Research report on the Nile Soft shell turtle (Trionyx triunguis)
in Nahal Alexander By Oren Rozner
Supervisor: Dr. Uri Shaines, Dept. Biology and Environmental Evolution, Haifa
University
.
Introduction
The soft turtles (Trionychidae) are a family of freshwater turtles that live mostly in
shallow waters, including streams and swamps. Their shell is flat, covered by epidermis
and with soft edges. The species found in Israel, the Nile soft shell turtle (Trionyx
triunguis), is widespread in Africa, and was also common in the coastal streams of the
Mediterranean Sea. Its characteristics with regard to conservation are as follows.
Global Threat: CR (C2a) – Critically Endangered
Regional Threat: CR (C2a) – Critically Endangered
Range: Tropical water systems in Africa, including the Nile. Coastal rivers along the
eastern Mediterranean coast, up to southern Turkey. The global threat category recorded
here relates only to the Mediterranean region. In Israel the species inhabits coastal streams
and rivers – Yarqon, Poleg, Alexander, Tanninim, Daliyya, Qishon, and Na’aman – and is
also found in the Hula Nature Reserve.
Historic Range: The species did not inhabit the Jordan River basin in the past: the Hula
Reserve population, whose size is unknown, was founded by Prof. Mendelssohn in the
1960s with turtles bred at the Tel-Aviv University Zoological Garden.
Typical Habitat: Freshwater and brackish streams and rivers. Egg deposition is on
riverbanks, in both sandy and heavy soils.
Threat and Disturbance Factors:
1. Over-utilization and partial drying up of streams in the Coastal Plain.
2. Illegal hunting, particularly by Thai workers.
3. Egg and juvenile predation by natural predators and stray dogs.
Population Size: The largest population known in Israel is in Nahal Alexander, and
comprises circa 50 turtles. This is a remnant of a larger population that throv in the
stream, but was harmed by floods in the winter of 1991-2, when more than 200 turtles
were injured and died. In the past, the population received artificial food supplements
(turkey carcasses), but this activity was stopped for veterinary reasons. Smaller
populations, with only a few turtles, can be found in other Coastal Plain streams.
Fluctuations in Population Size: African Softshells are known to breed in Nahal
Alexander, in the Hula Reserve and in Nahal Na’aman, but the growth rate of these
populations is unknown. The floods in Nahal Alexander in the winter of 1991-2 reduced
the stream’s population by about 80. [cf. 200, at top of page – Ed]
Isolation between sub-populations: The Coastal Plain river populations are isolated
from each other, but turtles can pass between them via the sea. The Hula Reserve
population is isolated.
1
Necessary Steps for Species Preservation:
Preventing illegal hunting.
Protecting nesting sites from predators.
Basic research on the life history of the African softshell, in order to determine
disturbance factors and necessary conservation steps.
Research objectives
1. Demography: Estimation of population size and composition: (breeding relation, age
distribution). Clarifying the genetic variability within the Nahal Alexander population
and, if possible, between the different populations of Israel and Turkey.
2. Movements: Habitat of the individual in the sea, and if possible, inland. Movements of
individual among sub-populations.
3. Fertility. Egg fertility ratio and its correlation with physical nest conditions in its
original place: humidity, temperature, depth of laying, soil composition. Determination of
correlation of fertility with individual female by DNA analysis.
Preliminary results
Shell mass and lenght
1.
Demography. The total number of eggs in the stream was estimated by counting
the number of eggs observed during the survey. Most of the eggs, and sometimes all, are
observed in “Eggs Bridge”. The maximum number of eggs counted was 53 (August,
2004). In additional, the number of turtles was estimated by capture, tagging, releasing
and recapture (capture-recapture). Until November 2004, 24 specimens were captured and
tagged. One individual was captured by chance by fishermen. Most of the turtles captured
were relative large: shell length from 66 - 79 cm, weight from 15.5 - 45 kg. (Fig. 1)
100.0
80.0
Shell mass
Shell length
60.0
40.0
20.0
W
0.0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Turtle number
Figure 1: Shell length and weight of 24 turtles captured in Nahal Alexander.
2
Atatur (1979) suggests differentiating between males and females by measuring tail
length, in the part from the plasteron to the cloacae, a section that is presumed to be
longer in males than in females. The tail was measured in all the exemplars trapped (Fig.
2) and the results showed correlation (Fig. 3). However, if the data are corrected by
deducting the size difference among the exemplars by checking the difference in the
length of the plastron to cloacae section in relation to the total tail length, the correlation
disappears (Fig. 4).
Figure 2: Measuring the tail length from the plasteron in a Nile Softshell turtle
trapped in Nahal Alexander.
Plastron to cloacae
length
30.0
25.0
20.0
15.0
10.0
5.0
25
23
21
19
17
15
13
11
9
7
5
3
1
0.0
Turtle number
Figure 3: Tail length (cm), from plastron to cloacae, in specimens with live weight
above 15 kg.
3
Plastron to cloacae
length / total tail length
1.00
0.80
0.60
0.40
0.20
0.00
1
3
5
7
9 11 13 15 17 19 21 23 25 27 29
Turtle number
Figure 4: Tail length (cm), from plastron to cloacae, in relation to the total length of
the tail, in specimens weighing above 15 kg.
2.
Movements. The movements inside the living area of the specimens in the
stream, changes in the movement model (for instance in relation to the seasons),
exemplars movement among the streams and in possible also overseas was checked by
attaching telemetric devices to five specimens. The devices were purchased from AVM,
the manufacturer.
