Once you’ve managed to get the first cohort of fish past the feeding challenges and disease risks, it’s eventually time to harvest them.

Deciding When to Harvest

Before worrying about knowing when to harvest, set a goal for the fish you plan to sell. Most aquaponics systems aim to raise fish to a weight of between 1.5 and 2 pounds, as this weight point offers the best balance between the cost of production and the sale price. Due to the slowing rate of growth as a fish gets larger, aiming for a weight above this range will take much longer than the amount of time it took to reach the two-pound mark. You don’t need every fish to reach the same weight, but at least 75% of the fish you harvest should be at the target weight or you’ll find it difficult to meet the standards of wholesale buyers or consumers. There’s relatively little demand in most markets for fish larger than two pounds, especially in the species that are well suited to aquaponics production.

Grading the Harvested Fish

Grader bars are the primary method for separating out the fish that are large enough to harvest, but you can’t count on this tool alone. Aim to weigh around 10% of the fish you harvest with a grader bar to verify they’re reaching your goal weight and not just the right size. Grading is an extensive process in mixed age rearing tanks, but goes relatively quickly in batch tanks where only a small percentage of the fish should be under the goal size. Grading bars are popular in aquaponics, but they’re not the only grading tool. Mesh bags are also a valuable tool for sorting out smaller and larger fish. Three or more mesh bags are stacked inside each other. The mesh size gets progressively smaller towards the outer mesh bags. The largest fish can’t leave the first and smallest bag while smaller fish escape into the outer bags. Then it’s simply a matter of lifting up the middle mesh bag to scoop out the largest fish without stressing the rest of them.

Purging the Fish

Before you can pull a single fish out and process it, you’ll need to process all the fish you’ve determined to be of the right size and weight. This is generally done in a separate tank, but can be done in-tank if you’ve sorted out any undersized fish. Fish that are harvested without purging tend to have an unpleasant muddy taste. You simply need to hold the fish for ten to 14 days without feeding them and change the water daily by 25%. Only use clean, fresh water that hasn’t circulated through the system or the same flavor compounds will continue to build up in the meat.

Hygienic Handling and Processing

Because most fish produced by aquaponics methods are destined to become human food, they must be handled hygienically from the moment you harvest them. Killing the fish is best accomplished with a tool known as a fish bat, which is a solid piece of hardwood you use to firmly hit the fish a few times in the center top of its head. This immediately kills the fish with no suffering or damage to the harvestable meat.

Once a fish is dispatched, the gills must be cut to release the blood. Hang the fish from its tail for a few minutes until the blood stops flowing freely. Many areas don’t allow for on-site processing, so many companies end their harvesting here by moving the fish to cold storage at this point. If you are allowed to further process on-site, you’ll need to slit the belly of the fish and remove its organs, then cut the head off or at least remove the gills with a sharp knife. Fish should be kept on ice from the moment bleeding ends to prevent spoilage. Freezing fish whole is usually the best option for preparing them for sale to both consumers and wholesale buyers.