Small fish have been analyzed in the RMP Exposure and Effects Pilot Study since 2005. Small fish are excellent indicators of biological uptake of contaminants, particularly mercury. Small fish have high site fidelity and are prey for higher trophic level organisms such as piscivorous birds, mammals, and fish. Each year, sites are selected based on site characteristics such as enclosed embayments, open bay sites, wetlands with differing mercury concentrations, sites in close proximity to mercury mines, and sites near wastewater treatment facilities.

The monitoring to date has established a clear spatial gradient in food web methyl mercury, with elevated concentrations observed in the Lower South Bay (near Alviso slough) and declining concentrations moving towards the Delta. Seasonal variation has been demonstrated to occur in Hg bioaccumulation in San Francisco Bay and elsewhere. However, seasonal variation differs among locations, with summer Hg peaks in two South Bay wetlands, spring peaks in Delta tributaries after large storm events, and variable patterns in North Bay salt ponds.

There is a need to better understand the patterns and magnitude of seasonal variation, in order to determine what times of the year present the greatest potential risks to piscivorous wildlife. In 2011, seasonal variation will be assessed at four sites around the bay, and is intended to complement previous and on-going work funded by the CalFed program and by the South Bay Mercury Program.

This study will examine methylmercury concentrations in fish less than one-year in age. Small fish tend to have small ranges in habitat and are a dominant food-source for piscivorous fish. The purposes of this study are:

  • to monitor small fish for the assessment of long-term trends in methylmercury bioaccumulation; and
  • to provide data on the seasonal variation of methylmercury uptake.

Applicable RMP Management Questions

This study will assist in answering the following questions that were developed as part of the RMP Mercury Strategy.

Small fish are the best tool for assessing inter-annual variation in food-web mercury in aquatic habitats.

Continued long-term and seasonal monitoring may aid in identifying high leverage processes, such as seasonal flooding events in high flow years, which are hypothesized to result in elevated biotic Hg exposure.

Continued long term monitoring is needed to determine whether specific management activities (e.g., source control through the MRP; wetland restoration activities) affect Hg biotic exposure in the Bay.

  1. Where and when is mercury entering the food web?
  2. What are the high leverage processes, sources, and pathways?
  3. What effects can be expected from management actions?


A report summarizing the findings from the multi-year sampling effort from 2008 and 2010 will be released in the summer of 2011.

Staff Involved

This project will be led by Meg Sedlak of SFEI. Darell Slotton and Shaun Ayers of UC Davis will perform field sampling and laboratory analysis.

Programs and Focus Areas: 
Clean Water Program
Data Services
Location Information