Siphakeme .Mnindwa
Leslie Ter Morshuizen, Correspondent
AS fish farmers, we feed our fish optimal quantities of high-quality 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 similar-sized 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 percent.
Even species such as tilapia (Oreochromis niloticus) will eat their tank mates during the early stages of life if there is sufficient difference in the size of the fish.
Perhaps even more surprising, if a size difference allows them the opportunity to do so, fish fingerlings will eat each other in preference to high-quality feed.
There are numerous benefits to size sorting fish including reducing cannibalism.
The benefits of size sorting to reduce cannibalism have been mentioned above, and for many species this is absolutely vital to ensure an acceptable survival rate among the fingerlings.
Ensuring equal access to feed
When fish of different sizes are held in the same tank, the larger fish will dominate the best feeding spots, satisfying their hunger before the smaller fish are able to feed.
Time aggravates the situation whereby the larger fish continue to feed and grow well, whereas smaller fish tend to have restricted access to feed and grow poorly, falling further behind their larger tank mates.
Having only fish of similar sizes in a tank means more equal access to feed, resulting in the even growth of the group.
Providing the correct feed
Small fish require smaller feed particles that contain a higher level of protein than do larger-sized feeds. If the fish in a tank are of mixed sizes, then either the feed will be too large for the smaller fish to feed on or too rich in protein and consequently unnecessarily expensive, for the larger fish.
By having fish that are similar in size, the optimal feed can be provided, both in terms of the particle size and the protein content.
Meeting market demand
The market often requires fish of a narrow size range, and fish that are outside of this range obtain a lower market price.
Holding fish of similar sizes in groups makes harvesting the entire group for marketing (when they attain the optimal size) far simpler than having to select individual marketable fish from a group. Such “partial harvesting” is also very stressful for the fish.
The alternative is that a group of mixed-sized fish is harvested and sold, with a portion of them receiving a suboptimal price.
Culling
A final benefit of size sorting is that it affords an opportunity in the early stages to remove fish that are growing at a rate that is suboptimal and not commercially viable.
These fish are then separated from the remainder of the fish and culled. This means that the remaining fish will all grow to market size within a financially acceptable period.
The term “grading” is sometimes used interchangeably with “size sorting” but this is misleading, as “grading” is a quality-based selection process.
“Size sorting”, on the other hand, does not consider quality; it merely selects according to girth. The term “size sorting” should thus be used when referring to the classification of fish based on their girth width.
There are many benefits to size sorting on a fish farm, including optimal growth and feed utilisation, and the reduction of cannibalism.
It is important to ensure that you know the requirements of the species of fish you are farming in terms of size sorting frequency.
According to the Food and Agriculture Organisation, Zimbabwe has relatively developed aquaculture and is retained to be one of the top-ten fish farming countries in Sub-Saharan Africa for a decade.
In 2014, the total production was estimated at 10 600 tonnes and much of the production were Nile tilapia raised in floating cages in Lake Kariba operated by the Lake Harvest company, one of the leading private aquaculture firms in Africa, which also operates in Uganda and Zambia.
The Lake Harvest is vertically integrating farming, processing, marketing and export. Trout is also produced in the Eastern Highlands for the urban markets and for recreational fishing.
There is a need to further explore the potential in aquaculture, especially small-scale pond fish farming of tilapias and African catfish, by improving fingerling and feed production and supply, as well extension services.
Development in aquaculture could help to improve the national food and nutrition security and lift the still low per capita fish consumption.
In 2014, imports of fish and fishery products were valued at US$27,9 million and exports US$15,3 million.
Livestock and fisheries make an underestimated contribution to national agricultural sector performance compared to the crop sector. The fishery sub-sector comprises capture fishery, aquaculture and recreational fishery components.
The largest fishery is on Lake Kariba and contributes almost 90 percent of the country’s fish production. Lake Kariba supports an open-water semi-industrial night fishery for the Kapenta small pelagics and an artisanal inshore by village communities around the lakeshore largely using gillnets.
Meanwhile, Zimbabwe needs to have an integrated policy to develop and manage its fisheries and aquaculture sectors. Such a policy would assist in the development and management of aquatic resources addressing overcapacity, impact of destructive land use practices, pollution and water weeds.
Because of a lack of statistics, difficulties in management, and unreported transfers of catch (blackfish) in the kapenta fishery, the actual production of this fishery and its contribution to the economy is underestimated.
Zimbabwe has considerable potential for aquaculture. It has an estimated 10 700 large-medium sized dams covering 3 910 square kilometres. Possibilities of small-scale pond fish farming of tilapias and African catfish are endless.
By improving fingerling and feed supply, as well as extension services and market access, through joint efforts of public and private sectors, the aquaculture sector in the country will soar.
n Leslie Ter Morshuizen designs and builds fish farms across sub-Saharan Africa, trains farmers to manage them optimally and has run his own operations. He is the founder of Aquaculture Solutions. He can be contacted on leslie@aquaculturesolutions.org.
—Farmer’s Weekly/FAO
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