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Introduction
Catfish of the family Clariidae are widespread in freshwaters of tropical and subtropical Africa and Asia (Na-Nakorn & Brumment, 2009). Economically important catfish species Clarias fuscus (Siluriformes: Clariidae) is a much valued species for consumption in many countries, especially in South China, where it is frequently consumed. The fish species occurs across South China due to its ease of spawning, tolerance of low dissolved oxygen and high stocking density and it is regarded as the most edible catfish species (Qin et al.,1998; Zheng et al.,1988). Despite its merits for aquaculture, it harbours a number of deadly pathogens, including viruses, bacteria and parasites. Monogeneans are common pathogens which occur naturally in an open culture systems (Bakke et al., 2002). Monogenea are hermaphroditic ectoparasites of aquatic vertebrates, predominantly fish where they can be found on the skin and gills (Euzet & Combes, 1998). There is an equilibrium between host and parasite and monogenean parasites cause few obvious problems in nature. However, these parasites can reach very significant prevalence in captivity, such that the number of affected fish and the parasite load per host are both increased, which makes the disease evident and severe (Thoney & Hargis, 1991). C. fuscus is abundant in the Pearl River, China (Lu, 1990) and frequented by Quadriacanthus kobinensis on the gills. The occurrence of monogenean parasites have previously been shown to exhibited significant seasonal variations and peak period (high prevalence and intensity) is the most harmful period (Xia & Wang, 1997). Research on infection dynamics of C. fuscus in the Pearl River is necessary to determine intervention measures to reduce severe losses in farming conditions. The results presented here stems from a survey on seasonality of Q. kobinensis in C. fuscus under natural conditions in the Pearl River.
Materials and Methods
Experimental design and host collection
In the Pearl River basin, spring lasts from March to May, summer from June to August, autumn from September to November, and winter from December to February of the following year. Fish samples were collected monthly from June 2012 to May 2013 with gill nets in the lower reaches of the Pearl River between latitudes and longitudes 23°02′N, 23°04′N and 113°18′E, 113°24′E. Up to 30 fish were collected and transferred live to the laboratory. The fish were measured and weighed; they ranged from 2 to 34 cm in total length and,1 – 470 g in weight. The river water surface temperature were measured daily
Parasite examination
The gills were excised immediately after fish killed by severing the spinal cord anterior to the dorsal fin and examined for gill parasites using a dissection microscope in separate Petri dishes containing water from the tap. The epithelium surface of the gill filaments were scraped with a needle and the scrapings were collected in a beaker filled with water. The contents was then stirred, allowed to settle and the supernatant decanted. This procedure was repeated until a clear suspension was obtained Sediments were examined with a dissection microscope for parasites For identification purposes, monogeneans were fixed on a slide either with a drop of Malmberg solution (ammonium picrate glycerine) or glycerine jelly. The water surface temperature were measured daily. The host fishes were divided into six size groups based on their body length (L), L≤20 cm, 20< L ≤22 cm, 22< L ≤24 cm, 24< L ≤26 cm, 26< L ≤28 cm and L > 28 cm to determine whether a correlation existed between fish length and the infection levels.
Statistical analyses
Prevalence, intensity, mean intensity and abundance of parasites were calculated as defined by Margolis et al. (1982). The significance test for seasonal changes in abundance, mean intensity, mean length of parasites and mean length of fish was carried out using one-way ANOVA (Bryman & Duncan, 1997). Statistics for each season of Q. Kobinensis includes variance-mean ratio S2/X, parameter of negative binomial distribution K, average degree of congestion M*, and diffusivity index I∮. T-test was used to detect the significance of different host size classes in the infecting effect of Q. kobinensis on the hosts. All statistical analyses were executed using SPSS 17.0 (SPSS Inc., Chicago, Illinois) at a significance level of 0.05.
Results
Seasonal variations in populations of Q. Kobinensis
A total of 6041 specimens of Q. kobinensis were collected from 341 individuals of C. fuscus between June 2012 and May 2013 (Table 1). The prevalence, intensity and other parameters related to Q. kobinensis infection are shown in Table 1. Prevalence values were above 50 % in all seasons and show a indistinct seasonal trend. Prevalence values were above 50 % in all seasons and show a indistinct seasonal trend. Prevalence value was highest (70.13 %) in the summer and lowest (53.26 %) in autumn. Mean intensity reached a peak in autumn (43.2) and its lowest level in spring (11.6), it somewhat increased in summer, but then decreased in winter. The maximum number of Q. kobinensis was 474 whereas the maximum values for abundance were recorded during summer (25.8) and the minimum values occurred during spring (7.3).
Infection indices of Q. kobinensis in Clarias fuscus during different seasons in Pearl River, China
mean water temp. (°C)
No. of fish sampled
Length of fish (Mean ± SE) (cm)
Prevalence (%)
No. of parasites
Mean Intensity (SE) Max
Abundance(SE)
Spring 19.8
91
22.1 ± 0.3
62.64
661
11.6(2.1)69
7.3(1.4)
13.0 – 30.0
Summer 30.7
77
23.2 ± 0.4
70.13
1984
36.7 (7.7)
221 25.8 (5.7)
15.1 – 31.9
Autumn 24.4
92
22.9 ± 0.5
53.26
2115
43.2 (11.7)
474 23.0 (6.6)
13.7 – 32.5
Winter 15.6
81
22.3 ± 0.3
60.49
1281
26.1 (5.9) 265
15.8 (3.8)
14.6 – 29.8
Aggregation Index of Q. kobinensis in host
The diffusion index (I∮) means crowding degree (M*) and variance-to-mean ratio (S2/X) were greater than 1, and the results showed that Q. kobinensis was in aggregated distribution in each season. Values for the negative binomial parameter (K) differ among seasons and showed different aggregation intensities. The population distribution pattern of Q. kobinensis in C. fuscus is shown in Table 2. Table 3 shows that most hosts across all seasons is only infected by a small number of Q. kobinensis. In contrast, a small number of host were infected by large amounts of Q. kobinensis.