3.
Fertility. During the summer, 2004, laying season a total of 13 nests was
identified and protected, in three locations along the Nahal Alexander. An additional nest
was discovered on the beach, an was transferred to the “Hofit” protected location. Nest
protection: out of 14 nests, 8 were predated by foxes and 6 survived. All the nests in
locations “Ha Dekel” and “Hofit” were predated, whereas in location “By the Stream” all
the nests survived. At the beginning, protection consisted of a metal net, with 7 x 5 cm
holes, in a form similar to a hat with a wide brim (width 20 cm). The “hat brims” were
buried in the soil to prevent digging by the predators (Fig. 5).
Figure 5: Nest protection broken through by foxes
4
After the failure of the first protection system, a new form was tried. An iron net sized 1 x
1 m was placed on the nest and covered by 5 cm sand. The net was fixed to the sand by Ushaped pegs with 40 cm long “legs”, the distance between the legs was 40 cm and the pegs
were places at 10 cm intervals. (Fig. 6)
Figure 6: Nest protected by an iron net with pegs.
In some of the nests, the protection was improved by adding a “hat” in the middle
of the net to attract the predator to the part most strongly protected, by this system as the
simple net, filed both. In addition, “Ha Dekel” and “Hofit” were fenced with two types of
net, with 10 x 10 and 7 x 5 holes. The sector with the larger holed net was fenced with
double layer of netting, to a height of 1 m, and with a 20 cm wide border buried in the
soil. At the fence top, the net was bent outward to prevent the predator climbing it.
However, all the systems failed to prevent predation, and according to tracks found, the
foxed climbed the fences.
No predation was registered in the “By the stream” location and no predator
activity was found there, despite the fact that predators have been observed in the area.
Nest temperature measurements.
When a nest was discovered, a digital thermometer was introduced into it and the
temperature was recorded twice a week, in the morning. The temperature measured inside
the nests was in the 26.8 - 32.5º range whereas the area temperature range was recorded
at 26.8 – 32.5ºC.
.
5
Temperature (°C)
40
35
30
25
20
15
10
5
0
Nest
Ambient
MIN
MAX
Fig. 7: Temperatures measured inside the nests and in the area.
Most of the measurement was conducted at the time when eggs were counted, between
the 0630 and 0900. The use of automatic data loggers is planned for 2005.
40
Temperature in °C
35
30
25
20
15
10
Nest
Ambient
5
0
1
4
7
10 13 16 19 22 25 28 31 34 37
Measurement number
Figure 8: Temperatures inside and outside the nest.
Humidity measurements in the nests.
As each nest was discovered, a soil sample was taken to measure the soil humidity.
Additional samples were taken from two nests, during the laying season, at nest opening,
and at the end of the laying season. The soil samples were dried at 60°C for a number of
days. The soil was weighed before and after the drying, and the data were used to
determine the percentage of fluids in the soil. The relative humidity recorded fluctuated
between1 and 21%. A total of 10 samples were taken into account for the calculations, to
compare the humidity at lying time and after hatching.
6
18%
Humidity in percentage
16%
14%
Eggs lying
Juvenile emergence
12%
10%
8%
6%
4%
2%
0%
Nest 45
Nest 98
Nest code
Figure 9: Humidity percentage at two nests, immediately after laying and after
juvenile emergence.
Emergence data
At the end of the estimated hatching period, the nests were opened. The results are
presented in Fig. 10. In the entire nest no developed (atrophied) eggs were found. May be
that part of the eggs were damaged in the process of finding the nests, and may be that
the broken eggs disturbed the development of the other eggs and the capability of the
juveniles to escape from the nest. The percentage of living juveniles reached 50%. And if
the sterile nest is not taken in account, the success % is 60%. The success % in the
previous year was: 2003, 75% -2002, 41%- 2000- 29%. In a survey conducted at Turkey
the success % was 69% (Gidis and Kaska, 2004).
.
Observations conducted in juvenile turtles in the Hula Nature Reserve.
A total of 26 juvenile turtles that emerged in the Hula Nature Reserve between 14
Aug. 2004 and 15 Aug. 2004 were measured on 15/8/04. They were released the same
day in the Nahal Sorek Stream.
Parameter Weight
(g)
Average
S.D
13.62
1.90
Visible Tailrests of plastron
yolk
(tail A)
(cm)
38%
1.25
0.14
Figure 10: One day old juvenile measurements.
7
Tailcloacae
(tail B)
(cm)
2.27
0.47
Shell
width
(CW)
(cm)
4.54
0.26
Shell
length
(CL)
(cm)
5.15
0.25
Contamination: After the nests were opened, egg samples were sent to the
laboratory for analysis, to test for contamination by heavy metals.
References
1. Atatür, M.K. (1979). Investigation on the morphology and osteology, biotope and
distribution in Anatolia of T. triunguis (Reptilia, Testudines), with some
observations on its biology (in Turkish). Ege Universities Fen Fakültesi
Monograf. Izmir: 1-75.
2. Pritchard, P.C.H. (2001). Observations on body size, sympathy, and niche
divergence in softshell turtles (Trionychidae). Chelonian Conservation and
Biology 4: 5-27.
3. Gidis, M. Kaska, Y. (2004). Population size, reproductive ecology and heavy
metal levels in eggshells of the Nile Soft Shelled turtle (Trionyx triunguis) around
thermal Lake Kukurtlu (Sulphorous), Mugla-Trurkey. Fesenius Environmental
Bulletin 13: 5. 405-412.
8