Mercury follows the fish

01 · The finding

Larvae grown on a mixed lake-fish side stream carried the mercury of the fish they ate. The practical lesson from KALARVO is to screen the raw material, not only the finished larvae.

KALARVO tested three Kainuu fish side streams as feed for black soldier fly (BSF) larvae, over three rounds and 70 recipes. This article covers one finding; the summary report covers the whole project.

02 · Measurements

What we measured

Larvae from the final round were frozen whole and analysed in an accredited laboratory, one composite per raw-material group, for four heavy metals: mercury, cadmium, lead and arsenic.

EU feed law sets maximum levels for feed materials at 12 % moisture, so the fresh larvae values are converted before comparison with the limit.

Larvae grown on

Lake-fish side stream (mixed wild fish)

Mercury, mg/kg fresh

0.097

Mercury at 12 % moisture, mg/kg

about 0.25 (0.19–0.31)

Against the 0.1 mg/kg feed-material limit

about 2.5 times the limit
Rainbow trout filleting side stream (farmed fish)

Mercury, mg/kg fresh

0.0059

Mercury at 12 % moisture, mg/kg

0.013

Against the 0.1 mg/kg feed-material limit

13 % of the limit

The lake-fish result is clear even before conversion: the fresh value alone is already at the level of the limit. The range reflects the about 25 % measurement uncertainty.

The trout larvae were a different story. Mercury was 16 times lower, and cadmium, lead and arsenic stayed below 10 % of their limits in both larva types.

03 · Source

Where the mercury came from

The lake-fish side stream was a mix of wild species delivered as one batch: bream and other cyprinids, zander, pike, perch and vendace. The rainbow trout was farmed fish. The result shows that mercury moves from the fish into the larvae.

04 · Frass

The frass carries less

Frass, the larvae's droppings mixed with uneaten feed, was also analysed from the lake-fish trays. Its mercury was 0.15 mg/kg of dry matter, about half of the larvae's 0.28 mg/kg on the same basis, and about 15 % of the organic fertiliser limit of 1 mg/kg.

In KALARVO, the mercury risk sits with the larvae and the feed route, not with the frass. Frass has its own hurdle, hygiene, covered in the next article.

05 · Screening

Screen the raw material

Mercury is a manageable risk once you know where to look:

Screen each batch. Measure mercury in each lake-fish raw-material batch and know its species mix before any feed or pet-food use.
Know your stream. The farmed-trout side streams gave larvae well within all four heavy-metal limits.
Check the route. The limits above apply to feed materials; other uses have their own requirements.

Mercury is not the only gate. Raw fish side streams are not a permitted substrate for insects reared for feed or pet food in Finland today; fishmeal is. For lake fish, mercury is a second, independent gate.

06 · What this means

What this means

Lake fish is not off the table. It was the most reliable raw material in the rearing trials, with no failed batches in three rounds. But the quality check has to start before the larvae. A mercury result on the incoming raw material tells you before rearing whether the larvae could ever go to a feed route.

If you have a fish or other side stream you want evaluated, get in touch.

Kajaani University of Applied Sciences (KAMK) procured black soldier fly larvae rearing trials on Kainuu fish side streams as part of the KALARVO project (Processing fish side streams into value products). Manna Insect carried out the trials as the contractor, responsible for the trial design, measurements and analysis. Co-funded by the European Union.