Abstract

Marine mammals and sea turtles often fatally interact with fishing gear.^1-4 If an animal is not found with gear, it can be difficult to determine the source of the interaction.⁵ In Virginia, pound net gear remains in the water for months at a time, and fishers apply diluted, copper-based anti-fouling paint to the nets prior to deployment in order to reduce fouling during the fishing season. At the time of the strandings, pound net leaders were constructed using a relatively heavy, twisted, nylon twine. In cetaceans, we can often identify the distinct pattern associated with pound net twine in the ligature lesions, but it is usually difficult to detect this pattern on the thicker skin of sea turtles. Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry which is capable of detecting metals and several non-metals at concentrations as low as 0.1 parts per billion (ppb).⁶ Here we test the use of ICP-MS as a forensic tool to detect elevated copper levels associated with anti-fouling paint on ligature marks suspected of being made by pound nets.

We tested three samples each (ligature, adjacent to ligature, and control > 5.0 cm from the ligature) from four stranded animals, one Kemp's ridley, one loggerhead sea turtle, and two bottlenose dolphins. Trace levels of copper (ng/mg tissue) were significantly elevated in the ligature lesions compared with the other samples on both sea turtles (lesion = 131.29 and 826.42 ng/mg tissue; adjacent = 2.24 and 7.05 ng/mg tissue) and one of the two dolphins (lesion = 19.99 ng/mg tissue; adjacent = 1.22 ng/mg tissue), and the trace levels of copper were similar between the sample adjacent to the lesion and the control area (0.38–6.23 ng/mg tissue difference). In the other animal, the ligature lesion had the lowest concentration of copper and the control had the highest, though the difference was only 0.235 ng/mg tissue. Values were similar for ⁶³Cu and ⁶⁵Cu isotopes. The loggerhead sea turtle was removed from a pound net, the other three animals stranded with ligature lesions consistent with pound net entanglement. All of the animals were in a moderate state of decomposition. Of the three that were not removed from a pound net, they stranded from 1.3 to 28.7 km from the closest pound net. The animal with the highest copper reading stranded the farthest from a pound net, suggesting that the copper persists in ligature marks despite an animal drifting some distance in seawater.

This pilot study suggests that trace metals associated with specific fishing gear may be detected using ICP-MS and may persist for some time in salt water. Next steps would be to increase our sample size over a range of species and decomposition levels with and without the distinct twisted twine impressions as well as net samples from both newly coated nets and nets that have soaked for weeks to months to determine the amount of copper transference that occurs during entanglement.

* Presenting author

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