1. Gill opercle erosion / absence

Classic sign in salmonids of heavy-metal exposure — especially aluminum and zinc. Chronic Al exposure in acidic or metal-rich streams interferes with calcium metabolism and ossification of craniofacial bones.

Opercular erosion has also been reported with chronic low-level lead and cadmium exposure, as these disrupt mineralization.

Less commonly, it can be linked to nutritional deficiency (e.g., phosphorus or vitamin C deficiency), but in your case the heavy-metal context makes contaminants the prime suspect.

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2. White blobs on gills

Could be several things:

Parasites: Ichthyophthirius multifiliis (“ich”) or Trichodina cause visible white cysts or plaques. However, “ich” usually shows up as salt-like specks across the skin and fins as well, not just gills.

Fungal infections (Saprolegnia) form cottony white patches on gills, often opportunistic in stressed fish.

Epithelial hyperplasia / mucous cell proliferation: under a dissecting scope, hyperplastic tissue on gill filaments can look like pale nodules. This is a very common chronic response to metal or pollutant exposure.

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3. Melanophore clustering

You’re right — generalized darkening is a stress response in salmonids.

Causes: toxic stress (heavy metals, hypoxia, parasites) or acute stress from capture.

Given that only a “decent proportion” show this while others are visually fine, the stressed group may be more heavily impacted or already compromised.

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4. Likely underlying mechanism

The metals you mentioned (Al, Zn, Pb, Cd) are well documented to damage gill epithelium:

Cause epithelial lifting, hyperplasia, lamellar fusion → impaired respiration/osmoregulation.

Increase susceptibility to parasites/fungi → explains the blobs.

Chronic exposure interferes with bone mineralization → opercle erosion/missing opercles.

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✅ What to check in histopathology:

Gill: epithelial hyperplasia, lamellar fusion, mucous cell proliferation, necrosis, parasite or fungal structures.

Liver: metal inclusion bodies, vacuolization, necrosis, melano-macrophage centers.

Opercle bone: resorption or mineralization defects.

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Bottom line

What you’re likely seeing is a combined heavy-metal toxicity effect (erosion of opercles + gill hyperplasia) with secondary opportunistic infection (fungal/parasite white spots), all compounded by stress from trapping.