Comparative Aspects

At the ecological level the heminth parasites common to both the trout and salmon parr share many spatial and temporal patterns. Thus, C. metoecus exhibit summer troughs and winter highs, unlike N. rutili, in both fish species. However, the abundance of C. farionis does not vary seasonally in the case of salmon parr although it does so in the trout. Likewise D. sagittata and N. rutili are more abundant in both trout and salmon parr at Station A than at Station B.

As with trout there were significant tendencies for the abundances of D. sagittata, C. metoecus and C. farionis, but not N. rutili, to increase with the length of salmon parr. However, unlike the trout, there were no statistically significant relationships between the abundances of any of the parasite species and the weight of the salmon parr. This absence of relationship can be attributed to the much smaller range in weight of the salmon parr compared with trout. With the exception of N. rutili and D. ditremum the abundances of all the parasite species in trout increased significantly with age. This was also the case with the helminth parasites of salmon parr with the exception of N. rutili and C. farionis. As with the trout the abundance values of the dominant parasite, N. rutili, were significantly positively correlated with both the condition factor and the adipose index. There was no evidence therefore that the helminth parasites had an adverse effect on the wellbeing of either fish species as measured by the condition factor and the adipose index.

However, the helminth communities of salmon parr and the brown trout (Tables 3 and 14; Figure 17) also differ in many important respects. Firstly, although the trout and salmon parr share five species (D. sagittata, C. farionis, C. metoecus, N. rutili and C. ephemeridarum) the trout has a richer helminth fauna as it is also parasitised by P. simile, D. ditremum, R. acus, C. truttae and species of Capillaria. Secondly, at the component population level both the Simpson and Shannon Wiener diversity indices and the equitability index are much higher for the trout than the salmon parr. In contrast, the Berger-Parker predominance index is higher for the salmon parr than trout. Thirdly, the Brillouin and maximum Brillouin indices are higher in trout than in salmon parr at both the component and infrapopulation levels. These results are to be expected as species richness, the mean number of parasites per fish and the maximum number of parasites per fish at the infrapopulation level are much higher for trout than salmon parr. In contrast, more salmon parr were without any parasites (13.3-14.3%) compared with trout (1.0-1.3%). With the exception of N. rutili, the dominant parasite in the case of the salmon parr, the abundance values of shared parasite species were much higher for trout. This trend was particularly marked in the case of D. sagittata.

Although it can be postulated that these differences are attributable to the salmon parr having a higher level of immunity or resistance than trout it is more probable that they are caused by differences in age distribution and the niches occupied by the two species (Figures 18 and 19). Thus, nearly all of the salmon parr are in the 1 or 2 year age category whereas many of the brown trout are much older. The evidence that parasite abundance increases with age

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