A comparative study of metazoan parasite communities of sentinel bluegill caged in two urbanizing streams, San Antonio, Texas

Abstract

Urbanization has deleterious effects on water quality and biota in stream systems. This project used caged bluegill (Lepomis macrochirus) to assess metazoan fish parasite communities in two urbanizing streams of the upper San Antonio River Basin, Bexar County, Texas. Field studies on Leon and Salado creeks were conducted during late summer in 1999 and 2000. Both study streams are non-point source polluted and have been declared hazardous for human and wildlife use. In both years, juvenile bluegill, obtained from a local aquaculturist, were placed in individual compartments of floating cages made from plastic-coated wire mesh; the fish were held in cages 10-22 days at middle and lower watershed sites to expose them to in-stream conditions and to allow parasite communities to become established. After removal from cages, fish were examined for metazoan parasites. In 2000, wild Lepomis spp. also were collected at the study sites for assessment of their parasites and tissue concentrations of trace metals. In both years, physical and chemical properties of the water were monitored at each of the four sites. Analysis of these data indicated differences in parasitism between upper and lower sites within streams (but not between streams), between caged and wild fish, and between sites with higher and lower levels of nitrate. Of the 120 fish examined for parasites, 96.7% were infected by at least one organism from among the 11 parasitic taxa observed; the overall average intensity of parasitism was 65.7 individual organisms representing 2.8 taxa. For caged fish, both diversity and equitability of parasite communities tended to be lower at downstream sites, which were more eutrophic; accordingly, parasite diversity and equitability were inversely correlated with nitrate concentrations. Ectoparasites were more prevalent in caged fish and endoparasites in wild fish. An Ergasilus sp. copepod and a Posthodiplostomum sp. trematode dominated the ecto- and endoparasite faunas, respectively. In both years, growth of caged fish was inversely correlated with intensity of infestation by a Cleidodiscus sp. monogenean and by the copepods Lernea sp. and Ergasilus sp. This study suggests that assessment of watershed health can benefit from comparative cage studies of parasite community development involving sentinel fish species.