Baitfish are sold by many different methods across North America. The two most often used are by weight or by displacement.
The displacement method, or "by the gallon" is most frequently used in the Northern U.S. and Canada. Generally, a 5 gallon plastic pail is used with graduation marks on the inside. Water is filled up to the 1st line, which is 1 gallon. Then the fish are strained through a dip-net, letting the water drain out, and the fish are poured into the pail up to the next line or any thereafter. There is also another method to measure out exactly 10 gallons of fish - 1 five gallon pail is filled full with water and then divided equally between two other pails, or into thirds. Then all three pails are filled with fish until full, equaling 10 gallons. (5 gal. Water / 10 gal. Fish = 3 five gallon pails) This method can also be used to measure exactly 5 gallons of fish or 15 gallons of fish.
1 U.S. gallon = 8.3 pounds, but often just estimated at 8 pounds.
1 Canadian gallon = 10 pounds (this is only used in Canada)
The weight method is just simply that. The fish are weighed by filling the pail with a random amount of water (usually about 1-2 gallons), zeroing the scale and then filling with fish to the desired weight. This is highly accurate, but more time consuming, and overage in measure is uncommon, where by displacement tends to be somewhat more generous.
Sometimes fish are sold by the piece or thousand. Actually, they are still being sold by either of the two other methods, but the unit of sale is changed to indicate number of fish.
1000 Small Fatheads = 4 pounds, or 1/2 gallon
With some of the large baitfish, specifically White Suckers, they are actually counted out fish by fish. Because of their large size, no other method is very accurate, and usually the numbers that are typically sold are not that high.
This may sound confusing, but with a little experience it's not a problem. Unfortunately there isn't an industry standard, but generally the same unit of measure is used throughout a specific region.
Here at Gollon Brothers Wholesale Live Bait Inc, our baitfish are sold by the gallon, or displacement method (except white suckers over 8"). We often convert to pounds for customers when desired.