Mercury Specs for Indonesian Tuna: Size-Based Playbook

A dockside, fork-length–based system to keep Indonesian yellowfin under the EU 1.0 mg/kg mercury limit. Includes practical cutoffs, decision rules, sampling plan, and spec wording you can copy-paste.

We turned a 1-in-5 rejection problem into 99% EU passes in 90 days using this system. Not lab tricks. Just rigorous fork-length sorting, clean data, and a lean verification plan that fits the reality of Indonesian docks.

Here’s the playbook our team uses for Indonesian yellowfin tuna. If you buy or export to markets with the EU tuna mercury limit, you’ll want to bookmark this.

The 3 pillars of reliable mercury compliance

Sort by size at the dock using fork length. Mercury in yellowfin rises with size and age. A simple, enforced cutoff keeps most lots comfortably under 1.0 mg/kg.
Verify with focused sampling. Don’t overspend on blanket testing. Target the size bands that actually drive risk.
Lock it into your purchase spec and supplier training. Consistent measurement beats last-minute lab rescues.

Our experience shows that if you get those three right, the rest is optimization.

Weeks 1–2: Map your fish and validate numbers

Start with your last three months of landings. For each lot, add fork length (FL), catch area, product form, and any mercury results you have. Build a quick regression: mercury vs fork length. You’ll see the same curve we see across Indonesian fleets.

What we see in Indonesian yellowfin tuna:

FL under 110 cm typically averages 0.35–0.6 mg/kg.
110–125 cm averages 0.6–0.9 mg/kg.
Above 125–130 cm, exceedance risk rises fast. Individual fish over 1.0 mg/kg are common.

The practical cutoff that keeps EU compliance high: FL ≤ 120 cm or round weight ≤ 35 kg. That’s our default when a buyer asks for “EU 1.0 mg/kg compliant yellowfin.” Some EU retailers now run internal targets at 0.8 mg/kg to protect margin. If that’s you, push the cutoff to FL ≤ 115 cm.

Takeaway: Set your initial rule at FL ≤ 120 cm for EU lots. Validate it on your data before you scale.

Weeks 3–6: Implement fork-length sorting and tagging

Training matters. Mercury decisions are only as good as your measurements.

How we measure fork length consistently at the dock:

Use a rigid fish board. Mouth closed. Snout touching the stop. Tape along the midline.
Fork length is snout tip to the inner fork of the tail. Not total length.
Record to the nearest cm. If exactly between, round up.
Tag graded fish immediately by color band: Low-risk (≤ 115 cm), Monitor (116–125 cm), High-risk (> 125 cm).

Decision rules we run on the dock:

EU-destined lots: accept Low-risk without hesitation. Sample the Monitor band. Divert High-risk to non-EU markets or alternate formats.
Domestic or markets with higher limits: you can keep > 125 cm, but don’t mix those fish into EU lots.

We apply these rules across our yellowfin lines like Yellowfin Saku (Sushi Grade), Yellowfin Steak and Yellowfin Cube (IQF). If your spec is sub-0.8 mg/kg, we’ll dial the cutoff tighter and bias yields to belly and mid-loin from smaller fish.

Weeks 7–12: Scale and optimize with a lean sampling plan

Here’s the thing. You don’t need to test every fish. You need to test the right fish.

A practical verification plan we’ve pressure-tested:

Lot definition: vessels or day’s landings by size band.
Low-risk band (≤ 115 cm): sample 1 per 2 metric tons. Minimum n=3 per lot.
Monitor band (116–125 cm): sample 1 per metric ton. Minimum n=5 per lot.
High-risk (> 125 cm): do not include in EU lots. If included for other markets, sample separately.
Composites: for screening, you can pool 5 subsamples within the same size band. If a composite is ≥ 0.8 mg/kg, retest individuals. Never pool across size bands.
Method: ISO 17025 lab using thermal decomposition amalgamation (e.g., DMA-80) or equivalent. Keep a 48-hour turnaround SLA.

We track trending by supplier and size band. If a supplier’s 116–125 cm fish average ≥ 0.9 mg/kg for two consecutive months, we shift their EU cutoff to 115 cm until corrective actions stick.

Need help tailoring the sampling plan to your fleet mix or buyer spec? Contact us on whatsapp and we’ll share a template you can drop into your QA SOP.

Common questions we get

What fork length cutoff keeps Indonesian yellowfin below the EU 1.0 mg/kg mercury limit?

We recommend FL ≤ 120 cm as a reliable operational cutoff for EU lots. If you need a tighter internal buffer (retail private label, baby food adjacency), use FL ≤ 115 cm. In our data set, that moves the lot average from roughly 0.7–0.8 mg/kg down to 0.55–0.65 mg/kg.

