On the half-life of thiocyanate in the plasma of the marine fish Amphiprion ocellaris: implications for cyanide detection

Abstract

The illegal practice of using cyanide (CN) as a stunning agent to collect fish for both the marine aquarium and live fish food trades has been used throughout the Indo-Pacific for over 50 years. CN fishing is destructive to all life forms within the coral reef ecosystems where it is used and is certainly one of many anthropogenic activities that have led to 95% of the reefs in the Indo-Pacific being labeled at risk for degradation and loss. A field-deployable test for detecting fish caught using CN would assist in combating the use of this destructive practice, however, no reliable and robust test exists. Further, there is little toxicokinetic data available on marine fish to support the development of such a test, yet such data is critical to establishing the concentration range and time scale over which such a test would be viable. This study presents the first direct measurement of the half-life of the metabolite thiocyanate (SCN) after pulsed exposure to CN in a marine fish. SCN was measured in the plasma of Amphiprion ocellaris after exposure to 50 ppm CN for three exposure times (20, 45, and 60 s) using HPLC-UV and a C30 column pre-treated with polyethylene glycol. Plasma SCN levels observed are dose-dependent, reflecting a longer time for conversion of CN to SCN as the dose of CN increases. SCN plasma levels reached a maximum concentration (1.2–2.3 ppm) 12–20 h after exposure to CN. The half-life for the elimination of SCN was 1.01 ± 0.26 days for 45 s exposure and 0.44 ± 0.15 days for 20 s exposure. Fish were also directly exposed to SCN (100 ppm for 11 days) and the observed half-life for SCN elimination was 0.35 ± 0.07 days. Plasma SCN levels did not return to control levels, even after 41 days when exposed to CN but did return to control levels after 48 days when exposed to SCN. The similar half-lives observed for CN and SCN exposure suggests that SCN exposure can be used as a proxy for measuring the rate of SCN elimination following CN exposure. In order for plasma SCN to be used as a marker for CN exposure, these results must be extended to other species and endogenous levels of SCN in wild caught fish must be established.

Where does it go? The fate of thiocyanate in the aquarium water and blood plasma ofAmphiprion clarkiiafter exposure to cyanide

“…Trial 1 CN exposures have more sampling data points at early 366 time points, resulting in a better fit of the elimination from the blood plasma rather than the 367 terminal half-life. 368 SCN concentration for A. ocellaris plateaued between two and four days at approximately 500 369 ppb for both 20 and 45 s exposure times (Breen et al 2019). In the first trial of the current study, 370 the SCN concentration plateaued between four and 12 hours at approximately 150 ppb for both 371 exposure times, likely due to the faster half-life and likely lower dose of CN ingested.…”

“…With the aquarium water test outside of the realm of possibility from a mass balance perspective, 107 testing for SCN in bodily fluids of exposed marine fish is the next logical step. Indeed, elevated 108 levels of SCN in the plasma of a marine fish after acute pulsed exposure to CN have been 109 recently reported (Breen et al 2019). SCN in the blood plasma of the laboratory cultured marine 110…”

“…This study reported an initial fast elimination rate of SCN in blood plasma followed by a 112 much slower elimination rate as evidenced by long residence times of low levels of SCN above 113 that found in the controls. Breen et al (2019) speculated that the observation of both a fast and 114 slow elimination rate might be due to multiple elimination pathways in marine fish. These rates 115 could also be governed by the availability of sulfur donors and the rate of diffusion from organs 116 and tissues with limited blood flow (Day et al 2018).…”

“…These rates 115 could also be governed by the availability of sulfur donors and the rate of diffusion from organs 116 and tissues with limited blood flow (Day et al 2018). Breen et al (2019) were the first to report the SCN concentration in the plasma of a marine fish 118 following CN exposure. The report was for a single species and the species used was from an 119 aquacultured stock with limited genetic diversity within the replicate fish.…”

“…Two-compartment 475 models are commonly used in pharmaceutical research for drug metabolism (Metzler 1971). As 476 CN is absorbed through the gills and/or the gut via the drinking response, it is metabolized into 477 SCN, and the SCN concentration increases in the blood plasma of the fish (Breen et al 2019). 478…”