Fish Parasites as Sources of Information on Pollution

As stated above it is difficult to use parasites as bioindicators of pollution because of the complexity and variability of their interrelationships with the environment. Despite these limitations various parasitologists have suggested that they may have some informational value as bioindicators of the following forms of pollution.

Heavy metal pollution. As heavy metals often bioaccumulate at higher concentrations in the parasites than in their fish hosts several authors (Riggs and Esch, 1987; Sures et al., 1994; Chubb, 1997; Landsberg et al., 1998) have advocated their use as sensitive biomarkers.

Acidification. Cone et al. (1993) found that the parasite communities in the American eel, Anguilla rostrata, inhabiting acidic water bodies with pH values of 4.5-5.0 were characterised by the absence of digenetic trematodes, lower species richness and fewer multiple infections compared with those in less acidic, limed waters. Subsequent studies by Marcogliese and Cone (1996) on the helminth parasites of eels show that component community diversity, as measured by species richness, Shannon-Wiener index and Hill's number, decreased when the pH was less than 5.4. These authors argued therefore that the parasitic helminth fauna reflects differences in the food webs in these contrasting habitats and conclude that parasitic assemblages may be good indicators of environmental stress. These findings are in accord with those in the present investigation as the parasitic community of the trout from the Pysgotwr, the most acidic river, had the lowest species diversity indices.

Eutrophication. In the initial stages an increase in eutrophication of lentic waters favours the growth of phytoplankton populations and hence those of organisms along the food chain which benefit from this including zooplankton, planktivorous fish and piscivorous birds. It is not surprising therefore that Kennedy et al. (1994), Kennedy (1995) and Yeomans et al. (1996) found that increased levels of eutrophication were positively correlated with population increases in fish parasites such as Ligula and trichodinids, respectively. One might also predict that communities of fish parasites would become more species rich and diverse with eutrophication increasing up to a certain level and then declining as the water bodies become hypertrophied.

Organic pollution. This may take the form of increased loading with waste organic matter of human or animal origin or organic pesticides. The former harm the environment by oxygen depletion whereas the latter act as toxicants. Yeomans et al. (1996) and Landsberg et al. (1998) discuss the possibility of using trichodinids as indicators of enhanced organic loading. However, as pointed out by MacKenzie et al. (1995) many of the studies associating changes in fish parasites with the effects of specific pollutants have been speculative and inconclusive.

Many authors, including MacKenzie et al. (1995), have suggested that fish parasites would be ideal bioindicators of specific pollutants, as they would be expected to be highly sensitive. However, it has also been suggested (Kuperman, 1993) that the monogenetic fluke, Diplozoon paradoxum, and the cestode Caryophyllaeus laticeps are suitable bioindicators of pollution by chemical products of the coal tar industry in Russia because of their insensi-tivity to the toxicants. As a result the parasites are found to be more abundant in the polluted zones. However, this author does not appear to have considered the possibility that this phenomenon might be attributable to the fish immunity being lowered by the toxicants. There are many conflicting reports regarding the effects of pollution on disease prevalence and mortality. Some of these indicate that pollution results in an increase in disease prevalence and mortality whereas others come to the opposite conclusion (Williams and Jones, 1994; MacKenzie et al., 1995; Landsberg et al., 1998). It is not therefore possible to generalise and further work on specific cases is needed before any firm conclusions can be drawn. The dissemination of pathogenic parasites by the stocking and dumping of fish may also be regarded as another form of pollution (Kennedy, 1975; Williams and Jones, 1994).

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