Harvesting and Grading in Aquaculture: A Guide
How farmed fish are harvested and graded: harvest methods, pre-harvest feed withdrawal, off-flavor purging, size grading, and minimizing handling stress.
Overview
Harvesting is the removal of fish from the culture system for sale or restocking, and grading is the sorting of fish by size. Both are stressful, labour-intensive steps that strongly affect product quality and fish survival. Good harvesting practice plans the method, the timing, the water quality during crowding, and the gentle handling needed to deliver healthy, marketable fish.
Harvest methods
- Seining: a seine net is drawn through the pond to crowd and capture fish, used for partial or complete harvest.
- Draining or draw-down: the pond water level is lowered to concentrate fish at the outlet or a harvest basin.
- Partial harvest: only market-size fish are removed and the rest grow on, common in catfish and tilapia.
- Complete harvest: the whole stock is removed at once, often by combining seining with draining.
- Traps and fish pumps: used to crowd, lift or move fish with less manual handling in larger operations.
Pre-harvest preparation
Feeding is normally stopped for a period before harvest so that the gut empties, which reduces waste, oxygen demand and fouling of the water while fish are crowded and hauled. Stopping feed also lowers the metabolic load during the handling that follows. Water quality is watched closely during crowding because dense aggregations rapidly deplete oxygen.
Off-flavor and purging
In pond-raised fish such as channel catfish, certain cyanobacteria release the compounds geosmin and 2-methylisoborneol (MIB), which are absorbed into the fish's fatty tissue and cause a muddy or musty off-flavor. Affected fish are held in clean water to depurate, a process that can take days to weeks depending on lipid content, water temperature and the severity of the episode. Fish are typically tested for flavor before harvest and held until judged on-flavor.
Grading
Grading sorts fish into size classes, usually with bar graders whose bar spacing lets smaller fish pass while retaining larger ones. Grading produces uniform batches for market, separates fish that have outgrown or lagged behind, and in some species reduces cannibalism by keeping similar sizes together. Grading is done as gently and quickly as possible because it adds handling stress on top of harvest.
Minimizing handling stress
- Crowd fish for as short a time as possible and keep oxygen high during crowding.
- Handle fish gently, keep them wet, and avoid harvesting during the hottest part of the day.
- Withhold feed before harvest to empty the gut and reduce water fouling.
- Move graded and harvested fish promptly into clean, well-oxygenated holding or transport water.