Overview
As apex marine predators, sharks are susceptible to mercury contamination through bioavailability and bioaccumulation of mercury via natural food web cycles. Mercury exists in the environment in many forms from natural elemental mercury (Hg) to toxic methyl mercury (MeHg). Some of this mercury is naturally occurring while some is derived as waste from anthropogenic sources such as power plants, incinerators, and chlor-alkali industrial plants. Bacteria can convert total mercury into the toxic form of MeHg, which is then available to the food chain.
Mercury contamination may affect sharks, in addition to other challenges they face. Before we can understand how Hg affects sharks, we must characterize mercury in muscle tissue for species captured in southern Florida and determine how much of that mercury is in the toxic form of MeHg. We also will develop a model to correlate total Hg and shark length, in hopes that length may be an estimator of mercury concentration for each species. This may be useful when considering human health advisories, such as the Federal Drug Administration’s 1.0 μg/g (ww) action level for human health concerns.
Groundwork
Mercury contamination is a common concern for consumers of seafood. Fish are more commonly consumed than sharks, so most of the research with mercury (Hg) has focused on different teleost fish species such as tunas, swordfish, and salmon. Recently, researchers have been concerned with Hg contamination in sharks; both for human health reasons and for the overall health of the animals. Sharks are top predators but differences in Hg between species exist. Three published studies have assessed mercury levels in Florida sharks (Adams & McMichael, 1999; Heuter et al., 1995; Nam et al., 2011). We are currently collecting data on multiple species over a large area to compare with previous studies and increase our understanding of how sharks accumulate mercury.
Recent Study Highlights & Selected Examples
Between 2006 and 2011, we collected and sampled 589 sharks of 11 different species. Our preliminary data suggests that Hg levels in Atlantic sharpnose averaged less than previous studies in Florida, whereas Hg values in Blacknose, Blacktip, Bull, Caribbean reef, Lemon, Sandbar, and Tiger sharks averaged higher than other studies in Florida. We also sampled Great hammerhead and Nurse sharks but no previously published studies sampled these species in Florida (See figure below).
We are developing relationships for each species between predicted Hg and total length to determine the threshold length that exceeds human health advisories. This gives greater resolution to broad-cast advisory levels, providing length so that consumers can decide if they are at risk when eating certain sharks.
From a shark conservation standpoint, we compare different species and look at their preferred prey types to better understand how mercury is moving through the food web and into sharks. Though there is a lot of research to be conducted still, we may trace this mercury back to its sources and help prevent it from finding its way into our oceans and into sharks.
RJD In Focus
we are developing relationships for each species between predicted Hg and total length to determine the threshold length that exceeds human health advisories
