Growth and Methylmercury Accumulation in Juvenile Chinook Salmon in the Sacramento River and Its Floodplain, the Yolo Bypass

Abstract

The literature indicates a strong correlation between inundation of previously oxidized soils, as can occur on a floodplain, and increased microbial methylation of mercury. There is special concern over the potential for increased methylmercury levels in the Yolo Bypass, a 24,000-ha floodplain for California's Sacramento River and its tributaries. The objective of our first study component was to compare methylmercury accumulation between juvenile Chinook salmon Oncorhynchus tshawytscha in the Yolo Bypass and those in the Sacramento River during a winter 2005 flood event. For each location, we tested accumulation in two groups of hatchery Chinook salmon juveniles: (1) free-ranging, coded-wire-tagged fish that were released into the floodplain and river and recaptured by downstream sampling and (2) fish that were reared in enclosures at fixed locations in both the river and floodplain. We found that free-ranging juvenile Chinook salmon in the floodplain accumulated 3.2% more methylmercury per day than did free-ranging fish in the river. However, fish in the floodplain grew 0.7% more per day than fish in the river. Variance in growth and in methylmercury content was significantly higher in the free-ranging fish than in the enclosure-reared fish, suggesting suboptimal rearing conditions in the enclosures. In a second study component, we analyzed methylmercury levels of free-ranging Chinook salmon released in the Yolo Bypass during hydrologically variable years (2001–2003 and 2005); the objective was to determine whether interannual differences in the primary source of floodwater to the Yolo Bypass were associated with different patterns of mercury accumulation in Yolo Bypass Chinook salmon. Fish in the Yolo Bypass showed different patterns of methylmercury accumulation in 2001, 2002, 2003, and 2005. Methylmercury accumulated linearly with time in years when Cache Creek provided the primary source of flood flow but followed a quadratic pattern in years when flood flow was dominated by the Sacramento River.