Fish side streams are not one raw material

01 · Three streams

Same conversion, different behaviour

Kajaani University of Applied Sciences (KAMK) procured three rounds of black soldier fly (BSF) rearing trials, 70 recipes in all, run in spring 2026 in a Finnish BSF producer's container facility. The fish raw materials were:

rainbow trout gut mass: the viscera from gutting;
rainbow trout filleting side stream: heads, bones, fins and skin with some flesh attached;
lake fish side stream: gut mass and filleting fraction of mixed wild lake fish, such as bream, pike-perch, pike, perch and vendace, delivered combined.

Bioconversion, the share of the mix's dry matter that ends up as larval dry matter, was in the same range for all three: about 16–18 %. The real question is which stream works reliably, and on what terms.

Failed batches

Lake fish side stream

none in three rounds

Trout gut mass

2 in round 1; 5 of 6 in round 3 (too wet)

Trout filleting side stream

1 in round 1
Recommended fish share

Lake fish side stream

30–45 %

Trout gut mass

10–30 %

Trout filleting side stream

20–40 %
Pre-treatment

Lake fish side stream

fine crushing

Trout gut mass

none; dry mix, no added water

Trout filleting side stream

fine crushing required
Role

Lake fish side stream

base raw material

Trout gut mass

supplement for large larvae

Trout filleting side stream

co-substrate

02 · Lake fish

Lake fish: the reliable base

Not one lake-fish batch failed in three rounds, and it had the widest moisture margin. The best fully measured result of the final round came from about 30 % lake fish with potato and wheat bran: 1.38 kg of fresh larvae per tray and 145 mg per larva with raw, unmarketable potato. With potato peelings it gave 1.21 kg and 130 mg, a difference within measurement noise. (Final-round shares include the added water: the fish was 27 % of the loaded mix.) The recommended share is 30–45 %.

Two catches: heads and bones that larvae do not eat need proper fine crushing, so the best result carries the heaviest pre-treatment. And the mercury in lake-fish larvae exceeded the general limit for feed materials, so raw-material batches must be screened (the mercury findings).

03 · Gut mass

Trout gut mass: highest ceiling, narrowest window

Gut mass produced the biggest larvae. In round 1, acidified gut mass at 30 % with potato and wheat bran gave 183 mg larvae. In the final round, a 10 % recipe without wheat bran averaged about 180 mg, and two of its trays left to day 8 reached 195 and 187 mg. It is homogeneous, mixes evenly and needs no crushing. The acidified form did as well as the oil-skimmed one, saving the skimming work.

It is also the wettest stream and the most moisture-sensitive. Run dry in round 2, gut mass at 45 % gave the round's best batches. With more water in round 3, five of its six batches failed: the larvae left the over-wet trays before they started eating. More gut mass only added water and risk, not size. The recommended share is 10–30 %, in a dry mix with no added water.

04 · Filleting side stream

Filleting side stream: a co-substrate that needs crushing

It worked at every share and tolerated moisture. Its limit was piece size: heads, bones and skin did not break down, causing lumps, ammonia and uneaten feed. Its larvae stayed smaller, 89–115 mg in the final round. The cleanest results came at 20 %; 40 % gave more mass but also lumps and odour. The recommended share is 20–40 %, finely crushed.

05 · Moisture

Moisture tolerance belongs to the raw material

Moisture ran through all three rounds. The water-to-bran pre-mix moved from 2:1 to 1:1, then to 1.5:1 plus water at loading. The same practice that drowned the gut mass suited the drier filleting and lake-fish streams. One water setting does not fit every fish side stream.

06 · What this means

What this means

Build the recipe around the raw material, not a fish percentage: crushed lake fish as the base, gut mass low and dry, the filleting side stream finely crushed. Raw fish side streams are not a permitted substrate for insects reared for feed or pet food in Finland today (fishmeal is), so the route to commercial use runs through confirmation trials and a written position from the Finnish Food Authority.

All results are in the summary report. If you have a side stream you want evaluated, get in touch.

About the project: 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.