Greetings all. this is to answer a couple questions that have appeared
on the Sarnissa site:
1. the cages made in Uganda: the factory imports nylon twine of
various sizes. Twine thickness is usually described in terms of "ply",
which is number of threads that are twisted together. The gill nets they
make are between 2 and 6-ply. We wanted 36 or greater ply netting to be
made but the machines could only make 24-ply. However, we boiled the
netting, which thickened it. In essence, it shrank it. There are very
few net factories in Africa these days, as most netting is imported.
However, I was fortunate to have access to this factory and work with
them to make the cage material. They also stretch and heat-set the
knots now, which is important because if a knot slips, then it opens the
mesh and fish may escape. The bitumen treatment is actually standard in
the US becasue it prevents catfish spines from catching on the netting.
We used it to make the net resistant to UV radiation. Sunlight degrades
nylon.
Our first cages were in fact hand-made with 72-ply nylon twine. Here,
they charge by the knot. So the cage was a bit more expensive than the
factory made cage but still not so much more. By the way, not all
"plys" are the same. Unscrupulous vendors who wish to benefit from lack
of experience of African buyers often give a certain ply which is much
less thick than what the reputable vendors sell. As for imported
netting, be cautious that you do not get polyester because it will
degrade in a matter of weeks. However, the poly propylene and
polyethylene are very good and usually cost less than nylon. However,
they should have their knots heat-set or their joints (in case of
knotless netting). the cage culturist should consider cost, longevity,
percent open area of the mesh, resistance to degradation by sun or by
organic loading in the water, abrasion on the fish (often not as much a
problem as you would think) and potential for fouling. I can give hours
of lecture on netting- it is my pet subject. But I do not wish to bore
the readers here.
2. Feeds: Dr Brummett is quite correct in stating that a complete diet
must be available in order to culture fish in cages. There are some who
say that one can feed kitchen scraps, etc, However, such practice is
what gave cage culture a bad name in the past because poor quality feed
results in rather high levels of pollution and very poor fish growth
rates. It is also highly desirable to have floating feed or at least
feed that sinks slowly. The things people do to keep the floating feed
in the cage are often problematic because they prevent water from the
surface - or first 40 cm from entering the cage and this is often the
best water. For people in DRC, if you are near the border with Uganda,
you will be able to purchase floating feed by the end of this year from
Uganda. It will have the proper vitamins, etc for cage culture.
3. Stiff cages: Hard cages made of wire mesh are Ok to start with and
to run some trials but most people switch to soft cages (netting) as
soon as they can. This is because the netting cages are easier to
manipulate, are less expensive and can of course work for large
volumes. However, hard mesh cages in either PVC-coated galvanized metal
or in stiff plastic are rather convenient if one is faced with a problem
from snapping turtles or tiger fish. I would say that if you need more
than 4 cubic meters volume, you'd better think about netting instead of
stiff materials.
4. Density: the only reason why lower density is used in big cages is
that the cages are too big. By this I mean that water quality in big
cages is very bad in the center if fish density is high. Fish do not
require the space people think. they require good water quality. People
have been able to culture very high densities of fish in small volumes
if the water quality can be maintained. In Uganda, we grow the fish to
market size in small cages and high density. Not just fingerlings.
5. Placement: 4 placed together like I saw in one drawing is a rather
bad idea. this is because each cage has only 2 sides with access to open
waters. But it is less bad than the 16 or so cages placed together in
which center cages have NO open sides. Instead , a chain of cages, each
separated by one cage width is much much better. I have a single photo
of this attached-not my best but all i have with me. I really like the
Ghana cage design but I think it is too deep. However, the frame is
well-done and very practical. It uses galvanized pipe. As Olivier has
stated , angle iron is also OK and has some advantages over a pipe. The
Chinese are using just iron bars (solid) but once again, they do not
have to worry about 20 cormorants landing on their cage frame, as we do
in Africa.The photos from Niger were pretty good examples. Note the
feeding rings as used in Brazil. Very important. their surface area
relative to total cage volume is important.
6. Other references: there is a very good manual on Low volume-high
density cage culture put out by the American Soybean Association- on
their web site. It was written for China but is very useful for Africa.
We are working on our own manual that will borrow heavily from theirs
(we have permission) but it will talk more about the special problems we
have encountered in Africa- especially with predators. It will also talk
more about conditioning fish for these cages as this seems to be where
most people fail in Uganda and neighboring countries. When I return to
the US next week, I will put on our website some presentations on cage
culture. The Uganda cage culture manual will probably be out in
September/October. If you do not see it, please remind me. However,
there are many ways to solve a problem and as long as people keep their
records and look at results, they should be able to make good decisions.
Now I think I am boring you all so I will sign off. will be happy to
answer questions.
--
Karen L. Veverica
Auburn University Dept. of Fisheries and Allied Aquacultures, USA
office: +1-334-844-9201
cell: +1-334-332-1560
Chief of Party
USAID Fisheries Investment for Sustainable Harvest (F.I.S.H.)
Uganda
phone +256- 782-970-622
www.ag.auburn.edu/fish/international/uganda/_______________________________________________
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