← Back to the archive★ Support my pages
DD-723NatureFiled Last updated

Fish Fattening

Short answer: 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.

VideoWrite-up4 sources
More like this★ Support my pagesCite this file

What should you know about Fish Fattening?

Quick facts. Each one stands alone and has a stable ID you can cite.

  1. Fattening farms feed about 10-20 kg of baitfish per kg of tuna weight gained.

    DD-723-F01 · Write-up · Exhibit B (What they eat)

  2. Daily rations are about 2-10% of the caged tuna biomass.

    DD-723-F02 · Write-up · Exhibit C (How much)

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

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!

Burned-in captions, read frame by frame (see transcript.md) · 17.5s Reel ·

Video coming soonFish Fattening · Instagram Reel
Fish Fattening: Wild Bluefin Tuna Are Fattened in Sea Cages Before Sale Instagram Reel · · 17.5s

The 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. Exhibit A

    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. Exhibit B

    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. Exhibit C

    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. Exhibit D

    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. Exhibit E

    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.

Questions people ask about Fish Fattening

Where do the tuna in fattening farms come from?

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

About how much baitfish does it take per kg of tuna gained?

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)

What's the daily ration as a share of caged biomass?

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

About what share of the energy eaten goes into growth?

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)

Which bluefin species is ranched in South Australia?

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

Test Your Knowledge

All quizzes →

5 questions, all from this case file. Answers are revealed instantly.

  1. Where do the tuna in fattening farms come from?

    1. Caught in the wild
    2. Hatcheries only
    3. Rivers
    4. Lakes
    Show the answer

    Answer: A. Caught in the wild 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

  2. About how much baitfish does it take per kg of tuna gained?

    1. 1 kg
    2. 3 kg
    3. 10-20 kg
    4. 100 kg
    Show the answer

    Answer: C. 10-20 kg 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)

  3. What's the daily ration as a share of caged biomass?

    1. About 0.1%
    2. About 2-10%
    3. About 50%
    4. About 100%
    Show the answer

    Answer: B. About 2-10% 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

  4. About what share of the energy eaten goes into growth?

    1. 5%
    2. 50%
    3. 75%
    4. 95%
    Show the answer

    Answer: A. 5% 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)

  5. Which bluefin species is ranched in South Australia?

    1. Southern bluefin
    2. Atlantic bluefin
    3. Yellowfin
    4. Skipjack
    Show the answer

    Answer: A. Southern bluefin 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

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/

Permalink: https://detailenjoyer.com/case/fish-fattening/ · Last updated

Markdown version

Cite a single fact by its ID, for example DD-723-F01 → https://detailenjoyer.com/case/fish-fattening/#DD-723-F01