Many different words and expressions are used in aquaculture literature for the practice of harvesting (i.e. cropping) fish. According to the number of harvests during a culture period they may be divided into:
Single harvest:
Batch harvest, complete/total harvest, major/main harvest.
Multiple harvests:
Partial harvesting, periodical harvesting, intermediate harvesting/fishing, intermediatory harvesting/fishing, intermittent harvesting, rotary harvesting, continuous harvesting, harvest on staggered basis, thinning down, thinning out and skimming off.
The majority of the expressions for multiple harvests are used to differentiate harvesting strategies and are related to semi-intensive or intensive farming systems. These systems include regular grading of the fish and multiple stockings in mono- or polyculture of species that do not breed in captivity or of one sex only (monosex culture).
In this document, particularly with regard to the practice of small-scale rural tilapia fish farmers in Zambia and Zimbabwe, who seldom or never drain their ponds completely, the following expressions are used:
Batch harvest: Fish are harvested after a certain culture period. The pond may be drained completely and all the fish harvested, i.e. complete batch harvest or partially drained and most (>50%) of the fish biomass harvested, i.e. incomplete batch harvest.
Intermittent harvesting: Small amounts of fish are harvested at intervals followed by a complete or incomplete batch harvest, or no main harvest.
Previous research on intermittent harvesting strategies is limited, and has focussed entirely on the bio-technical feasibility of such a strategy under intensive fish farming systems. However, within the ALCOM region, the majority of fish farmers practise extensive small-scale fish farming in ponds as a subsidiary activity to agriculture. The socio-economic aspects of intermittent harvesting under these conditions have never been investigated, but some of the more likely advantages and disadvantages of the strategy are outlined below.
There are obviously good reasons why fish farming households adopt an intermittent harvesting strategy in terms of socio-economic and nutritional factors.
Initial investigations suggest that farmers adopt this strategy because it:
provides an accessible supply of animal protein to the household
allows for a more varied diet
makes available small amounts of cash through the sale of fish
in some households, smaller fish may be preferred
can be an easier method to harvest fish in undrainable ponds
The possible disadvantages of adopting an intermittent harvesting strategy are:
total fish production may be lower under some intermittent harvesting strategies
more labour hours are required for some intermittent harvesting methods than for incomplete/complete batch harvesting methods
Since not all households practise intermittent harvesting, there must be socio-economic factors which determine a household's choice of harvesting strategy. The factors likely to influence the adoption of this strategy within the household are:
characteristics of fish farmer (e.g. education, age, gender, work experience)
household constitution
type of agriculture farming system
availability and allocation of resources (labour, cash, food crops)
nutritional requirements during the agricultural year
cash requirements of the household during the year
availability of alternative sources of animal or vegetable protein
adoption by near-peers
ownership and/or control of fish ponds
External factors likely to influence whether an intermittent harvesting strategy is adopted are:
proximity to a market and/or access to fish traders
local demand and supply for different species and sizes of fish
price of fish and alternative animal protein
local nutritional habits
access to and quality of extension services
This section describes maximizing production through intermittent harvesting strategies in intensive farming systems where the number of fish is generally controlled.
In a classical fish culture system in stagnant water, ponds are stocked with juvenile fish of the same size. The total biomass is harvested after a certain culture period. The magnitude of the optimal standing crop depends on the natural productivity of the pond and can be enhanced by increasing the nutritional level through feeding and/or fertilizing to the point where the rate of increase of biomass starts to decline. This optimum is specific for the amount of nutrients applied and species reared.
The total yield may be increased by multiple harvest once the optimal standing crop has been reached and by allowing it to attain the optimum once again (see fig. 1). The productivity of the pond probably decreases gradually due to deterioration of the pond condition (e.g. sub-optimal oxygen level; accumulation of growth inhibiting factors; decrease of natural food organisms). This farming system allows high initial (stocking) biomass. Although often described in aquaculture handbooks, few studies confirm this theory.
Another strategy used in intensive farming systems to maximize production is to maintain the biomass as close to its optimum during the whole culture period. These culture strategies, such as rotation culture (culture of different size classes at the same time) and multigrade culture (fish of the same size are regularly harvested and transferred to other ponds to grow) require both regular harvest and restocking and are commonly used in Asia.
The above described model for intensive farming systems becomes more complex when tilapia species are cultured. These species, currently used by small-scale farmers in rural areas in Southern Africa, breed freely in captivity which makes the control over the number of fish and hence their individual growth, more difficult. While this prolific breeding, resulting in high densities of fish in mixed sex culture, justifies intermittent harvesting to maximize yields (by giving more living space and food for the remaining population) it is likely to reduce production if in-breeding is not taken into consideration.
In Southern Africa, cultured tilapias such as Oreochromis andersonii, O. macrochir., O. mossambicus and Tilapia rendalli generally spawn when water temperature is above 20–21°C. (Maar et al, 1966; Balarin and Hatton, 1979). Thus, breeding may occur up to 8–10 months a year.
The bio-technical advantages and disadvantages of intermittent harvesting strategies for extensive and semi-intensive, mixed sex tilapia culture, are summarized below:
Advantages
maximize production under certain conditions
fry production is reduced if intermittent harvest is undertaken with a seine net as it can disturb the tilapia nests
avoid loss of food and production by allowing longer intervals between two batch harvests.
Disadvantages
requires year round water supply
reuse of self produced fingerlings may not be possible due to one or a combination of the following factors:
inbreeding
intermittent harvesting strategies which remove only big fish (selective fishing)
low nutrition levels which stimulate early spawning (lack of fertilizers and/or direct food items)
difficulties in controlling the number of fish of each size class and total fish biomass. (Little is known about population dynamics in mixed sex tilapia culture systems since the majority of semi-intensive and intensive systems use monosex culture systems).
Current knowledge on the socio-economic and bio-technical aspects of intermittent harvesting strategies under small-scale extensive and semi-intensive mixed-sexed tilapia culture is clearly limited. If such a practice is to be encouraged and improvements in the strategy developed and introduced to farmers, investigations are required on the:
socio-economic profiles of fish farmers and fish farming house-holds who practise different harvesting strategies;
reasons why farmers practise intermittent harvesting and the external factors influencing their choice of harvesting strategy;
the advantages and disadvantages in socio-economic and nutritional terms (i.e. resource availability and allocation, protein availability, cash requirements) of adopting an intermittent harvesting strategy;
biotic and a-biotic factors controlling breeding (nutrition, temperature, oxygen, pH, soil, etc).
harvesting techniques in order to determine
quantity of fish to harvest (crowding effect)
quantity of each size class
fishing methods
Pilot activities to be undertaken by ALCOM to investigate these socio-economic and bio-technical aspects of different harvesting strategies are described in the following chapter.
