Physicochemical Factors that may Influence the Distribution and Abundance of the Parasites and their Fish Hosts

The nature of the parasitic community in aquatic ecosystems is determined by interactions involving the definitive host fish, the intermediate hosts and the physicochemical factors (Wisniewski, 1958; Dogiel, 1961; Chubb, 1970; Kennedy, 1978c; Esch et al, 1988; Kennedy and Hartvigsen, 2000). The major physicochemical factors that may influence the distribution and abundance of the parasitic fauna in aquatic ecosystems are summarised in Table 4 and Figure 1 and their possible involvement is discussed below.

4.2.1.Water flows and water types

As a result of evolutionary pressure, trout and salmon have become adapted to live in the sediment-producing erosion zone, which is the first of three major zones in river classification systems (Dobson and Frid, 1998). In this zone, which is the major source of both water and sediment, the slope is typically steep and deposition of sediment is therefore localised or ephemeral because of the high shear velocity. The water types in this zone are diverse and include successions of riffles, fast reaches, small pools and backwaters with depositing sediments. In the older classification systems, based on biological parameters, which were developed for temperate-zone rivers, this was described as the trout zone (Carpenter, 1928). The second major zone, known as the sediment transfer zone, is characterised by a reduction in gradient. As a result the sediments, consisting of sand and gravel, are transported with little net loss or gain. Here the pools and backwaters are larger, the flow more uniform and riffles and fast reaches are absent. This zone is inhabited by minnows, grayling and some trout and is succeeded by the sediment deposition zone, where roach, bream and barbel predominate.

The presence of localised, often ephemeral, depositing zones in reaches occupied by trout in rivers is of paramount importance because it determines the distribution of both the free-living stages of the parasites and their hosts. The parasitic stages, which emerge from the eggs, are planktonic in nature and are therefore particularly vulnerable to currents. This consideration may explain the following gradation in the prevalence and intensities of infection of D. sagittata in brown trout at the three stations: Station A > Station B > Station D. Thus, the water flow in Station A in the Tregaron Bog area is hardly detectable much of the time because of the very gentle gradient. The Teifi at Station B is a much larger river than the Pysgotwr at Station D and because it has a gentler gradient and a reduced shear force it has a much greater surface area of sheltered microhabitats consisting of backwaters, pools and undercut banks with depositing sediments. It is likely that the low flow at station A is also responsible for the fact that it is the only station where D. ditremum was found. The almost lentic conditions prevailing at this station are favourable to both the planktonic coracidium larvae of this species and its copepod intermediate host. The completion of the life cycle of B. claviceps may also be facilitated by the fact that the eel has a preference for sheltered habitats, with depositing sediments, frequented by copepod intermediate hosts.

The intermediate hosts of other parasites including C.farionis, C. metoecus, P. simile, N. rutili, C. truttae and Capillaria species (Table 5) are also generally found in association with depositing sediments. The highly erosive nature of the substrate may, therefore, be partly responsible for the absence of P. simile, N. rutili and C. truttae from Station D. In contrast to the other parasites the trout at Station D are more heavily infected with C. metoecus than the trout at the two stations on the Teifi. This suggests that the second intermediate hosts may be mayflies or stoneflies that are well adapted to live in more turbulent waters with eroding sediments. The species involved might include Ecdyonurus torrentis, Baetis rhodani, Paraleptophlebia submarginata and Leuctra spp. as Awachie (1968) found these to be infected with dead or dying metacercariae of C. metoecus. However, Gammarus pulex, which Awachie (1968) considered to be the major second intermediate host of C. metoecus, was absent from the Pysgotwr.

It would appear, therefore, that some trematode species, such as C. metoecus are better adapted for life in fast-flowing waters than others. This is also the case with some of the nematode parasites of freshwater fish. Thus, Moravec (1994) lists Cucullanus truttae, Paraquimpera tenerrima and Cysdicoloides ephemeridarum as being adapted to live in regions of rivers where the water is fast-flowing, while Raphidascaris acus is better adapted to still or fast-flowing waters. However, these parasites may be locally distributed because the intermediate hosts have preferences for particular water types. Thus, C. ephemeridarum is abundant in the more erosive habitats that support mayflies but tends not to coexist with Cystidicola farionis, which uses gammarids as intermediate hosts. Likewise, C. truttae is restricted to stretches of river where depositing conditions provide favourable habitats for its intermediate host, which are larval lampreys. It is not surprising that none of the nematode species described by Moravec (1994) as being planktophilous because they use copepods or brachiurids as intermediate hosts and therefore are characteristic of still or very slow-flowing waters were found in the Teifi or Pysgotwr.

