the concern expressed by
Jacobsen (1984) would appear to be well-founded, and there
is a distinct possibility that this population will become
extinct.
The distribution pattern of the crocodiles in Loskop Dam
indicates that most of the crocodiles observed occurred in the
river-like area at the inlet of the Olifants River to the dam.
This is possibly due to the higher water levels, caused by
the raising of the dam wall, and which has rendered the vast
majority of all the other shoreline areas in the dam unsuit-
able for crocodiles. The impact of this is that the population
is now concentrated in those areas where the possible effects
of pollution from the inowing river are likely far worse
than anywhere else in the dam, due to the dilution effect of
the main water body in the dam (Oberholster et al., 2010).
Therefore the altered spatial distribution of crocodiles, caused
by the loss of suitable habitat as a result of the raising of the
dam wall, has placed them in an area where they would likely
experience higher levels of pollution than elsewhere in the
dam. As ingestion and drin king are likely the major routes
of exposure to contaminants in crocodilians, it is likely that
the crocodile’s food source, which inhabits this same area,
contributes towards placing them in areas at risk of pollu-
tion. This could very well be a critical factor in the episodes
of periodic die-off of crocodiles that have been witnessed in
Loskop Dam.
The results of 8 surveys carried out since 2001 show that
the distribution pat tern of crocodiles in Loskop Dam does
not vary between winter and summer periods. Distribution
patter ns and movements in crocodilian populations are usu-
ally associated with important population milestones, such as
the onset of mating and nesting during the summer months.
Denite seasonal distribution patterns are known to occur
in the larger Nile crocodile population of the Flag Boshielo
Dam downst ream from the Loskop Dam (Botha, 2005). The
total absence of any seasonal variation in the dist ribution of
the Loskop Dam crocodile population supports the hypothesis
that no crocodiles in the large and ver y large size class cur-
rently occur in the Loskop Dam. It also indicates that impor-
tant behaviour and population milestones do not now occur
in the Loskop Dam population, and indicate that this is an
unstable population, unlikely to survive if current conditions
persist into the fut ure.
Conclusion
The total number of Nile crocodiles in the Loskop Dam
has declined over the last 25 to 30 years. As importantly,
there are now no surviving large animals over 2.1 m TL in
the entire Loskop Dam population; this presents a crisis for
future breeding seasons as animals of this size represent the
sexually mature animals in the population. Surveys conrm
the complete absence of any dominant animals in the popu-
lation since at least 2001. The recorded crocodile die-off
events between 2005 and 2007 have further depleted the
Nile crocodile population in this aquatic system. Crocodile
mortalities in the Loskop Dam during this period have been
ascribed to pansteatitis, which is associated with the intake of
rancid sh after several sh dieoff events, which appears to
have resulted from cumulative impacts of acid mine drainage

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into the dam (Holman, 2008; Paton, 2008; Oberholster et
al., 2010). Water quality monitoring in the Olifants River
indicates that the quality of the water has deteriorated since
the 1970s as a result of industrial, mining and agricultural
activities (De Villiers and Mkwelo, 2009). Therefore, it is
likely that the current-day distribution pattern of crocodiles in
the river inow zone of Loskop Dam exposes them to a wide
array of contaminants and likely a more concentrated mix of
pollutants than they would experience in the main body of
the dam where pollutants would be more diluted. However,
it is important to note that dermal exposure is not the likely
route of contaminant intake; rather a diet of contaminated sh
and other vertebrates from this same ecosystem would con-
tribute to the contamination of this top predator. However,
this hypothesis must be veried by further study. It is clear
that the experimental re-introduction of crocodiles to the
population has failed to stabilise or cont ribute to its growth
and similar re-introductions should not be considered while
current conditions in Loskop Dam persist.
Acknowledgements
We thank the Mpumalanga Tourism and Parks Agency, the
Centre for Wildlife Management at the University of Pretoria
and Koos de Wet for assistance with this project. We thank
Peter Ashton of the CSIR for providing suggestions that
improved an early draft of this manuscript.