The seasonal fluctuation of aggregation indices of Q. kobinensis in host
Infection intensity
S2
S2/X
K
M*
I∮
Spring
0 – 69
189.69
26.13
0.29
32.39
4.42
Summer
0 – 221
2512.97
97.52
0.27
122.29
4.70
Autumn
0 – 474
3999.04
173.95
0.13
195.94
8.45
Winter
0 – 265
1192.03
75.40
0.21
90.21
5.65
The seasonal frequency distribution indices of Q. kobinensis in host
Frequency indices
No. of Q. kobinensis
Spring
Summer
Autumn
Winter
0
38.71
29.87
46.74
39.51
1 – 4
31.18
27.27
19.57
14.81
5 – 8
7.53
6.49
4.35
8.64
9 – 12
3.23
6.49
4.35
3.70
13 – 16
6.45
3.90
3.26
6.17
17 – 20
3.23
1.30
1.09
4.94
>20
9.68
24.68
20.65
22.22
Significance test of infection intensity among the different host body length groups of Q. kobinensis
The highest prevalence occurred in 28 cm<L fish, and infection intensity was also the highest in this size class. Results of the significance test for infection intensity is provided in table. 4. In terms of infection intensities between size classes, hosts of 28 cm < L differed significantly from 24<L≤26 cm, but no significant differences existed between the other groups.
Discussion
Monogeneans prefer fishes, amphibians, reptiles and mammalians has host (Pan et. al., 1990), and occur especially on gills of fishes. Monogenea has a high degree of host specificity (Whittington 1998). A total of 4 monogenean species, Clariotrema austrofujianensis, Clariotrema meridionalis, Q. kobinensis and Gyrodactylus fusci have previously been reported on the gills of C. fuscus in natural systems in China (Zhang, 1984). However, only Q. kobinensis and G. fusci were found on the gills of C. fuscus in our investigation, the result confirms that of Lim (1998) in Southeast Asia. We haven′t calculated the statistical parameters for G. fusci because Q. kobinensis was absolutely predominant .There were only 93 specimens of G. fusci collected and it was far less than the number of Q. kobinensis. The seasonal population dynamics of a monogenean is related to complex environmental parameters especially water temperature (Chubb,1977) as e.g., MO (1992) found Gyrodactylus salaris through most of the year except 2-3 months when temperature had fallen to almost 0 °C; Jansen & Bakke (1993) found the infra populations of G. salaris increased markedly on all salmon parr with the spring rise in water temperature and lower temperatures in winter affect the population growth of the parasite negatively; Yang et. al. (2006) found infection levels of Pseudorhabdosynochus coioidesis and Pseudorhabdosynochus serrani were low during summer and prevalence and mean intensity were high in autumn and early winter. Water temperature reached peak in summer (30.7 °C) and decreased in winter (15.6 °C) in our investigation. In present study, prevalence was high in summer (70.13 %) and low in autumn (53.26 %); abundance was also high in summer (25.8) but low in spring (7.3); mean intensity was high in autumn (43.2) and low in spring (11.6). The seasonal population dynamics of Q. kobinensis in C. fuscus over four seasons showed that mean abundance, mean intensity and prevalence did not differ significantly which were due to relatively warm water ( 15.6 – 30.7 °C) . The result shows that there was no clear effect of temperature on the probability of being infected corroborating the finding of Hendrichsen et. al. (2015) which given the lower water temperature (mean 5.4 °C). Perhaps another reason of irregular seasonal population dynamics of Q. kobinensis was the relatively stable population structure of hosts (the seasonal population dynamics of C. fuscus will be published later) which means new hosts immigrate or previous hosts emigrate or die, in the mean time new parasites enter the hosts or previous parasites emigrate or die. (Lv. et. al., 2000). The distribution of Q. kobinensis in a host over the year shows an aggregated distribution, and the degree of aggregation varied little. The seasonal fluctuation of aggregation indices of Q. kobinensis in host showed that the majority of hosts were not infected and a large numberof Q. kobinensis parasited a small amount of hosts, so K is lower, and the population density of parasites is higher. The aggregated distribution minimize the effect of parasite on host populations and may be related to the fact that parasitization makes the host weak and thus more susceptible to other parasites (Li & Wang, 2002). The observed frequency distribution indices in this study showed that Q. kobinensis was found to be infected during the sampling months of whole year, and their infection levels were relatively low during Autumn. Both experimental and theoretical studies indicate that host body length positively correlate with mean intensity, prevalence, number of parasites and abundance. Khidr (1990) found prevalence and intensity of E. cichlidarum increase significantly with increasing host size. Tombi & Bilong Bilong (2004) also recorded a positive correlation between parasitic infections and host size and attributed this to increasing gill surface area and water flow through the gill chamber. Madanire-Moyo et al. (2011) also reported that prevalence values increased with increasing host size. However, there is no correlation between host body length and the mean intensity and prevalence in our study. The relationship between Q. kobinensis and the hosts C. fuscusin the Pearl River basin needs further research.