The increase in availability of depositing sediments in the second and third major river zones favours the distribution and abundance of macrophytes, the molluscan and crustacean intermediate hosts of helminth parasites and cyprinid and other non-salmonid fish. It can therefore be hypothesised that species richness and diversity of the helminth parasites would increase at the supracommunity level but that the converse would be the case with the helminth parasites of trout because of the reduction in the densities of both definitive and suitable intermediate hosts.

Lentic water bodies may be more favourable for the transmission of the helminth parasites in some fish. Thus, Landry and Kelso (1999) found that median parasite abundances in the largemouth bass, Micropterus salmoides, were higher in lakes than in rivers or swamps although the abundance of Proteocephalus amploplitis was higher in riverine sites. As species diversity is higher in the littoral zone of lentic water bodies than in the profundal zone it can be postulated that the helminth parasites of fish occupying these zones would show a similar trend. There is evidence in support of this hypothesis as Knudsen et al. (1997) found that the normal morph of the Arctic charr, Salvelinus alpinus, which feeds in the littoral zone, has a much richer helminth fauna than the dwarf morph that inhabits the profundal zone.

Fish inhabiting lentic water bodies also tend to have a higher prevalence of allogenic helminth species than fish in lotic water bodies. There are two main reasons for this. Firstly, the larvae of allogenic species are more planktonic in nature than those of autogenic species and birds, which are generally the definitive hosts. Secondly, birds are more abundant in lentic than in lotic habitats. As trout have evolved to exploit lotic habitats their helminth communities are characterised by a dominance of autogenic species. In contrast, allogenic species are more prevalent than autogenic species in cyprinid fish as they have evolved to exploit lentic habitats and the depositing conditions encountered in lowland reaches of lotic ecosystems.

Kennedy (1978c) attempted to relate the parasitic fauna of brown trout in nine British lakes to various physicochemical parameters. However, the only significant correlations were those between the size of the lake and the number of parasite species harboured by trout and between the altitude of the lake and the number of species present. The relationship between the parasitic fauna of the fish and the size of the water body is in accord with the findings of Dogiel (1961) and is probably attributable to greater habitat and biological diversity. The negative relationship between lake altitude and parasitic fauna was attributed to the higher altitude lakes being relatively smaller than those at lower altitude. However, there were no significant relationships between the number of parasitic species and geographical position, age, degree of isolation or CaC03 levels of the lakes. Kennedy (1978c) and Esch et al. (1988) therefore concluded that helminth communities of freshwater fish were stochastic assemblages whose compositions are largely determined by high rates of dispersion of the parasites and host fish by birds and human activity. Although these conclusions were based on relatively small samples it is evident that research aimed at relating the parasitic fauna to limnological conditions should be carried out in relatively undisturbed habitats.

4.2.2. Water temperature and photoperiod

As temperature limits the growth and development of poikilothermic organisms including helminth parasites it would seem logical to hypothesise that the helminth communities should be richer in tropical freshwater fish than in temperate zone fish because the average temperatures will tend to be higher in the former case. However, Choudury (2000) and Poulin (2001b) found the opposite trend in contrast to the majority of animal and plant assemblages. The reasons for this apparent anomaly are not known and require investigation. Observations made by Thomas (1966) in a tropical lake in Ghana may provide some clues. These show that species richness is much higher among taxa with high dispersive potential such as the Odonata, Hemiptera, Coleoptera and Diptera than among taxa with poorer dispersive powers such as the Ephemeroptera, Trichoptera, Annelida, Crustacea and Mollusca. The most plausible explanation for this phenomenon is that water bodies in savanna areas of Africa are often ephemeral due to severe seasonal drought. The selective mechanisms would therefore favour taxa with high dispersive powers. In contrast Platyhelminthes and the organisms involved as intermediate hosts of helminth parasites, such as the Mollusca and Crustacea, would be disadvantaged.

As both the fish hosts and parasites are poikilothermic it is to be expected that their reproduction and population dynamics would also be affected by seasonal changes in temperature in the temperate zones. In the case of the salmonid hosts there is ample evidence in support of this hypothesis. Thus, experimental work on the brown trout has shown that although growth occurs in the temperature range of 4-19 °C the optimum growth occurs between 13 and 14 °C (Elliot, 1989a). A growth model for the brown trout (Elliot, 1989b) which assumed that the initial size of the fry and the water temperature are the

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