References
BAYLISS P, WEBB GJ W, WHITEHEAD PJ, DEMPSEY K and
SMITH A (1986) Estimating t he abundance of saltwater crocodiles,
Crocodylus porosus Schneider, in tidal wetlands of the Northern
Territory: a mark-recapture experi ment to cor rect spotlight counts
to absolute nu mbers and the calibration of helicopter and spotlight
counts. Au st. Wildl. Res. 13 309-32 0.
BAYLISS P (1987) Survey methods and mon itoring withi n croco -
dile management prog rammes. In: Webb GJW, Manolis SC
and Whitehead PJ (eds.) Wildlife Management: Crocodiles and
Alligators. Surrey Beatty and Sons, Chipping Nor ton.
BOTHA PJ (2005) The ecology and population dynamics of the Nile
crocodile Crocodylus niloticus in the Flag Boshielo Dam, Mpuma-
langa province, South Africa. M.Sc. Thesis. Universit y of Pretoria.
CAMPBELL TW (2006) Clinical patholog y of reptiles. In: Mader DR
(ed .) Reptile Medicine and Surgery (2nd edn.). Saunders Elsevier, St
Louis.
COTT HB (1961) Scientic results of an inquiry into the ecology and eco-
nomic status of the Nile crocodile (Crocodylus niloticus) in Uganda
and Northern R hodesia. Proc. Zool. Soc. London 29 211-25 6.
DE VILLIERS S and MKWELO ST (2009) Has monitori ng failed the
Olifants River, Mpumalanga? Water SA 35 (5) 671-676.
GAMES I (1990) Growth curves for the Nile crocodile as estimated
by skeleton-chronology. Proc. 10th Working Group Meeting of the
Crocodile Specialist Group of the Species Sur vival Commission of
IUCN. IUCN. Gland, Switzerland.
GRAH AM A (1968) The Lake Rudolf crocodile (Crocodylus niloti-
cus Laurenti) population. Unpublished Report, Kenya Game
Depar tment, Nairobi.
GUILLETTE LJ Jr. and CR AIN DA (eds.) (2000) Endocrine Dis-
rupting Contaminants: An Evolutionary Perspective. Taylor and
Francis, Inc., Philadelphia.
HOLMAN J (2008) Coal giant recognises water management responsi-
bility. Engineering News Online, 18 July 2008. URL: http://www.
engineeringnews.co.za (Accessed 14 April 2010).
HUTTON JM (1984) The population ecology of the Nile crocodile,
Crocodylus niloticus Laurenti, 1768, at Ngezi, Zimbabwe. Ph.D.
Thesis, University of Zimbabwe, Harare.
HUTTON JM and WOOLHOUSE MEJ (1989) Mark-recapture to
assess factors af fecting the proportion of a Nile crocodile popula-
tion seen during spotlight counts at Ngezi, Zimbabwe and the use
of spotlight counts to monitor crocodile abundance. J. Appl. Ecol.
26 381-395.
JACOBSEN NHG (1984) The distribution and status of crocodile
populations in the Transvaal outside the K ruger National Park.
Biol. Conserv. 29 191-2 00 .
JACOBSEN NHG (1988) The Nile crocodile. In: Branch WR (ed.)
South African red data book – reptiles and amphibians. South
African National Scientic Report No. 151. CSIR, Pretoria.
JOANEN T and McNEASE L (1975) Notes on the reproductive biol-
ogy and captive propagation of the American alligator. Proc. Ann.
Conf. Southeastern Assoc. Game and Fish Comm. 29 4 07 - 415.
KATSU Y, MYBURGH J, KOHNO S, SWAN G E, GUILLETTE LJ
Jr. and IGUCHI T (2006) Molecular cloning of estrogen receptor α
of the Nile crocodile. Comp. Biochem. Physiol. Part A: Mol. Integr.
Physiol. 143 340-346.
LANG JW (1987) Crocodilian behaviour: Implications for manage-
ment. In: Webb GJW, Manolis SC and Whitehead PJ (eds.) Wildlife
Management: Crocodiles and Alligators. Surrey Beatty and Son s,
Chipping Norton.
LANG JW (1990) Social behaviour. In: Ross CA (ed.) Crocodiles and
Alligators. Merehurst Press, London.
