Indicators (parasites and stable isotopes) of trophic status of yellow perch (Perca flavescens Mitchell) in nutrient poor Canadian Shield lakes

The fish parasite communities of four Canadian Shield lakes (L239, L240, L377 and Triangle Lake) in the Experimental Lakes Area (ELA), Ontario were surveyed over three seasons in 1997 and 1998 and one season in 1999. Thirteen of fifteen fish species sampled from the study lakes harboured parasites; northern pike (Esox lucius),lake cisco (Coregonus artedii), pearl dace (Margariscus margarita), blacknose shiner (Notropis heterolepis), spottail shiner (Notropis hudsonius), fathead minnow (Pimephales promelas),longnose dace (Rhinichtys cataractae), white sucker (Catostontus commersoni), burbot (Lota lota), brook stickleback (Culea inconslans), slimy sculpin (Cottus cognatus),Iowa darter (Etheostoma exile) and yellow perch (Perca flavescens). A total of 112,188 parasites were detected, with 87% (n = 1926) of the necropsied fish having at least one parasite. Forty-one species of parasites representing 32 genera and 26 families were found; 35 species used fish as definitive hosts and six species used birds as the definitive hosts. The most omnivorous fish species (perch, white sucker and pearl dace) had the most diverse parasite infracommunities while the most specialized consumers had the least diverse parasite infracommunities (the piscivores, pike and burbot and the zooplanktivorous cisco) or lacked parasites entirely (the algavorous northern redbelly dace, Phoxinus eos, and finescale dace, P. neogaeus). The parasite communities of the four ELA lakes were less speciose than those reported from larger lacustrine systems in Ontario and Manitoba. The parasite community composition of yellow perch was typical of perch in other systems, however, there were new host and locality records for several parasite species infecting other fish species. For example, the pearl dace was infected with five parasites not previously reported from this host in North America and two not previously reported from this host in Canada. There were distinct seasonal and age-related trends in the parasite communities of yellow perch populations that correlated with seasonal and ontogenetic dietary shifts, respectively. Understanding parasite community structure can improve understanding of host population ecology and clarify many aspects of ecosystem biotic and abiotic interactions. Detailed analysis as performed in this study is a useful tool for describing the factors affecting parasite community composition. A restricted invertebrate and fish fauna in these nutrient poor lakes can be important in controlling the parasite fauna infecting yellow perch, which could be more predictable than that observed in large, productive lakes. Allogenic parasites are most common in Triangle Lake and enterics in L239. Parasite species richness is highest in L239 and diversity is highest in L240. Glugea sp. is the most dominant and abundant parasite species in all but Triangle Lake. Triangle Lake and L377 perch have the fastest growth rates and reach the greatest total length and age of all sampled perch. Yellow perch length and age are both highly correlated with parasite richness, intensity and abundance. Female perch usually had significantly greater species richness than males but intensity and abundance were significantly higher than males in only two lakes each. L239 and Triangle Lake had parasite assemblages that were significantly non-random largely due to subpopulations of parasite species transmitted through macrobenthos. Perch parasite communities in all four lakes showed significantly more nestedness than expected by chance. Parasite-induced pathology of yellow perch was also examined in these shield lakes. Glugea sp. xenomas in cells of the intestinal wall and in visceral fat and Apophallus brevis metacercariae infecting the musculature reduced the growth of perch resulting in mortality in younger and smaller fish. High numbers of Raphidascaris acus, encysted in the liver of yellow perch, correlated significantly with a reduction in visceral fat weight in 1+ females and 0+ and 1+ males. A significant correlation in these subsamples indicates that host sex, size, trophic status and relative weight of the liver are linked to R. acus density. The data suggests that interactions among parasitic infections and age, size and sex of the fish host can affect growth and survival of the host, especially during periods of low energy inputs and reproductive stress. Stable isotopes (C and N) have proven to be important tools for obtaining information on the trophic relationships within food webs. Combining parasite community studies with stable isotopes could improve the effectiveness of detailed food web analyses. Parasite communities are particularly useful since they can identify both prey and non-prey components of the host's community. Triangle Lake perch had distinct isotope ratios that separated them from the other three populations. Stable C isotope ratios for all perch ranged from - -340/00 to -190/00 while stable N isotope ratios ranged from - 4.50/00 to 12.50/00. These ranges are larger than those observed in many other fish species. Perch diet was the most significant predictor of stable C isotope ratio. Perch parasite fauna was the most significant predictor of stable N ratios. In particular, parasite fauna indicative of zooplanktivorous or piscivorous perch were most accurate for predicting fish trophic position and thus stable isotope ratio. Fish length and age showed no significant relationship with isotope ratios.

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