How reliable is fish size as a predictor of mercury in yellowfin tuna?

Very. Our Indonesia-only regression across 1,800 fish shows strong correlation. Past ~90 cm FL, mercury rises about 0.015–0.02 mg/kg per additional cm on average. Individual outliers exist, which is why we verify the 116–125 cm band.

Do catch area or season in Indonesia change the size-based cutoff?

They add noise. They don’t rewrite the rule. We see small basin differences between the Indian Ocean and the Western Pacific, and modest seasonal shifts tied to prey. But after you control for size, those effects are second order. Keep the same cutoffs and adjust sampling intensity by supplier and area if you notice sustained drift.

How many samples per lot do I need to verify mercury compliance cost-effectively?

For mixed lots under 20 tons, n=5 to n=8 targeted samples usually strikes the right cost-risk balance. Use more samples when your lot skews toward 116–125 cm. Use fewer when you’re firmly ≤ 115 cm. Always stratify by size band.

What’s the correct way to measure fork length consistently on the dock?

Snout to fork, mouth closed, flat board, nearest cm. Train with photos, not paragraphs. We keep a laminated card on every board. Two measurers agree on the first 20 fish at the start of each shift to align.

Can trimming or selecting loins reduce mercury enough to pass a 1.0 ppm spec?

Not reliably. Mercury binds to muscle protein, so it’s relatively uniform across loins. You might see slightly lower values in belly portions compared to dorsal tops, but the differences won’t rescue an over-1.0 mg/kg fish. The real lever is fish size. If you must trim, do it for yield and texture, not for mercury.

How should I word a purchase specification to enforce a size-based mercury limit?

Here’s a practical template you can adapt:

Species and origin: Yellowfin Tuna (Thunnus albacares), Indonesia, FAO 57/71.
Size control: Fork length ≤ 120 cm or Round Weight ≤ 35 kg for all fish included in EU-destined lots.
Mercury: Total mercury < 1.0 mg/kg per EU regulation. Internal target < 0.8 mg/kg.
Sampling: Stratified by size band. Low-risk band 1/2 MT (min n=3). Monitor band 1/MT (min n=5). No high-risk fish in EU lots.
Method: Accredited ISO 17025 lab. Thermal decomposition amalgamation or equivalent.
Traceability: Each carton linked to vessel/date/size band. Fork length records retained 12 months.
Corrective action: If two consecutive lots exceed targets, implement FL ≤ 115 cm until corrective actions verified.

5 mistakes that kill mercury compliance programs

Measuring total length, not fork length. You’ll think you’re safe at “120 cm TL” and then fail. Use FL.
Mixing high-risk fish into EU bins “to fill the container.” That’s how you lose a whole lot to one outlier.
Sampling by habit, not by risk. If the lot is all ≤ 115 cm, save the money. If it’s heavy in 116–125 cm fish, sample more.
Treating mercury like histamine. Trimming and icing won’t change it. Only fish size and sourcing do.
Vague purchase specs. If your spec just says “Hg < 1.0 mg/kg,” expect arguments later. Add fork length limits, sampling rules, and corrective actions.

Quick dockside QA checklist

Fish boards calibrated and labeled “Fork Length.”
Color-tagging system by size band. Staff aligned at start of shift.
Separate totes for ≤ 115 cm, 116–125 cm, and > 125 cm. Never mix bins.
Real-time tally sheet: count by band, average FL, vessel, and set.
Pull targeted samples as you build the lot. Don’t wait until boxing.
Hold-and-release protocol tied to lab ETA.

If a lot won’t clear the EU limit, we redirect to other product lines like Mahi Mahi Portion (IQF), Cobia Fillet (IVP / IQF), or Wahoo Portion (IQF / IVP / IWP) that naturally run low mercury and fit family retail programs.

Resources and next steps

Build your 90-day plan: Set FL ≤ 120 cm for EU. Train measurement. Start stratified sampling. Trend by supplier.
Tighten to ≤ 115 cm if your brand targets < 0.8 mg/kg.
Watch the 116–125 cm band like a hawk. That’s where most surprises live.

We run this system across Indonesian fleets every day. If you want our fork-length SOPs, sampling templates, or to spec an EU-compliant yellowfin run for Yellowfin Steak or Yellowfin Saku (Sushi Grade), Call us and we’ll walk you through what works for your lanes.

One last thought. The market is quietly rewarding suppliers who can guarantee < 0.8 mg/kg without drama. Size-based sorting is the simplest way to get there. Start now, and you’ll feel it in your approval rate within a month.