Mercury burden in round goby and yellow perch in relation to riparian wetlands of the Upper St. Lawrence River
To examine if riparian wetlands are a source of mercury to fish, 174 yellow perch (Perca flavescens) and 145 round goby (Neogobius melanostomus) were collected in 2019 from eight wetland and seven non-wetland habitats throughout the Upper St. Lawrence River to examine mercury concentration differences between habitat types. Mercury levels were significantly (p < 0.01) higher in fish collected at wetlands than those collected from non-wetland habitats for both yellow perch and round goby. Perch had mercury concentrations of 74.5 ± 35.4 ng/g dry wt in wetlands compared to 59.9 ± 23.0 ng/g dry wt in non-wetlands. Goby had mercury concentrations of 55.4 ± 13.8 ng/g dry wt in wetlands and non-wetland concentrations of 41.0 ± 14.0 ng/g dry wt.
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Fish were sampled in the nearshore of the Upper St. Lawrence River adjacent to 8 wetlands and 7 non-wetland sites. Distances between contrasting sites (wetland vs non-wetland) were measured in Google Earth Pro 18.104.22.16848 (64-bit) and compared to assess potential movement of fish among sites. Rules for determining distances among sites were as follows: (1) sites being compared must be on the same contiguous shoreline, assuming that the young fish will not be crossing the main channel to nearby islands; (2) sites on islands without any sites on the same contiguous island or nearby island separated by shallow water will not be compared to other sites given the assumption (1) above; (3) straight lines will be used when possible, but exceptions will be made for peninsulas where a conservative number of straight lines will be used to estimate distance traveled around the land feature. The shoreline riparian zone in wetland sites was categorized as marsh, meadow, emergent cattails, or cultivated. The overwhelming shoreline categorization was natural, with two wetlands categorized as having maintained shoreline. The riparian zones of non-wetland sites were cultivated, with one composed of deciduous forest. Fish were collected by hand using a 9.5 m x 1.8 m bag seine net with a mesh size of 6.4 mm. The seine net method consisted of 3-5 hauls per site. Collected fish were identified, counted, and measured for total length. Yellow perch and round goby were separated into polyethylene bags (Whirl-Pak) and placed in a cooler until return to the laboratory where they were frozen (-20 °C) until future processing and analysis. Up to 25 individuals of each species per site were collected for determining Hg content. Sample selection was done at random, although fish < 1 cm were released due to their size not being suitable for collection as they are not large enough to determine species accurately. Lethal methods were employed by cutting the brain cavity of the fish with a stainless-steel knife or flicking the anterior end until death. Fish individuals were homogenized in a stainless-steel blender after being measured for length. Homogenized tissue from entire individual fish were analyzed for mercury using the DMA-80 Direct Mercury Analyzer (Milestone Srl, Bergamo, Italy). The operating procedure developed was based upon the mercury analysis component of EPA Method 7473. DOLT-5 dogfish liver standard reference material from National Research Council Canada (440 ± 180 ng/g Hg; percent recovery 83.4%) was used in each sample batch in duplicate. Samples were run in duplicate weighing between 0.09 and 0.1 g. The relative standard deviation (RSD) of samples ranged from 0.01 - 53.5% with an average RSD of 9.06%. Dry weight calculations were done by drying two replicates of each sample at 60°C in a drying oven. Samples analyzed in wet weight. Results were converted after Hg analysis of wet tissue on the basis of dry weight and reported as Hg per unit dry biomass.