Introduction

The parasites of aquatic organisms are ubiquitous and major components of ecosystem, which are closely related to several characteristics of the biotic and abiotic environment in which they exist (Lacerda et al. 2017). Most vertebrate species serve as a host of one or more parasitic species (Lagrue et al. 2011). Among vertebrates, fish have the highest rates of parasitic infestation because of the unique characteristics of their aquatic environment, which facilitate the spread, reproduction and life cycle completion of each parasitic group (Emily et al. 2018). The backwaters of Kerala possess high species richness and diversity of organisms, including parasites of fish populations. Parasites are components of most ecosystems and are involved in numerous food webs.

Parasitic copepods constitute one of the major group among parasitic arthropods that affect wild fish are of commercially significance as they affect host survival or cause changes in the flesh. Ergasilid copepod damages the gills and cause epizootics in cultured and wild populations of fishes (Lester and Hayward 2006). According to Kabata (1988), parasitic copepods belong to the suborder Siphonostomatoida (75%), Poeilostomatoida (20%) and Cyclopoida (5%). Most of the parasitic copepods are external parasites commonly found on the gills, skin and operculum and these copepods negatively affect the fish production, both from the wild and fish farms, thus making them difficult to market (Nike et al. 2013). Knowledge about the parasitic fauna of fishes in Kayamkulam backwater is scare. Therefore the aim of the study was to evaluate the parasitic copepod infestation on commercially important fishes of Kayamkulam backwater such as Liza parsia (Hamilton, 1822), Oreochromis mossambicus (Peters, 1852) and Etroplus suratensis (Bloch, 1790).

Materials and methods

Study area

The study area, Kayamkulam backwater or Kayamkulam estuary (latitudes 9°2′N and 9°16′N & longitudes 76°25′E and 76°32′E) is a shallow brackish water lagoon. Kayamkulam backwater occupies area in both Alapuzzha district and Kollam of the total 1652.33 hectares. For the purpose of study, three stations were considered as sampling areas within the estuary: Ayiramthengu (Station 1), Valiyazheekal (Station 2) and Choolatheruvu (Station 3). The first sampling station (S1) is the Ayiramthengu mangrove and this mangrove area was declared as environmental hot spot after ravaged by the Tsunami in 2004. Station 2 is at Valiyazheekal near to pozhi, connecting to the Arabian Sea. Kayamkulam backwater opens into the Lakshadweep Sea through Valiyazheekal Azhi. Station 3 is the Choolatheruvu, a part of Kayamkulam backwater near to National Thermal Power Corporation (NTPC).

Parasitological examination

The fishes Liza parsia, Oreochromis mossambicus and Etroplus suratensis from the three stations were obtained by the assistance of fishermen. Between February 2015 and January 2017, a total of 2305 individuals from different species were examined for parasitic copepod. Scraping from the skin, fins, tail and operculum of the fishes were done for the detection of parasites. After dissection, stereo-microscopic observation was made on gills for the presence of parasitic copepod.

Parasitic species identification was based on morphological features according to Pillai (1985), Yamaguti (1985), Kabata (1988), Boxshall and Halsey (2004) and Ho and Lin (2004). The parasite number and place of their attachment were recorded. The collected parasites were fixed in glacial acetic acid (Pritchard and Kruse 1982) and preserved in ethanol (70%). The parasitological terms such as prevalence, mean intensity and mean abundance follow Bush et al. (1997) and Margolis et al. (1982). Results of all parameters were expressed as mean ± standard deviation. Data were analysed using one-way analysis of Variance (ANOVA) using Statistical Package for Social Sciences (SPSS version 20). Tukey’s HSD test was used to determine differences between groups at 95% confidence level.

Results

During the present study, parasitic copepods belonging to the family Bomolochidae, Ergasilidae (Poecilostomatoida), Caligidae (Siphonostomatoida) and Lernaeidae (Cyclopoida) were collected from the Kayamkulam backwater. A total of 2305 fishes (Liza parsia-768, Oreochromis mossambicus-725 and Etroplus suratensis-812) were examined for the presence of parasites. Among the examined fishes, 299 fishes (Liza parsia-160, Oreochromis mossambicus-102 and Etroplus suratensis-37) were infested with different parasitic species. The prevalence, mean intensity and mean abundance of infestation were recorded to calculate the degree of infestation during the year February 2015 to January 2017 (Tables 1, 2).