LOSKOP IRRIGATION BOARD (2009) Loskop water scheme.
Loskop Ir rigation Board, Groblersdal. UR L: http://www.loskopbe-
sproeingsraad.co.za (Accessed 2 Apr il 2009).
MACGREGOR J (2006) The call of the wild: Capt ive crocod ilian
production and the shaping of conservation incentives. TR AFFIC
Inter national, Cambridge.
MAGNUSSON WE, CAUGHLEY GJ and GRIGG GC (1978) A
double-su rvey estimate of population size from incomplete counts.
J. Wildl. Manage. 42 (1) 17 4 -176 .
MAGNUSSON WE, VLIET K A, POOLEY AC and WHITAK ER R
(1990) Reproduction. I n: Ross CA (ed.) Crocodiles and Alligators.
Merehurst Press, London.
MILNES MR, BER MUDEZ DS, BRYAN TA, EDWARDS TM,
GUNDERSON M P, LARKI N IV, MOORE BC and GUI LLETTE
LJ Jr. (2006) Contamina nt-induced feminization and demasculin i-
zation of nonmammalian ver tebrate males in aquatic environ ments.
Environ. Res. 100 3 -17.
MILNES MR and GUILLETTE LJ Jr. (2008) Alligator tales: New
lessons about environmental contaminants from a sentinel species.
Biosci. 58 (11) 1027-1036.
OBERHOLSTER PJ, MYBU RGH JG, ASHTON PJ and BOTHA A-M
(2010) Responses of phytoplankton upon exposure to a mixt ure of
acid mine drainage and hig h levels of nutrient pollution in Lake
Loskop, South A frica. Ecotoxicol. Environ. Saf. 73 (1) 326-335.
PATON C (2008) Dam dirt y. Financial Mail 28 November 2008.
32-39.
PHELPS RJ, FOCARDI S, FOSSI C, LEONZIO C and RENZONI A
(1986) Clorinated hydrocarbons and heavy metals in crocodile eggs
from Zimbabwe. Trans. Zimb. Sci . Assoc. 63 8 -15.
POOLEY AC (1969) Preliminary st udies on the breed ing of the Nile
crocodile Crocodylus niloticus in Zululand. Lammergeyer 10
22-44.
ROSS JP (ed.) (1998) Crocodiles. Status Sur vey and Conser vation
Action Plan (2nd edn.). IUCN/SSC Crocodile Specialist Group.
IUCN, Gland, Switzerland.
SKAA RE J U, INGEBRIGTSEN K , AULIE A and K ANUI TI (1991)
Organochlorines in crocodile eggs from Kenya. Bull. Environ.
Contam. Toxicol. 47 12 6-130.
STIRR AT SC, LAWSON D, FREELAND WJ a nd MORTON R (2001)
Monitoring Crocodylus porosus populations in the Northern
Territory of Australia: a retrospect ive analysis. Wildl. Res. 28
547-554.
SWANEPOEL DGJ (2001) The raising of the A rabie Dam wall and
the impacts on the Nile crocodile population. Unpublished Repor t,
Depar tment of Water Affairs and Forestry, Pretor ia, South Africa.
TRANSVAAL NATURE CONSERVATION DIVISION (1985)
Nineteenth Annual Report: 1983/84. Transvaal Provincial
Administration, Pretoria.

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108
TRANSVAAL NATURE CONSERVATION DIVISION (1986)
Twentieth Annual Repor t: 1984/85. Transvaal Provincial
Administration, Pretoria.
WEBB GJW and MESSEL H (1979) Wariness in Crocodylus porosus.
Aust. Wildl. Res. 6 227-234.
WHIT WORTH J (1971) Notes on the growth and mating of A merica n
alligators Alligator mississippiensis at the Ca nnon Aquarium,
Manchester Museum. Int. Zoo Yearbook 11 14 4.
WOODWARD A R and MOORE CT (1993) Use of crocodilian n ight
count data for population trend estimation. Proc. 2nd Regional
Conference of the Crocodile Specialist Group, Species Sur vival
Commission, IUCN, 12-13 March 1993, Darwin, Australia.