Taylor, D. L.; Kutil, N. J.; Malek, A. J.; Collie, J. S. Mercury bioaccumulation in cartilaginous fishes from Southern New England coastal waters: Contamination from a trophic ecology and human health perspective. Mar. Env. Res. 99, 2014, 20-33. DOI: 10.1016/j.marenvres.2014.05.009
Sharks and Skates as Seafood?
While smooth dogfish may not be on your list of favorite seafood, cartilaginous fish (mainly sharks and skates) may increasingly find their way onto your dinner plate due to the decline of more traditional fisheries. Spiny and smooth dogfish are two of the most common species of shark found in New England waters, and they’ve long been considered “trash fish” by local fisheries: because of their high abundance and low demand in seafood markets, they’re often discarded as bycatch.
Dogfish and winter skates have been fished as food for some time and little skates have been harvested for use as bait , but sharks and skates have long been regarded as challenging meats to prepare correctly. From time to time, interest in sharks and skates as seafood has popped up – if only this interest could be sustained, fishers could make a good profit selling these species, commonly discarded in the hunt for more desirable species like cod. While increased demand for these species as a food item could help struggling seafood industries, recent mention of using dogfish in federal food programs begs the question: Is it safe to eat shark?
The Mercury Question
Sharks often feed high on the food chain and there are concerns that, like other top predators such as tuna and salmon, they may accumulate high concentrations of methyl mercury. Consumption of seafood is the main way that people are exposed to methyl mercury and elevated levels of this toxin are the primary driver of fish consumption advisories. Inorganic mercury is usually introduced to the environment when it is emitted to the atmosphere from waste incineration or fossil fuel combustion. Mercury from nearby or distant sources can eventually be deposited into ocean waters, where it ends up stored in sediments. This is where innocuous inorganic mercury is converted by bacteria to its methylated, neurotoxic form, methyl mercury. When this methyl mercury leaves the sediment and enters the food chain, it can accumulate in wildlife, magnify up the food chain, and become a threat to public health.
In this study, researchers from Roger Williams University and the University of Rhode Island set out to measure mercury concentrations in common species of sharks (smooth dogfish and spiny dogfish) and skates (winter skate and small skate) along the southern New England coast to determine whether these species are safe for human consumption.
Researchers collected sharks and skates off the coast of southern New England from May to October from 2009 through 2012 during bottom trawl surveys and hook-and-line fishing expeditions.
Muscle samples from sharks and skates were analyzed for total mercury and the stomachs of the sharks and skates were removed and analyzed to to identify what these animals had been eating. Researchers also collected samples of species known to be common prey for skates and sharks and analyzed them for total mercury in order to better understand what prey items might be contributing most significantly to mercury levels in sharks and skates.
The authors found that, while concentrations of mercury in each fish species varied with location and time of capture, smooth dogfish contained the greatest average mercury concentrations, followed by spiny dogfish, with skates containing the lowest mercury levels, as shown in Figure 3.
For all species, mercury content was directly related to size and age: larger, older specimens had accumulated greater concentrations of mercury than smaller, younger sharks and skates. This is depicted in Figure 4. This finding confirms that mercury is accumulating in these species faster than it can be excreted from the body, resulting in an increase in concentration per unit weight over time, a phenomenon referred to as bioaccumulation.
Some other studies on other fish species have documented the phenomenon of “growth dilution” where faster-growing species accumulate mercury more slowly because their size is increasing much faster relative to their mercury intake. In this study, however, faster-growing species, namely smooth dogfish and little skates, had higher concentrations than slower-growing species in a given size range, contradicting the theory of growth dilution in cartilaginous fishes. The authors hypothesize that this may be because faster-growing species prefer different prey and have increased energy demands, requiring that they eat more and subsequently increase their dietary uptake of mercury.
To further explore how differences in dietary preferences among species might influence mercury levels, the researchers catalogued stomach contents in a subset of their catch and compared their findings to the average concentrations of mercury measured in common prey species (Figure 5). Spiny dogfish consumed the most prey species found further from shore, such as fish and squid, with butterfish as the most important prey item. In contrast, smooth dogfish mostly fed on invertebrates from the seafloor, with cancer crabs and unidentified fish being the primary prey items.
The authors note that the smooth dogfish’s elevated mercury levels are due in part to their higher trophic level (they eat higher up on the food chain) and preference for prey that contain higher mercury levels, like cancer crabs, while spiny dogfish’s primary prey of choice (butterfish) was less contaminated. However, spiny dogfish also consumed scup and squid, which explain why their mercury levels are greater than those measured in skates. For skates, invertebrates on the seafloor made up the majority of the diet, with amphipods, expected to contain very low concentrations of mercury, as the primary prey item.
Are sharks and skates safe for human consumption?
The main way that people are exposed to methyl mercury is through eating fish. Chronic exposure in adults can lead to elevated concentrations in the body, which can cause neurological problems such as difficulty speaking and walking, and muscle weakness, which can become permanent with prolonged exposure. Exposure in the womb can harm fetal neurological development, in some cases resulting in severe disability.
Cartilaginous fish like the ones in this study are increasingly being used as a food source due to declines in traditional fisheries, leading the authors to wonder whether these types of fish are safe for human consumption. They found that 24.1% of smooth dogfish and 1.6% of spiny dogfish exceeded mercury levels considered safe by the US FDA (1.0 parts per million wet weight), while skates remained below this threshold, and were considered safe. 87.0% of smooth dogfish, 32% of spiny dogfish, and less than 2% of skates contained concentrations above the US EPA’s somewhat lower human health threshold level (0.3 parts per million wet weight).
Skate: It’s What’s for Dinner!
From this study, it appears that the consumption of smooth dogfish, especially, could pose a significant human health risk. Using the relationship they found between body size and mercury concentration, researchers estimated that dogfish will typically contain levels of mercury above what is considered safe by the US EPA once they’ve reached about a foot in length (22.5 inches for smooth dogfish and 27.9 inches for spiny dogfish) – a relatively small size for these species.
On the other hand, the study shows that skate seem like a relatively safe seafood option. Interested in adding some New England skate to your diet? Share your thoughts in the comments.
Previously on oceanbites:
Top predators reveal the extent of mercury pollution in Canadian waters by Samantha DeCuollo
I am the founder of oceanbites, and a postdoctoral fellow in the Higgins Lab at Colorado School of Mines, where I study poly- and perfluorinated chemicals. I got my Ph.D. in the Lohmann Lab at the University of Rhode Island Graduate School of Oceanography, where my research focused on how toxic chemicals like flame retardants end up in our lakes and oceans. Before graduate school, I earned a B.Sc. in chemistry from MIT and spent two years in environmental consulting. When I’m not doing chemistry in the lab, I’m doing chemistry at home (brewing beer).