What Goes Wrong Inside a Closed Tank of Fish?
Extension services in the United States, which advise fish farmers, describe the trouble in a few steps [1] [2]. Fish take oxygen from the water, so the oxygen runs down. They give off carbon dioxide, which makes the water slightly acidic. They also produce ammonia, a waste that is poisonous to them. Packed together, shaken around and stressed, they use up oxygen faster. A Kentucky State University sheet says the two limits on how many fish a tank can hold are enough dissolved oxygen and the build-up of waste such as ammonia and carbon dioxide [2].
Temperature matters too. Fish are cold-blooded, so their bodies speed up in warm water and slow down in cold water. One fact sheet says a fish's metabolic rate roughly doubles for every 18°F (10°C) of warming and drops by half for every 18°F of cooling. A slower fish uses less oxygen and makes less carbon dioxide and ammonia, so keeping the water cool helps, for example with ice or gel packs. The same sheet also says that cold water can hold more dissolved oxygen: about 12.1 parts per million at 45°F (7°C) against 10.0 at 60°F (16°C) [1].
So the whole job comes down to three questions: where does fresh oxygen come from, where does the waste go, and how is the water kept cool? Different people in different centuries answered them differently.
How Did People in Old Edo Carry Live Fish to the Fish Market?
In Japan, the city of Edo is the old name of Tokyo. In the Edo period (the 1600s to the 1800s), a "nama-bune" was a river boat with a fish tank built into it. A city explainer sheet from Tokyo's Edogawa ward says these boats loaded live fish at Choshi, a port on the coast, and went up the Tone River. In winter the fish continued overland to Edo. In summer the boats came down the Edo River and through two canals to the Nihonbashi fish market [3].
The sheet does not say why the route changed with the seasons. What it does say is that fresh fish were the point, so haulers chose the shortest distance and the shortest time [3]. It does not describe how water was handled inside these boats, so that part is unknown from this source. For a clear look at how water was swapped, we need a later boat.
Why Did a Fish-Carrying Boat Have Holes in Its Hull on Purpose?
In 1905 a fish dealer in Akashi, a port town on the Seto Inland Sea, designed a new kind of live-fish boat. A professional ship-engineering society records that it had a fish tank, an upright bow, and, for the first time on such a boat, a petroleum engine, which cut the time to reach the big market in Osaka a great deal. The design later became a government-approved standard and was called the "Akashi-type nama-bune" (a live-fish carrier). Over 7,000 were built in total, mostly around the Seto Inland Sea. That figure is a running total over many years, not the number sailing at once [4].
The clever part was the water. A newspaper report on a museum exhibition says the tank at the bow had many holes in the bottom and lower sides, which the crew adjusted with wooden plugs, so fresh seawater kept flowing in [5]. In other words, the boat changed its fish's water for free while it moved. The last boat of this type, built about 43 years before, was scrapped in 2023, and a city library gathered its logbooks and building photos into reports for a museum [4].
On a road, though, there is no sea to pour through the tank. That is where trucks need a different trick.
What Does a Live-Fish Truck Carry to Replace the Sea?
Unlike a boat with holes below the waterline, a truck usually carries its water with it and has to keep that water in good condition. Japan's transport ministry defines a "live-fish truck" in its vehicle rules as a vehicle built only to move seafood alive. For vehicles in that category, the tank must be separated from the driver's seat by a wall, it must have equipment to feed oxygen into the water, and it must be built so that water and foam do not slosh out or splash around while driving [6].
One way to supply oxygen is to carry cylinders and feed oxygen into the tank during the trip. Fish farm advisers in Kentucky stress checking it: dissolved oxygen should be checked often at the start of a trip with a meter, to confirm the aerators and oxygen flow are working [2]. Even so, the cylinders have downsides, which a prefecture in northern Japan decided to test.
Can Fish Be Trucked Without Oxygen Cylinders?
The Yamagata Prefectural Fisheries Experiment Station says carrying oxygen has its own hassles. Cylinders must be refilled by specialists. Cylinders of 20 liters or more require carrying a fire extinguisher and a hazard card. And it says cases of fish dying because the oxygen ran out or a valve was set wrongly "keep happening." So the station tried air instead [7].
In their test, each one-ton tank held fish and two 30-centimeter air stones (porous stones that turn air into small bubbles), fed by a blower. Ice was added from time to time to hold the water temperature down. Cherry salmon were carried at up to 46.8 kilograms of fish per cubic meter of water (about 47 kg per 1,000 liters) at 10°C, and Japanese flounder at up to 38.1 kg per cubic meter at 25°C, with no deaths from lack of oxygen [7].
There are limits, and the report is open about them. Those densest runs were short, 59 km in 58 minutes and 20 km in 27 minutes. A 156 km, 136-minute test used lower densities, so the report does not show the same load would work on a long trip. The authors only say that fish at or below these densities "can be considered possible" to carry this way. The station also notes that an 80-watt power inverter stopped from time to time when it ran a 72-watt blower, so equipment needs some spare capacity [7].
Why Can Fish Still Struggle When the Water Has Plenty of Oxygen?
Oxygen is only one of the three worries. The Niigata Prefecture inland-water fisheries station studied ornamental koi shipped in sealed bags. It found that even with plenty of dissolved oxygen, more fish looked unwell once dissolved carbon dioxide went above 280 milligrams per liter. From this it suspected that carbon dioxide was the big problem [8]. (That is a suspicion drawn from the test, not a proven cause.)
The station then combined a commercial gas-absorbing product with an anesthetic called 2-phenoxyethanol. The result: a box of 15 liters could carry 5 to 6 kilograms of koi instead of 4 kilograms, or the same fish could travel in about 20 percent less water. The station warns that this does not fit every size, and it applies to koi in bags and boxes, not to trucks in general [8].
The sedative side deserves care. The Kentucky sheet notes that anesthetics can calm excitable fish and reduce injuries, but they can also make a fish's body react in ways similar to stress [2]. These chemicals are used by trained fish farmers under rules and are not something to try with a pet fish at home.
What Can You Look at Yourself, and What Is Still Unknown?
Look at a tank in a fish shop or an aquarium. You may spot a bubbling air stone (oxygen), a thermometer or a chiller (temperature), and a filter (which helps remove waste from the water). Those three jobs are the same ones the boat holes, the truck's oxygen and the cold ice were all doing. If you see a live-fish truck, notice where the tank sits and keep a safe distance.
Plenty remains open. The Yamagata test did not show whether air alone could work on long trips. The koi result may not carry over to other fish. And different species need different water, a point also made in Florida's fish-transport guidance [9]. The original reports listed below are short and readable, and they show the exact conditions behind each number.