Massachusetts conducted its first statewide examination of mercury in freshwater fish in 1994, followed in 1999 by an investigation of fish in a region of the state predicted to have high atmospheric deposition of mercury. A study of historical mercury deposition into one lake in this region through the analysis of a sediment core, using radioisotope dating techniques, complemented that study.
In 2001, Massachusetts established a long-term monitoring network of lakes to track changes in mercury contamination of fish. This coincided with reductions in mercury use and emissions in Massachusetts and the surrounding region. A 2006 report - Long-Term Monitoring of Massachusetts Fish Tissue Mercury, 1999-2004 file size 1MB file size 3MB - presents the results from the first five years of monitoring, highlighting changes in fish tissue mercury concentrations that have taken place in the high mercury deposition area while emissions from major point sources of mercury have declined substantially in Massachusetts and across the Northeast.
Other studies included one of seasonal variation in fish tissue mercury concentrations, to provide perspective on this type of variance in measurements and help better design monitoring studies. Massachusetts also conducted a comparative food web mercury study in two similar lakes located near each other, but with different levels of mercury in top predator fish. This increased understanding of the ecological basis for varying patterns seen in different lakes.
Wildlife is an integral part of any pond ecosystem. A first step toward addressing the risks mercury poses to animals that live in and around the water - fish-eating birds such as loons, for example - is to better understand their exposure from the food chain. As part of its overall program, Massachusetts has compiled information on mercury in wildlife.
Sediment and water quality of the lakes where fish have been studied were analyzed, and that data along with individual fish tissue mercury concentrations are available from the Fish Mercury Research Data Portal. This database contains total mercury concentrations in edible tissues (dorsal muscle). Almost all the mercury in these fish is in the methylmercury form; however it is analyzed as total mercury. The identified reports may be consulted for details of analytical methodologies employed in particular parts of the program.
Users of this data set are cautioned to be aware of (and to control for as necessary in analyses that they perform) the confounding effect of fish size (age) on tissue mercury concentrations when comparing data sets. This is because older, larger fish of predominantly predaceous species generally have more mercury than younger, smaller fish. Groups of a species of fish from different locations or times being compared may have different size compositions and hence different mercury concentrations due in part or wholly to size differences. In order to control for the effect of varying size of fish in species where there is a size-concentration relationship, either an Analysis of Covariance must be run on the data controlling the data for the effect of the covariate, fish weight, or individual fish tissue mercury concentrations can be standardized to a concentration of a theoretical fish of standard length and then these values can be compared. Methods for performing these adjustments are described in Section 2.4 of the Long-Term Monitoring of Massachusetts Fish Tissue Mercury, 1999-2004 file size 1MB file size 3MB .