It turns out these sedentary creatures can answer some mysteries of the deep.
Research Need
One specific area of the continental shelf off South Carolina and Georgia fascinates both oceanographers and fisheries scientists. Known as Scamp Ridge, this area regularly sees summertime water temperatures about 2°F less than nearby areas. It’s also thought to be an area of significant upwelling — the process when cool, deep, nutrient-rich water is brought closer to the surface.
Scientists would love to confirm that these upwelling events are occurring and, if so, whether upwelling changes the mix of trace metals on the shelf. Trace metals are of particular interest because they can help us better understand carbon cycling and overall ocean health.
To study how upwelling affects the presence of trace metals, oceanographers would need a natural recording device at these unique locations that would have been collecting data for years. However, the sediments on the shelf aren’t suitable for these types of analyses.
But fishery scientists and oceanographers got together and figured out that spotted moray eels frequent the Scamp Ridge area — and, even better for purposes of this study, moray eels spend most of their relatively long lives in one place.
If scientists could analyze the chemical composition of moray eel otoliths (the calcium carbonate structures in the fish’s head that assist in balance and buoyancy), the moray eel might serve as a stationary recording device to document upwelling over time.
What did they study?
Scientists from Skidaway Institute of Oceanography obtained otoliths from 48 spotted moray eels from an existing offshore fish-trap survey conducted from 2005 to 2006.
About one-third of the moray eel samples came from Scamp Ridge. To test their theory that this upwelling site might expose the eels to different trace elements than at sites without upwelling, they also compared the sites’ samples to samples from north and south of Scamp Ridge.
The team used mass spectroscopy to measure the trace elements magnesium, strontium, barium, vanadium, manganese, copper, zinc, and lithium in the moray eel otoliths. They also obtained bottom temperatures and salinity data for each site.
What did they find?
Trace elements in moray eel otoliths in samples from the middle of Scamp Ridge were like the chemical signature of the deep-water upwelling that occurs there. Likewise, the otolith profiles at the two sites without upwelling were similar to each other, but not to the Scamp Ridge samples.
In short, the moray eels acted like water chemistry recording devices, just as the team expected.
While the distinct cause of upwelling at Scamp Ridge is a mystery, it is located immediately downstream of the offshore deflection of the Gulf Stream at the Charleston Bump. This geological formation likely allows sustained eddy-driven upwelling of cool water to Scamp Ridge.
Anything else?
Twelve species of moray eel have been reported from the South Atlantic Bight. Long-term fisheries surveys have determined that three species (spotted moray, reticulate moray, and ocellated moray) make up greater than 92% of morays collected in our region of the coast.
While most morays are captured in waters of about 160 to 330 feet deep, individuals of each species regularly appear in mid-shelf waters of 100 to 200 feet.
So what?
As you might imagine, marine geochemists typically analyze the rocks, sediments, and minerals of the seafloor to study the environment. This study demonstrates that the otoliths within sedentary fish can provide information about oceanographic conditions that would not otherwise be available.
If you’re interested, the research team provides detail about the behaviors and patterns of each trace metal that the team studied.
Reading
Herbert L. Windom and William B. Savidge. 2023. Sources and transport pathways of trace metals to the outer continental shelf off South Carolina and Georgia, USA revealed from the otoliths of moray eels. Continental Shelf Research 282 (2024). 105331. https://doi.org/10.1016/j.csr.2024.105331
Photo credit: NOAA Office of Ocean Exploration and Research.
The text from Hook, Line & Science is available to reprint and republish at no cost, but only in its entirety and with this attribution: Hook, Line & Science, courtesy of Scott Baker and Sara Mirabilio, North Carolina Sea Grant.
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