Assessing the contribution of multiple stressors affecting small-bodied fish populations through a gradient of agricultural inputs in northwestern New Brunswick, Canada

Non-point source discharges, such as agricultural runoff, are often complex mixtures of chemical and non-chemical stressors. The complexity of runoff is compounded by its sporadic releases and few studies have attempted to assess the impacts of runoff on aquatic biota. In this study, an effects based approach was used to examine survival and reproduction of slimy sculpin (Cottus cognatus) in the intensive potato-farming areas of northwestern New Brunswick, Canada. Using non-lethal methods, fish were collected during the ice-free months through a gradient of agricultural intensity. These data were correlated with waterborne levels of pesticides, water temperatures and precipitation data. Results indicate that both adult and young-of-the-year (YOY) fish are longer and heavier in the downstream sites draining areas of higher agricultural intensity. Precipitation has a significant negative relationship with %YOY in the agricultural areas but not in the upstream forested area, indicating that contaminants are present in runoff caused by intensive rainfall events. Our results indicate that YOY sculpin may be at higher risk in the agricultural areas in years of heavier summer rains where peaks in pesticide levels occur. This study expands the existing knowledge base and development of non-lethal methods to define cause–effect relationships.