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Copper and Zinc Accumulated Differently Across Tissues in Fish and White-throated Kingfishers from River Kalpani

White-throated Kingfisher with a fish in Salai, Uttar Pradesh, India
Research · Ecology

A study from River Kalpani in northern Pakistan measured copper (Cu) and zinc (Zn) in two freshwater fish species and in white-throated kingfishers (Halcyon smyrnensis). Copper and zinc are essential trace elements, but excessive accumulation can become harmful. The researchers examined how the metals were distributed among tissues and whether the pattern was consistent with transfer from aquatic prey to a fish-eating predator.

Samples came from five sites in the Mardan and Nowshera districts between December 2017 and March 2018. The material comprised 64 fish — 30 spotted snakeheads (Channa punctata) and 34 zig-zag eels (Mastacembelus armatus) — together with tissue samples from 15 white-throated kingfishers. Copper and zinc were measured separately in several tissues using flame atomic absorption spectrophotometry, which atomises the sample in a flame and measures how strongly the metal atoms absorb light.

Map of sampling sites along River Kalpani in Khyber Pakhtunkhwa, Pakistan
Five sampling sites along River Kalpani in the Mardan and Nowshera districts of Khyber Pakhtunkhwa, Pakistan. Figure from Ahmad et al. (2026), ICCK Journal of Intelligent Biotechnology, CC BY 4.0. Source.

Different tissues carried different metal loads

In both fish species, copper and zinc generally accumulated most strongly in the liver, followed by the gills, with lower concentrations in other tissues. The authors suggest that the high liver concentrations may be related to metallothioneins, proteins involved in binding and handling metals. They found no consistent increase or decrease in concentrations from upstream to downstream sites.

In the kingfishers, copper and zinc occurred in feathers, bones and internal organs. Feathers and liver were among the tissues with the highest concentrations. As in the fish, there was no continuous upstream-to-downstream pattern.

Higher bird means, but no clear overall species difference

Mean copper and zinc concentrations were generally higher in kingfisher tissues than in fish tissues. The authors interpret this pattern as consistent with trophic transfer through the food web. At the same time, the analysis did not support a clear overall separation in metal levels among the species across all sites, so the higher bird values did not form a uniform difference between trophic levels.

The concentrations measured in the kingfishers remained below the avian toxicity thresholds used by the authors, who found no indication of immediate toxic risk. They nevertheless argue that the generally higher concentrations in the birds justify continued monitoring and suggest that white-throated kingfishers could serve as bioindicators of metal contamination in freshwater ecosystems.

Sources and image credit

  1. Ahmad, S., Ilyas, M., Gul, S., & Khan, M. Q. (2026). The ecological ripple: Investigating Cu and Zn transfer across trophic levels from fish to birds in River Kalpani, Pakistan. ICCK Journal of Intelligent Biotechnology, 1(1), 1–8.Primary open-access research article under CC BY 4.0. Five River Kalpani sites yielded 64 fish and 15 White-throated Kingfishers. Mean copper and zinc concentrations were generally higher in bird tissues, although the overall interspecies difference across all sites was not statistically significant; bird values remained below the toxicity thresholds used by the authors.
  2. FishBase. Channa punctata (Bloch, 1793) — Spotted snakehead.Taxonomic name check. The primary paper uses Channa punctatus; the accepted name used here is Channa punctata.

Image: Charles J. Sharp / Wikimedia Commons — white-throated kingfisher consuming a fish at Salai, Uttar Pradesh, India, 24 November 2017; Wikimedia Commons Quality Image; representative species image, not a study bird · CC BY-SA 4.0 · Source

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