# Fish Fattening (DD-723)

> Bluefin tuna 'fattening' or ranching is capture-based aquaculture: wild tuna are caught, usually by purse seine, towed in cages to farms and fed for months before harvest, mainly for the Japanese sashimi market. In the Mediterranean, farms feed defrosted sardines, herring, mackerel and squid. Daily rations run about 2-10% of the caged biomass, and one Greek farm reported 5-8% of body weight a day at the start, falling to 2-3% near harvest. Feed conversion is poor, around 10-20 kg of baitfish per kg gained, because only about 5% of the energy goes into growth. Much of the gain is fat, which raises the price.

- Department: Nature
- First published: 2026-06-08
- Last updated: 2026-06-08
- Permalink: https://detailenjoyer.com/case/fish-fattening/
- Author: Marcus Easterling (DetailEnjoyer), The Detail Department

## Quick facts

- **DD-723-F01**: Fattening farms feed about 10-20 kg of baitfish per kg of tuna weight gained. _(Write-up · Exhibit B (What they eat))_
- **DD-723-F02**: Daily rations are about 2-10% of the caged tuna biomass. _(Write-up · Exhibit C (How much))_

## Write-up

Wild bluefin tuna are fattened in sea cages before sale. Farms feed them roughly 15-20 kg of baitfish for every kg gained.

1. **Catch, then farm**: Fattening is capture-based aquaculture. Wild bluefin are caught, mainly by purse seine, transferred alive to towing cages, then moved to farm cages, where they're fed for several months before slaughter.
2. **What they eat**: Farms feed mixed, defrosted small pelagic fish such as sardine, pilchard, herring, mackerel and bogue, plus squid, usually 1-3 times a day. A diver often watches and signals when the tuna stop feeding.
3. **How much**: Daily feed is about 2-10% of the estimated caged biomass, depending on water temperature and fish size. One Greek farm estimated 5-8% of body weight a day at first, falling to 2-3% by harvest.
4. **Poor conversion**: Feed conversion ratios are about 10-15:1 for small fish and 15-20:1 for large ones. Bluefin are warm-bodied, constant swimmers, so only about 5% of the energy they eat goes into growth.
5. **Fat, not just size**: A 40-50 kg tuna can gain 30-40% of its weight in 6-7 months, a 150-200 kg fish much less. Much of the gain is fat, which is prized in sashimi markets.

## Related case files

- [CrabMaxxing (DD-735)](https://detailenjoyer.com/case/crabmaxxing/): Another food-farming case
- [Dipping Dipper (DD-040)](https://detailenjoyer.com/case/dipping-dipper/): Another animal case
- [Sheep Dipping (DD-602)](https://detailenjoyer.com/case/sheep-dipping/): Another look at how animals are farmed
- [Gyotaku (DD-671)](https://detailenjoyer.com/case/gyotaku/): Another case about fish and Japan

## Reel vs. record

- **Missing species**: southern bluefin tuna are also ranched this way, notably in South Australia, not only Atlantic and Pacific bluefin.
- **Precision**: 'several percent' is right. Rations are about 2-10% of the caged biomass a day, highest early and falling to 2-3% near harvest.
- **Missing context**: feed conversion is poor, at about 10-20 kg of baitfish per kg gained, because only about 5% of the energy goes into growth (FAO).
- **Missing context**: farmers aim for fat content as well as weight. Fattier fish sell for more in sashimi markets.
- **Sustainability**: because the fish are wild-caught, farms depend on regulated wild quotas (managed by ICCAT for Atlantic bluefin) and on large amounts of wild baitfish.

## Video: Fish Fattening: Wild Bluefin Tuna Are Fattened in Sea Cages Before Sale

- Duration: 17.5 s · Published: 2026-06-08
- File: https://detailenjoyer.com/assets/videos/DZVS3Swxh5o.mp4

### Transcript

DID YOU KNOW? In tuna fattening operations, wild-caught Atlantic bluefin tuna or Pacific bluefin tuna are transferred to large offshore sea cages and fed enormous quantities of oily baitfish to rapidly increase their weight before harvest. A single adult tuna can consume several percent of its body weight in food each day. NOW YOU KNOW!

## Questions & answers

**Q: Where do the tuna in fattening farms come from?**  
A: They're wild-caught, then farmed. _(Source: Write-up · Exhibit A (Catch, then farm) · FAO (2009), Capture-based aquaculture: bluefin tuna (chapter); FAO: Cultured Aquatic Species Information Programme, Atlantic bluefin tuna)_

**Q: About how much baitfish does it take per kg of tuna gained?**  
A: Feed conversion is about 10-20:1. _(Source: Write-up · Exhibit D (Poor conversion) · FAO (2009), Capture-based aquaculture: bluefin tuna (chapter); FAO/GFCM-ICCAT Working Group on Sustainable Bluefin Tuna Farming/Fattening Practices (2005))_

**Q: What's the daily ration as a share of caged biomass?**  
A: About 2-10% a day. _(Source: Write-up · Exhibit C (How much) · FAO (2009), Capture-based aquaculture: bluefin tuna (chapter); ICCAT Collect. Vol. Sci. Pap. 65(3): Weight growth of Atlantic bluefin tuna as a result of a 6-7 months fattening process in the central Mediterranean)_

**Q: About what share of the energy eaten goes into growth?**  
A: About 5%, because bluefin are warm-bodied, constant swimmers. _(Source: Write-up · Exhibit D (Poor conversion) · FAO (2009), Capture-based aquaculture: bluefin tuna (chapter); FAO/GFCM-ICCAT Working Group on Sustainable Bluefin Tuna Farming/Fattening Practices (2005))_

**Q: Which bluefin species is ranched in South Australia?**  
A: Southern bluefin. _(Source: Write-up · Exhibit A (Catch, then farm) · FAO (2009), Capture-based aquaculture: bluefin tuna (chapter); FAO: Cultured Aquatic Species Information Programme, Atlantic bluefin tuna)_

## Sources

- [FAO (2009), Capture-based aquaculture: bluefin tuna (chapter)](https://www.fao.org/4/i0254e/i0254e08.pdf)
- [FAO: Cultured Aquatic Species Information Programme, Atlantic bluefin tuna](https://www.fao.org/fishery/culturedspecies/BFT/en)
- [ICCAT Collect. Vol. Sci. Pap. 65(3): Weight growth of Atlantic bluefin tuna as a result of a 6-7 months fattening process in the central Mediterranean](https://www.iccat.int/Documents/CVSP/CV065_2010/n_3/CV065030787.pdf)
- [FAO/GFCM-ICCAT Working Group on Sustainable Bluefin Tuna Farming/Fattening Practices (2005)](https://www.fao.org/4/y8870e/y8870e0g.htm)

## Cite this case file

Easterling, Marcus. "Fish Fattening." The Detail Department (DetailEnjoyer), case file DD-723, last updated Jun 8, 2026. https://detailenjoyer.com/case/fish-fattening/