Helminth Parasites and other Aquatic Organisms as Pollution Bioindicators

As a result of ever-increasing levels of anthropogenic environmental pollution it has become obligatory to monitor physicochemical parameters to evaluate water quality on a routine basis and also when pollution incidents are suspected. However, because of their episodic or transitory nature, pollution incidents may easily be overlooked when using such methods. It has therefore been stressed by many aquatic biologists including Hynes (1960) and Mason (1996) that the physicochemical investigations must be complemented by using bioindicators under both field and laboratory conditions. Biomonitoring also has the added advantages of making it possible to identify the pollution source and of predicting the possible consequences to the community of the type of pollution involved.

There has been much discussion regarding the kinds of organisms that might be used as bioindicators of pollution. It may be suggested that the following criteria should be met when selecting bioindicators:

1. It should be possible to predict the kind of aquatic community that one might expect in a non-polluted aquatic environment from the relevant physicochemical data and then use these data as a basis for comparison with polluted habitats.

2. The biomonitoring method should be robust and have general application in a range of habitats.

3. The selected organisms should be highly responsive to the particular pollutant or the type of pollution being investigated.

4. The eco-physiological, biochemical and behavioural responses shown by the organisms to the pollutant should be understood.

5. The bioindicators selected should be readily accessible and identifiable.

6. It should be possible to apply the method without causing undue environmental damage.

7. The biotic index used should be cost effective with respect to the resources available.

Many parasitologists including Khan and Thulin (1991), Poulin (1992), MacKenzie et al. (1995), Yeomans et al. (1996), Kennedy (1997), Chubb (1997) and Landsberg et al. (1998) have discussed the possibility of using fish parasites as bioindicators of pollution. Unfortunately, although fish parasites, in common with other aquatic organisms, may respond to pollution, the arguments presented below show that they are not ideal bioindicator species.

As pointed out by Kennedy (1997) fish parasites fail to satisfy even the first of the above criteria. Thus, Hartvigsen and Kennedy (1993) state that 'the predictive value of their studies have foundered on the erratic and unpredictable occurrence of many fish helminth species'. This situation led Kennedy (1978a, b, 1981,1985) and Kennedy et al. (1986) to argue that helminth communities in freshwater fish are fundamentally stochastic assemblages, their composition being dependent on chance introductions of parasites into localities and on chance colonisation and extinction events. These arguments are supported by Esch et al. (1986) and Dobson (1990).

While it is to be expected that the free-living stages of the parasites would be highly susceptible to pollutants this does not mean, unfortunately, that they are ideal candidates for biomonitoring for the following reasons. Firstly, parasites are renowned for their exceptionally high rates of reproduction. As reproduction may be spread over a long period it may well not be seriously affected by pollution, as this is often episodic in nature. Furthermore, once the endoparasites have entered their intermediate or definitive hosts they receive a measure of protection from the direct effects of environmental pollution. Although fish ectoparasites will potentially be in greater danger than endoparasites from exposure to pollutants mucus secreted by the host will provide them with some protection. In addition the high level of mobility of the host fish enables them to move away from heavily polluted areas. As a result it is possible to envisage metapopulations at different stages of development surviving in spatially discontinuous habitats despite pollution. In the case of digenetic trematodes the quantification of the suprapopulation, which includes all developmental phases of a parasite at a particular time and place, is an extremely difficult task, particularly in cases where all the definitive and intermediate hosts have yet to be identified. The life history strategies adopted by helminth parasites help to explain their evolutionary success. Unfortunately, as this tends to makes them less responsive to pollution than free-living organisms their use as bioindicators is further contraindicated.

Unlike free-living organisms relatively little is known about the eco-physi-ological, biochemical and behavioural responses shown by the various stage in the life cycle of the parasites to the pollutants. This is mainly due to the short life span of the free-living stages and the difficulty of culturing the parasitic stages. As a result it is difficult to predict how the various phenotypes in the life cycle might respond to particular pollutants.

The use of fish parasites as bioindicators of pollution also fails to satisfy the fourth criterion, namely that the bioindicators selected should be readily accessible, widely distributed and identifiable. Unlike free-living invertebrates, fish hosts such as trout and hence their parasites tend to be restricted to the upper reaches of rivers and are replaced by coarse fish in the lower reaches. As fish parasites generally exhibit overdispersed distributional patterns it is necessary to sample a large number of fish in order to obtain reliable data. Identification may also be difficult as sibling species such as C. farionis and C. metoecus may coexist as adults. Larval stages generally present even greater problems with identification.

Unfortunately, the sampling of fish, such as trout, in order to obtain para-sitological data to monitor pollution is unacceptable as like other salmonid species they are under threat of extinction. Furthermore the gathering of such data would be very time consuming and not cost effective with respect to resource availability. It must be concluded therefore that as none of the above criteria are met the possibility of using fish parasites for biomonitoring pollution is hard to justify.

It is not surprising therefore that other biotic indices, such as the Saprobien index (Mason 1996), the BMWP indices (Mason, 1996) and those based on the River Invertebrate Prediction and Classification System (RIVPACS) (Wright et al., 1993) which meet the above criteria are preferred. All these indices were developed to monitor the effects of organic pollution, which has become an ever-increasing problem due to rising human population, farming intensification and the disposal of human sewage into river systems. The Saprobien index recognises four stages in the oxidation of organic matter: polysaprobic, a-mesosaprobic, (3-mesosaprobic and oligosaprobic. It is taxonomically demanding because it makes use of 2000 taxa which are individually graded on a 1-8 scale, based on increasing tolerance to organic pollution, and 1-4 on the basis of relative abundance. Although widely used in continental Europe it has received little support in Britain or America. It is not surprising therefore that it has been superseded by the BMWP and RIVPACS indices for biomonitoring organic pollution in lotic ecosystems. As shown by Metcalfe (1989) these indices and other related forms have evolved from the Trent biotic index.

Table 30 shows that all the scores for Station 1 in the Teifi bog area are lower than at other stations further downstream. However, the scores for Station 1 although relatively low are still indicative of good water quality whereas those for the other stations are exceptionally high, particularly those where the BMWP and average score per taxon (ASPT) scores exceed 200 and 6.0, respectively. There are two possible explanations for the relatively low biotic indices for the bog area site. Firstly, they may be attributable to the toxic effects of heavy metals, including zinc and lead, which have been shown to be present at higher concentrations in the sediments in the bog area than further downstream. Secondly, the lower values in the bog area may be the result of lower habitat diversity compared with the downstream sites.

Despite the fact that the scores for Station 1 are within acceptable Environment Agency standards the lower indices and the higher levels of heavy metals in the sediments compared with downstream stations are clearly causes for concern. In view of this it may be suggested that the Environment Agency should focus more on both chemical and biomonitoring of sediments in areas which are susceptible to pollution by heavy metal or organic toxicants. The existing methods used by the Environment Agency rely heavily on monitoring

Table 30 The number of aquatic invertebrate families encountered, the Biological Monitoring Working Party (BMWP) score and the average score per taxon (ASPT) obtained for samples taken at the various stations in the upper Teifi in May, 1998

Station number

Table 30 The number of aquatic invertebrate families encountered, the Biological Monitoring Working Party (BMWP) score and the average score per taxon (ASPT) obtained for samples taken at the various stations in the upper Teifi in May, 1998

Station number

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