Excreted thiocyanate detects live reef fishes illegally collected using cyanide: a non-invasive and non-destructive testing approach

Cyanide fishing is a method employed to capture marine fish alive on coral reefs. They are shipped to markets for human consumption in Southeast Asia, as well as to supply the marine aquarium trade worldwide. Although several techniques can be used to detect cyanide in reef fish, there is still no testing method that can be used to survey the whole supply chain. Most methods for cyanide detection are time-consuming and require the sacrifice of the sampled fish. Thiocyanate anion (SCN(-)) is a metabolite produced by the main metabolic pathway for cyanide anion (CN(-)) detoxification. Our study employed an optical fiber (OF) methodology (analytical time 3.16 µg L(-1)) of SCN(-) in seawater. Given that marine fish exposed to cyanide excrete SCN(-) in the urine, elevated levels of SCN(-) present in the seawater holding live reef fish indicate that the surveyed specimens were likely exposed to cyanide. In our study, captive-bred clownfish (Amphiprion clarkii) pulse exposed for 60 s to either 12.5 or 25 mg L(-1) of CN(-) excreted up to 6.96±0.03 and 9.84±0.03 µg L(-1) of SCN(-), respectively, during the 28 days following exposure. No detectable levels of SCN(-) were recorded in the water holding control organisms not exposed to CN(-), or in synthetic seawater lacking fish. While further research is necessary, our methodology can allow a rapid detection of SCN(-) in the holding water and can be used as a screening tool to indicate if live reef fish were collected with cyanide.publishe

Schematic representation of experimental procedures for cyanide (CN⁻) pulse exposure and depuration of <i>Amphiprion clarkii</i>.

<p>C – control specimens not exposed to CN⁻; PE 12.5 – Specimens pulse exposed during 60 s to 12.5 mg L⁻¹ of CN⁻; PE 25.0 - Specimens pulse exposed during 60 s to 25.0 mg L⁻¹ of CN⁻; Circles with a white background represent specimens surviving until the end of the depuration period, while light and dark grey backgrounds represent specimens dying at least 30 min after the pulse exposure and during the depuration period, respectively.</p

Concentrations (µg L⁻¹) of thiocyanate (SCN⁻) excreted during the depuration period of <i>Amphiprion clarkii</i>.

<p>Values are averages ± standard error of SCN⁻ excreted by fish not exposed to cyanide (control group) or pulse exposed to 12.5 or 25.0 mg L⁻¹ of cyanide. The grey dashed line represents the detection limit (3.16 µg L⁻¹) of the optical fiber methodology employed to determine SCN⁻concentrations on the water samples collected from the jars holding <i>A. clarkii</i> during the depuration period. Different superscript letters in columns in the same day post-exposure (DPE) represent significant differences at p<0.05.</p