Fish dissection

Testing mercury levels in Florida’s marine fishes

by Derek Tremain and Doug Adams

Fish dissection
Removing muscle tissue from a Spanish mackerel for mercury testing.

Mercury, a toxic metallic element, has been shown to bioaccumulate in fish tissue, and if humans eat contaminated fish, they can potentially consume significant levels of mercury. Testing fish flesh for mercury content began in 1989, the same year the Fisheries-Independent Monitoring (FIM) program began and continues today. In the early years, fish samples were sent to the Florida Department of Environmental Protection for testing, but in 2006 the FIM program purchased a new state-of-the-art instrument (DMA-80 Direct Mercury Analyzer) that now allows fisheries scientists from FIM’s Indian River field laboratory to analyze more fish samples for mercury content and reduce the processing time. To date, FIM staff have collected, processed, and documented mercury information from more than 80,000 individual fish yielding almost 300 species, all of which were collected through the statewide sampling program.  These individual specimens represent all major groups from primary consumers and prey species to apex predators at the top of the food web and include many popular sport fish such as seatrout, red drum, snappers, groupers, common snook, mackerels, and tunas to name just a few. The majority of marine and estuarine fishes examined contained low levels of mercury, but several important recreational and commercial fishery species such as sharks, tunas, mackerels, and cobia have shown elevated levels. Many of the samples processed by FIM scientists are provided to the Florida Department of Health (FDOH) to develop and update fish consumption advisories. These advisories provide specific guidelines regarding Florida marine fishes, and recent updates can be found on the FDOH website.

The venom in lionfish spines remains active, so fisherman are urged to clean their catch with an abundance of caution. Photo by John Stevely Florida Sea Grant Extension Emeritus

One species of recent interest, with almost no previous mercury information available, is the invasive Indo-Pacific lionfish (Pterois spp). Given the recent management strategy to encourage divers and fishers to harvest and eat this species, FIM scientists analyzed mercury levels in this species from throughout Florida’s coastal waters and published the research in a recent paper. The results suggest that lionfish contain very low levels of mercury, similar to species that currently fall under the FDOHs least restrictive consumption guidelines. However, lionfish populations are in the relatively early stages of establishment in Florida and larger and older individuals, with potentially higher mercury levels, will likely become available to consumers. Therefore, a routine part of the FIM program is to continue to monitor this species, and others, to identify any changes in mercury levels that may occur over time.

In the past few months, FIM staff and partners at the Florida Department of Environmental Protection have analyzed nearly 3,000 samples that include many fishery species such as mangrove snapper, red snapper, red porgy, common snook, black sea bass, spotted seatrout, and red drum. Future research relating mercury levels to fish age, feeding ecology, and the trophic structure (food web) of Florida’s marine and estuarine ecosystems will help us better understand concentrations of this element in marine fishes.