Table 1 Prevalence, Mean intensity and Mean abundance of parasitic copepod on commercially exploited fishes from Kayamkulam backwater during February 2015–January 2016
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Table 2 Prevalence, Mean intensity and Mean abundance of parasitic copepod on commercially exploited fishes from Kayamkulam backwater during February2 016- January 2017
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  1. a)

    Prevalence Prevalence is the proportion of infested host among all the hosts examined. The prevalence of infestation by parasitic copepod on different fishes during the study period were presented in Tables 1 and 2.

    a.1:

    Prevalence of parasitic copepod infestation on fishes during February 2015–January 2016. During the period February 2015 to January 2016, the prevalence of parasitic copepod on L.parsia at station 1, station 2 and station 3 varied from 18.4–26.5% 7, 12.5–18.6% and 21.6–40%. The prevalence of infestation of parasitic copepod in L.parsia during different season is shown in Fig. 1 and the parasites showed maximum effect in the monsoon period (40%) at station 3 (Fig. 1). The prevalence of parasitic copepod infestation in O. mossambicus at station 1, station 2 and station 3 were varied from 8.1–20.9%, 4.7–17.1% and 14.3–24.4% respectively. The higher prevalence was found in station 3 during monsoon season (24.4%) (Fig. 2). Regarding the prevalence of parasitic copepod infestation on E. suratensis, higher value was recorded from station 3 during monsoon period (11.6%) (Fig. 3). The infestation of copepod parasites on E. suratensis was low during the study period while the parasitic incidence in L. parsia was found high.

    Fig. 1
    figure 1

    Seasonal fluctuation of Prevalence of parasitic copepod on Liza parsia during the year Feb 2015–Jan 2017

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    Fig. 2
    figure 2

    Seasonal fluctuation of Prevalence of parasitic copepod on Oreochromis mossambicus during the year Feb 2015–Jan 2017

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    Fig. 3
    figure 3

    Seasonal fluctuation of Prevalence of parasitic copepod on Etroplus suratensis during the year Feb 2015–Jan 2017

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    a.2:

    Prevalence of parasitic copepod infestation on fishes during February 2016–January 2017. During the period February 2016–January 2017, the higher prevalence of parasitic copepod in L. parsia was reported in monsoon (31.8%) at station 3 and the prevalence of infestation at station 1 (25%) and station 2 (14.9%) were found higher during the monsoon period (Fig. 1). In O. mossambicus, the higher prevalence of parasites was reported from the post monsoon (25.6%) at station 3 (Fig. 2). In E. suratensis, the higher prevalence of parasites was observed in monsoon (6.8%) at station 2 (Fig. 3).

  2. b)

    Mean Intensity Mean intensity is mean number of parasites from infested hosts. Mean intensity of infestation on fishes were summarized in Tables 1 and 2.

    b.1:

    Mean Intensity of parasitic copepod infestation on fishes during February 2015–January 2016. Seasonal variation of mean intensity of parasitic copepod in fishes from three stations during the year February 2015–January 2016 was determined. In L. parsia, higher mean intensity was reported in monsoon (13.2) at station 3 (Fig. 4). In O. mossambicus, maximum mean intensity was found in the pre monsoon period (5.5) at station 2 (Fig. 5). There was no incidence of parasitic copepods in O. mossambicus in the month of March at all the stations. The mean intensity of parasitic copepods in E. suratensis was varied 0–3. The higher mean intensity was reported from station 2 and station 3 (3) during premonsoon period (Fig. 6). In all the stations, the infestation with parasitic copepods on E. suratensis was poor (Fig. 6).

    Fig. 4
    figure 4

    Seasonal fluctuation of Mean intensity of parasitic copepod on Liza parsia during the year Feb 2015–Jan 2017

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    Fig. 5
    figure 5

    Seasonal fluctuation of Mean intensity of parasitic copepod on Oreochromis mossambicus during the year Feb 2015–Jan 2017

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    Fig. 6
    figure 6

    Seasonal fluctuation of Mean intensity of parasitic copepod on Etroplus suratensis during the year Feb 2015–Jan 2017

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    b.2:

    Mean Intensity of parasitic copepod infestation on fishes during February 2016–January 2017. During the period February 2016 to January 2017, the mean intensity of parasitic copepod in L. parsia at station 1, 2 and 3 were ranged from 2.9–4.4, 4.2–5.1 and 4.4–5.7 respectively. The higher value of mean intensity was observed at station 3(5.7) during post monsoon (Fig. 4). In the case of O. mossambicus, maximum mean intensity was observed during pre monsoon period (4) at station 1 and 2 (Fig. 5). Throughout the study period, mean intensity of parasitic copepods in E. suratensis was very low in all the stations. However station 1 and 3 shows highest mean intensity of parasitic copepods in E. suratensis during post monsoon period (3.5 and 3.3) (Fig. 6).

  3. c)

    Mean Abundance Mean abundance is the mean number of parasites present in all hosts examined. Mean abundance of infestation during February 2015 to January 2017 was summarized in Tables 1 and 2.

    c.1:

    Mean Abundance of parasitic copepod infestation on fishes during February 2015–January 2016. Mean abundance of parasites on L. parsia at station 1, 2 and 3 during February 2015–January 2016 was ranged from 1.4–2.4, 1.1–1.9 and 2.7–5.3 respectively. Higher mean abundance was reported from station 3 (5.3) during monsoon (Fig. 7). In O. mossambicus, higher mean abundance was observed in monsoon period (0.9) at station 3 (Fig. 8). The mean abundance of parasitic copepod on E. sursatensis was varied from 0 to 0.3. Higher mean abundance was reported from station 3 (0.3) during monsoon period (Fig. 9).

