C P

D. sagitatta C. farionis C. metoecus N. rutili Nematodes

-0.022 0.703 -0.001 0.921 +0.007 0.278 +0.049 0.001

+0.079 0.008 +0.030 0.094 +0.091 0.029

+0.050 0.160 -0.186 0.024

+0.487 0.000

A consideration of the life history strategies can help to explain the large number of significant positive associations between the parasites. As explained earlier the transmission sites for all the helminth species tend to be located in microhabitats where the currents are relatively low and where the sediments are depositing. The low current speed is favourable for oncomiracidial transmission and the intermediate hosts of the other helminth parasites live in depositing sediments. This helps to explain why the monogenetic trematode, D. sagittata tends to be positively associated with the other species. Overlapping transmission sites may also explain other cases of positive associations between helminth parasites in salmonid fish including N. rutili and C. farionis (Dorocu et al., 1995). However it is important to bear in mind that positive associations do not rule out the possibility that competition may be occurring between the coexisting parasites.

When all the trout are taken into account the only parasites which were significantly negatively associated were the most abundant parasites, namely C. metoecus and N. rutili. In other cases when the entire gut is taken into account the significant negative associations involve parasites which tend to occupy the same region of the gut such as C. metoecus/C. truttae and N. rutili/C. farionis in the trout and C. metoecus/C. truttae in the salmon parr. On the other hand when the pyloric and post-pyloric regions of the gut are considered separately significant negative associations are also found between parasites which may tend to coexist in the same region of the gut or not. When taken together these results strongly suggest the existence of interspecies competition between the helminth parasites in the gut of salmonid fish.

Other negative associations involving helminth parasites which have been reported include tapeworms and acanthocephalids in the gut of Coregonus albula, the monogenean Octobothrium merlangi and the copepod Clavella

Table 35 A summary of the results of regression analyses carried out on the numbers of parasites (log (1 + x) transformed) of the brown trout located in the pyloric region and post-pyloric region of the intestine
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