Distinguishing and employing sister species of fish in assessment of stream quality

Biological indicators (bioindicators) can be individual species or species groups used to assess habitat quality. Unfortunately, conservationists often lack information on species distribution, how to differentiate between similar species, and environmental conditions associated with the presence of a species. We addressed these problems using two “sister” species of fish, the Mottled Sculpin (Cottus bairdii) and the Slimy Sculpin (Cottus cognatus), as stream quality indicators in the Manistee River watershed in the Huron-Manistee National Forests in Michigan, USA. We determined the abundance and distribution of these species and related their presence to concurrent in-stream measurements of temperature, dissolved oxygen, pH, conductivity, turbidity, and stream quality score based on macroinvertebrate diversity. To be certain of identification, we sequenced the Cytochrome c Oxidase Subunit I (CO1) molecular marker for specimens and used it as a DNA barcode to determine a specimen’s species. Cladistic analyses of CO1 unambiguously supported recognition of Mottled Sculpin and Slimy Sculpin as distinct species, confirming initial 87.5% correct identification using morphological characteristics, with uncertainty limited to juvenile fish. Field determinations increased to 100% correct identification as investigators gained more experience. Both species were most abundant in headwater regions, decreased downstream, and were sympatric at several locations. Mottled Sculpin were more likely to be found at stream locations with lower conductivity, pH, and stream quality scores, whereas Slimy Sculpin presence was more strongly associated higher levels of DO and lower levels of turbidity. Such findings are important because Mottled Sculpin are a designated management indicator species of the US Forest Service in the Huron-Manistee National Forests, but may be ineffective as a habitat quality indicator when used alone. Concurrent use of Mottled Sculpin and Slimy Sculpin as a management indicator sister-species complex could allow sufficient landscape coverage to permit habitat assessment if species-specific differences in environmental tolerances are precisely determined.

Weak effects of a microsporidian parasite on mottled sculpin in Michigan streams

Despite the widespread occurrence of microsporidian parasites in vertebrates and many invertebrates, research regarding their effects on wild populations remains uncommon. We used several metrics of individual growth, condition, and population characteristics to evaluate the effects of a microsporidian infection (Glugea) on six infected populations of mottled sculpin (Cottus bairdii) compared with 10 nearby uninfected populations. Infected individuals (n = 129) were significantly heavier in each age class when compared with uninfected fish (n = 331), yet other measures of growth and condition indicated little influence of the parasite. No significant parasite-associated effects were found when comparing mortality rates, mean age, sex ratios, and history of population genetic bottleneck between infected and uninfected populations. Additionally, we did not detect significant differences in environmental variables or fish assemblages between streams with and without the parasite. Given the disfiguring nature of infection and the strong effects of microsporidian parasites on other hosts, the lack of significant effects caused by parasitism was unexpected and is likely attributed to strong host density-dependent population dynamics or relatively low host infection intensities.