Optimising fish farming through size sorting

Aquaculture specialist Leslie Ter Morshuizen, owner of Aquaculture Solutions, designs and builds fish farms and aquaponics systems across sub-Saharan Africa and trains farmers to manage them optimally. He shares his expertise on the practice of size sorting in fish farming.

- Leslie Ter Morshuizen

As fish farmers we feed our fish optimal quantities of highquality feed to achieve rapid growth so that they attain market size as quickly as possible. A small challenge with this process is that the fish do not all grow at the same rate, necessitating intervention by the farmer.

The intervention we apply is to size sort the fish. This is achieved by confining the fish in a small area and offering an escape that requires them to swim through mesh or between parallel bars. Often this involves netting the fish out of their tank in small groups and placing them inside the net or size sorter that has the appropriate aperture sizes. The size of the gap is accurately determined for the species and age of fish, and allows the smaller fish to swim through, while larger fish cannot fit through the allowed space. This effectively divides them into two groups based on girth.

BENEFITS OF SIZE SORTING

Virtually all species of food fish require size sorting, with those that are cannibalistic needing the most frequent separation into groups of similarsized individuals. For example, catfish (Clarias gariepinus) need to be sorted weekly during the hatchery phase and monthly thereafter to avoid a huge reduction in the number of fish being cultivated. Failure to size-sort a batch of catfish can result in cannibalism, reducing the original quantity of fish by more than 99%. Even species such as tilapia (