    Fig. 7
    figure 7

    Seasonal fluctuation of Mean abundance of parasitic copepod on Liza parsia during the year Feb 2015–Jan 2017

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    Fig. 8
    figure 8

    Seasonal fluctuation of Mean abundance of parasitic copepod on Oreochromis mossambicus during the year Feb 2015–Jan 2017

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    Fig. 9
    figure 9

    Seasonal fluctuation of Mean abundance of parasitic copepod on Etroplus suratensis during the year Feb 2015–Jan 2017

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    c.2:

    Mean Abundance of parasitic copepod infestation on fishes during February 2016–January 2017. From February 2016 to January 2017, mean abundance of infestation on fishes examined was varied 0.5–1.4 (L. parsia), 0.2–0.9 (O. mossambicus) and 0–0.2 (E. suratensis). In L. parsia, higher mean abundance was reported in monsoon (1.4) at station 3 (Fig. 7). Mean abundance of parasites on O. mossambicus was highest in post monsoon (0.9) at station 3 (Fig. 8). In E. suratensis, higher mean abundance was recorded during monsoon (0.2) and post monsoon (0.2) at station 2 and station 3 respectively (Fig. 9). The study showed that mean abundance was high in L. parsia than O. mossambicus and E. suratensis (Fig. 10).

    Fig. 10
    figure 10

    Gill of fish infested with parasitic copepods

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Prevalence, Mean Intensity and Mean Abundance of parasitic copepod infestation on commercially exploited fishes of Kayamkulam backwater were summarized in Tables 1 and 2. The results of the study were expressed as Mean ± standard deviation. Statistical analysis of data showed that the infestation on L. parsia was varied significantly among stations during the year 2015–2016. The prevalence (P < 0.05), mean intensity (P < 0.05) and mean abundance (P < 0.01) of parasitic copepods on L.parsia were varied among stations during February 2015–January 2016. Tukey’s HSD test shows that 95% confident that prevalence, mean intensity and mean abundance of parasitic copepod on L. parsia from station 1 and station 2 were similar, yet these parameters differ from station 3 during 2015–2016. Prevalence (P > 0.01) and mean intensity (P > 0.01) of infestation on O. mossambicus were not varied significantly among stations. But the statistical analysis showed that mean abundance (P < 0.05) of parasites on O. mossambicus was varied significantly among stations during 2015–2016. Tukey’s HSD test shows that mean abundance of parasitic copepod on O. mossambicus was similar in station 1 and 2. Prevalence (P > 0.01), mean intensity (P > 0.01) and mean abundance (P > 0.01) of parasitic copepod in E.suratensis were not varied among stations during February 2015–January 2016 (Table 1). From February 2016 to January 2017, prevalence (P > 0.01) and mean intensity (P > 0.01) of parasitic copepod infestation on L. parsia were not varied significantly among stations. But the statistical analysis showed that mean abundance (P < 0.05) of parasites on L. parsia was varied significantly among stations. There was no significant difference in the prevalence, mean intensity and men abundance of parasitic copepod on O. mossambicus and E. suratensis between the stations (P > 0.01) during 2016–2017 (Table 2).

Discussion

The parasite fauna in the host fishes is determined by the environmental conditions, both natural and man-made, as also the composition of host species at a given time (Santhosh and Radhakrishnan 2009). The parasitic fauna of fish can vary in their abundance depending on the host species, its level in the food chain, the age, size and sex of individual fish, and other biotic and abiotic factors (Takemoto and Lizama 2010). Copepods are common parasite fauna of fishes which have been studied extensively in coastal and neritic waters, where as they have become pests of fish species of commercial importance and several factors such as host factors and external factors (temperature, rainfall, salinity etc.) influence the abundance of parasite species on host fishes (Vinoth et al. 2010). According to Iyaji et al. (2009), biotic factors like age and size of host, host size and parasite size, host specificity, feeding habits of host, host sex and immune system can influence profoundly in the assemblages of parasites on fish hosts. Pollution and other anthropogenic activities of the aquatic environment may affect a parasitic community directly by acting on free-living parasitic stages or on ectoparasites, or indirectly by acting on the intermediate host populations (Moller 1987).

During the two year study period, prevalence, mean intensity and mean abundance were highest in L. parsia collected from station 3 of the study area followed by O. mossambicus and E. suratensis. Prevalence, mean intensity and mean abundance of parasitic copepods on L. parsia were statistically different between stations; however there was no significant difference in prevalence, mean intensity and mean abundance of parasitic copepods on O. mossambicus and E. suratensis between stations. Concerning the variation of parasitic copepod infestation on different host fishes, the parasites were more frequently found in L. parsia and parasites were absent in E. suratensis during some months and they were least infested. Results obtained from the present study reveals that high infestation on L. parsia and low infestation rate on E. suratensis were reported elsewhere in Kayamkulam backwater. Feeding habit of the host is significant factor for determining the nature of parasitic fauna in host (Razia Beevi and Radhakrishnan 2012). The study showed variability in the parasitic copepod infestation on different host fishes could be attributed to host specificity of parasite fauna. Most of the parasitic copepods are highly host specific. Highest number of parasites was reported from L. parsia and most of the recovered parasites were stenoxenic.