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Species-specific effects of turbidity on the physiology of imperiled blackline shiners Notropis spp. in the Laurentian Great Lakes

Increased sedimentary turbidity associated with human activities is often cited as a key stressor contributing to the decline of fishes globally. The mechanisms underlying negative effects of turbidity on fish populations have been well documented, including effects on behavior (e.g. visual impairment) and/or respiratory function (e.g. clogging of the gills); however, the longterm physiological consequences are less well understood. The decline or disappearance of several blackline shiners Notropis spp. in the Laurentian Great Lakes has been associated with increased turbidity. Here, we used non-lethal physiological methods to assess the responses of 3 blackline shiners under varying degrees of threat in Canada (Species at Risk Act; pugnose shiner N. anogenus: endangered; bridle shiner N. bifrenatus: special concern; blacknose shiner N. heterolepis: common) to increased turbidity. Fish were exposed for 3 to 6 mo to continuous low levels of turbidity (~7 nephelometric turbidity units, NTU). To test for effects on respiratory function, we measured both resting metabolic rate (RMR) and critical oxygen tension (the oxygen partial pressure at which the RMR of fish declines). No change in RMR was detected across species or in clear versus turbid treatments. However, critical oxygen tensions were negatively affected by longterm exposure to low levels of sedimentary turbidity in the 2 imperiled species, viz. pugnose and bridle shiner, but not in the more common blacknose shiner. Variation in effects of turbidity on respiratory performance suggests differential sensitivity to turbidity, which may contribute to the current conservation status of the 3 species.