CHAPTER 1 General Introduction
2.4 RESULTS
as for the respective sub-regions within these systems from 1971-2009. This was done to determine any changes in habitat use by the NGR Nile Crocodile population over the last 40 years.
2.3.3.2 Lake Nyamithi
Due to the non-independence of variables we used regression analysis to check for significant influences of continuous environmental variables (water level, temperature, rainfall) on the density of Nile Crocodiles in Lake Nyamithi.
each system remained relatively constant (but not significantly so) over consecutive count years (ANOVA F(10, 12) = 0.00, p = 1.00) despite significant increases in population size in each system from 1971-2009 (Fig. 4).
The proportion of crocodiles counted within each system (Phongolo and Usuthu) remained (significantly) constant (ANOVA F(1, 22) = 160.28, p = 0.00) from 1985 – 2009 and was unrelated to changes in total population size (ANOVA F(11, 12) = 0.00, p = 1.00) during this period. That is to say, despite successive population increases in each system the proportion of crocodiles counted in each system remained much the same. The only changes in the proportion of crocodile counted in each system did occur at times of low crocodile density (1971-1973), and when one river system experienced flooding while the other did not (1989; DWAF station WH4009 11/08/1989, Fig. 4). This is thought to be due to a) NGR having a low density of crocodiles in the early 1970‟s and one system (Phongola) being favored by crocodiles over the other (Usuthu) and b) due to the disparate water levels biasing the results of counts conducted in 1989.
Excluding the counts of the 1970‟s, the Phongola system on average accounted for 79%
of the NGR Nile Crocodile population and the Usuthu system for 21% from 1985 – 2009 (Fig 4).
2.4.2 Distribution of abundance of Nile Crocodiles within river systems 2.4.2.1 Usuthu River system
Historically Lake Banzi and Shokwe Pan (Fig. 5) accounted for the majority of the Usuthu crocodile population with less than 1% found in the Usuthu River over the last 25 years.
The same pattern is visible currently with Banzi having around 75% of the population and the remaining 25% in Shokwe. This relationship has persisted since 1988 to the present with Banzi
having a significantly greater proportion of crocodile over the years (ANOVA F(2, 33) = 50.76, p = 0.00). The Usuthu system is therefore made up of two primary components, Lake Banzi and Shokwe Pan. Significantly more Nile Crocodiles were counted in Lake Banzi than in Shokwe and the Usuthu River (ANOVA F(2, 33) = 15.14, p = 0.0002). The Usuthu is a wide, shallow river with many sandbanks and is therefore not prone to survey bias. The low number of crocodiles counted here is most likely a function of habitat suitability. The northern bank of the river borders on Mozambique and is not protected, possibly elevating human disturbance to levels unsuitable for crocodiles. However, in order for crocodile to be present in Shokwe they almost certainly have to use the Usuthu River and thus it plays an important role in connecting the different habitats available to Nile Crocodiles in the Usuthu river system.
2.4.2.2 Phongola River System
Since 1971 the majority (61%) of Nile Crocodiles in the Phongola river system were found in the Phongola floodplain during the winter period when the surveys took place. There is a significant difference in the number of Nile Crocodiles counted in the Phongola floodplain compared with Lake Nyamithi from 1971-2009 (ANOVA F(1, 22) = 6.34, p = 0.02). Despite successive, and often dramatic population increases, the proportion of crocodiles counted in the Phongola floodplain and Lake Nyamithi remained relatively constant and distinct from one another (ANOVA F(1, 22)= 17.30, p = 0.0004, Fig. 6). Unusually, in 2009 the majority (69%) of crocodiles were counted in Lake Nyamithi.
2.4.2.3 Lake Nyamithi
Although the Phongola floodplain contained more crocodiles than Lake Nyamithi during the winter surveys, Lake Nyamithi held a greater density of Nile Crocodiles than the floodplain.
Generally Nile Crocodiles moved from the floodplain and back into Lake Nyamithi as water levels in the floodplain dissipated and temperatures decreased with the onset of winter (Fig. 8.).
Numbers of crocodiles at Lake Nyamithi climb steadily from an average of 61 (SE ± 5.5) in March to a peak of 306 (SE ± 13.5) in July. Numbers then oscillated around 273 (SE ± 13.5) in August dropping to 192 (SE ± 24.3) in September 2009 and to 61 (SE ± 1.4) by December.
Total crocodile density in Lake Nyamithi was significantly and negatively related to average monthly ambient temperature (GLIM ANOVA F4, 25 = 18.5, p = 0.006) (Fig. 10).
Rainfall and water level had strong negative effects on the total number of crocodiles in Lake Nyamithi but these effects were not significant. However, Nile Crocodiles show differential habitat use according to size (TL) (Cott and Pooley, 1971; Botha, 2005; Radloff et al., 2012a;b) and it is necessary to delineate the population into various size classes (Swanepoel, 1999) in order to identify environmental variables specific to each size class (Radloff et al., 2012a).
None of the environmental variables measured had a significant influence on the number of Nile Crocodiles counted in the < 1.5 m; 2.5 – 3.5 m; 3.5 – 4.5 m and 4.5 m+ size classes in Lake Nyamithi. Environmental variables had a significant effect on the number of crocodiles counted in the 1.5 - 2.5 m size class only. Minimum average monthly temperature and rainfall both had a significant and negative effect on the number of crocodile between 1.5 - 2.5 m TL counted over the 2 years. A summary of these results is presented in Table 1. Although not always significant, environmental variables had varying impacts on the number of Nile Crocodiles counted in each size class and are discussed below in greater detail.