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Determination of Geographical and Seasonal Variations of Heavy Metals in Swordfish (Xiphias gladius) and Yellowfin Tuna (Thunnus albacares)

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Overview

This study used secondary export-company testing data to map geographical variation in heavy metals in swordfish and yellowfin tuna associated with the Sri Lankan export fish market.

Key numbers

Source units are ppm/mg/kg as mapped categories unless otherwise stated.

  • Hg accumulated in swordfish: n=302, collected June to September 2017.
  • Heavy metals in yellowfin tuna: n=33, collected January 2015 to August 2017.
  • Cd, As, and Pb were not detected in the fish samples according to the abstract.
  • Mercury was the main accumulated metal; mapped swordfish Hg categories were 0.00-0.60, 0.60-0.80, 0.80-1.00, and 1.00-2.00 ppm.
  • The highest mercury accumulation was identified at longitudes 64-70 and latitudes 0-8.

Methods

The study used secondary heavy-metal analysis data supplied by two unnamed Sri Lankan fish-export companies, with export-company testing conducted once per three months over January 2015 to August 2017. The authors then traced log-sheet waypoints to particular catching vessels and used QGIS to map Hg values against fish species, dividing the catch area into four stations (station 1: 64-70°E, 0-8°N; station 2: 76-80°E, 0-8°N; station 3: 88-92°E, 14-18°N; station 4: 82-90°E, 6-14°N). Mercury map color bins were 0.00-0.60 ppm (blue), 0.60-0.80 (green), 0.80-1.00 (yellow), and 1.00-2.00 (red). The paper does not describe any of the underlying analytical chemistry — no instrument (AAS, ICP-MS, CV-AAS), no limit of detection, no certified reference material, and no digestion or sample-preparation procedure is reported. This limits the analytical defensibility of the non-detect claims for Cd, As, and Pb and of the Hg values themselves.

Implications

The source is useful for geographic/seasonal swordfish mercury context in the Indian Ocean export fishery off Sri Lanka. It provides per-station exceedance proportions against the 1 ppm and 0.8 ppm thresholds (station 1: 40.33% > 1 ppm, 81.93% > 0.8 ppm; station 2: 9.73% > 1 ppm, 29.55% > 0.8 ppm; station 3: 10.81% > 1 ppm, 72.97% > 0.8 ppm) and seasonal exceedance proportions at station 2 across June, July, August, and September 2017. The Cd, As, and Pb non-detect flag should be cited only with the caveat that the source provides no LOD or analytical method, so non-detect cannot be quantified against any specific instrument floor. The mapped categories are not individual sample values.

Update history

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