SESSION 3: CULTURE
Chairman's OverviewH, F. HENDERSON
One of the major difficulties in comparing different culture systems is
the lack of an appropriate yardstick for performance. One of the more
obvious examples is the expression of the productivity of a system as the
biomass per unit of time per unit area. In extensive aquaculture in large
lacustrine systems, this unit would seem to have the same sort of validity
as in comparative studies of the ecology of aquatic ecosystems in general.
In pond systems it still seems a useful figure for judging the most economical
use of space, but has somewhat doubtful validity in comparing biological
efficiencies unless depth is brought into the picture.
As soon as we move into cage and pen systems, however, one hardly
bothers to calculate such figures. From the point of view of the culturist,
input-output information is of more interest. Unfortunately the commonly
quoted feed conversion ratio is not a very satisfactory biological index
unless the water content of the feed is taken into consideration. Growth
rate and production are of direct interest to both the scientists and the
culturists, but are difficult to compare for different lengths of growing
season. Dr. Coche gives special attention to these problems, providing
several different measures of performance to compare cage culture systems.
Recognizing that it may not be very useful to compare the efficiency of the
use of space, say, between pond and pen culture, it does nevertheless seem
worthwhile to define standard performance criteria for culture systems.
These criteria should obviously extend to economic as well as biological
factors.
The reviews prepared by Dr. Coche and Drs. Hepher and Pruginin suggest
that pen and cage culture of tilapias have advantages for the developing
countries while pond culture seems to require rather complex management
for success, and may be better suited to the developed countries. It is clear,
however, that the main technical and managerial complexities lie in the
production of seed rather than in the growout phase, particularly the hybrid
seed which seems to be required for profitable production in temperate
climates. The question of suitable pond and cage culture systems should
also be considered in both high and low technology contexts. Offhand
it would seem to me that cage design and construction would offer more
problems in developing countries than pond construction. However, in-
expensive pre-fabricated cages could conceivably be developed.
Dr. Coche remarks that keeping cage floors well above the bottom of the
water body seems to reduce the incidence of disease. He also states, how-
ever, that caged fish may be under greater stress than pond fish from crowd-
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