5.1. Wild zooplankton
5.2. Production of copepods
5.3. Mesocosm systems
5.4. Literature of interest

5.1. Wild zooplankton

5.1.1. Introduction

5.1.2. Collection from the wild

5.1.3. Collection techniques
5.1.4. Zooplankton grading
5.1.5. Transport and storage of collected zooplankton

5.1.1. Introduction
Zooplankton is made up of small water invertebrates feeding on phytoplankton. Even
though “plankton” means passively floating or drifting, some representatives of
zooplankton may be strong swimmers. The yearly plankton cycle consists of various
phytoplankton species blooming in response to a particular sequence of changes in
temperature, salinity, photoperiod and light intensity, nutrient availability, and a
consequent bloom of zooplankton populations. Phytoplankton and zooplankton
populations are therefore intimately linked in a continuous cycle of bloom and decline
that has evolved and persisted throughout millions of years of evolution.
Studies on the stomach contents of fish larvae caught in their natural environment clearly
show that almost no fish species can be regarded as strongly stenophagic (specialized in
feeding on only a few or just one zooplankton species), though some specialization may
occur (i.e. due to size limitations for ingestion).
There are three obvious advantages of using wild zooplankton as a live food source for
the cultivation of the early larval stages of shrimp or fish species:
· As it is the natural food source, it may be expected that its nutritional composition

20:5n-3 8.6 9.5 8.4 8.7 8.6
22:5n-3 1.0 Tr. 0.6 0.3 0.8
22:6n-3 10.6 14.1 11.8 10.0 7.0
* dry weight basis
Table 5.2. Free fatty acid composition (FFA; area% of total lipid) of wild
zooplankton compared to freshly-hatched Artemia nauplii (AF grade) (modified
from Naess and Bergh, 1994).

Wild zooplankton Artemia
14:0 3.4 0.8
16:0 16.9 12.6
16:1n-9 0.7 0.9
16:1n-7 1.7 4.0
16:2n-4 0.3 0.2
18:0 3.7 7.4
18:1n-9 2.9 22.5
18:1n-7 3.3 10.6
18:2n-6 2.0 6.8
18:3n-3 1.5 20.3
18:4n-3 1.5 2.3
20:1n-9 0.2 0.7
20:1n-7 0.6 0.1
20:4n-6 0.8 2.3
20:4n-3 0.5 0.6
20:5n-3 21.1 3.6
22:0 0.5 1.1
22:1n-11 0.0 Tr.
22:5n-3 0.8 0.1
22:6n-3 32.9 0.2
Sum (n-3)PUFA 58.3 27.1

Phenylalanine 2.1 1.5
Isoleucine 2.4 1.5
Leucine 4.5 2.5
Lysine 6.6 3.9
Total FAA 116.6 45.9

5.1.2. Collection from the wild
Zooplankton can be collected from seawater bodies as well as freshwater lakes or ponds.
For aquaculture purposes, approximately 80% is of marine origin. Around 25 species of
copepods, mysids and euphausids are commercially harvested. Leading countries in using
wild zooplankton in industrial aquaculture are Norway (annual catch ranges between 20
to 50 tonnes), Canada and Japan. The global annual catch of planktonic crustaceans
(essentially krill) is around 210,000 tonnes, but only a small percentage is used as a direct
food source in aquaculture (live or deep frozen).
On the Mediterranean and Atlantic coasts of France, densities of copepods (which make
up 85% of the zooplankton) may range from 500 copepods per m
3
in winter (November-
February) to more than 10,000 per m
3
in spring and summer. On average 1,000 copepods
per m³ are found in the littoral zone; this figure may, however, be higher in lagoons and
estuaries. In some eutrophic brackish water fjords in Norway, for instance, abundant
numbers of the copepod Eurytemora may be found, including 6 to 30.10
6
adults, 15 to
25.10
6
copepodites, and 25 to 50.10
6

5
larvae per day on such a short tow.
5.1.3. Collection techniques

5.1.3.1. Plankton nets
5.1.3.2. Trawl nets
5.1.3.3. Baleen harvesting system
5.1.3.4. Flow-through harvesting
5.1.3.5. Plankton light trapping

Harvesting techniques depend strongly on the location of the harvesting site and should
meet the following criteria:
· capable to operate on a continuous basis without surveillance;
· easy to transport and to set up;
· relatively cheap in purchase and maintenance;
· available on site;
· designed for the required quantities and zooplankton sizes.
5.1.3.1. Plankton nets
The following mesh sizes may be used to collect the various sizes of freshwater
zooplankton:
· 80 µm for small species of rotifers and larger infusorians. These are an excellent starter
feed especially for the fry of some fishes that need small food in the early stages (tench,
grass carp, silver carp, big head, carp);
· 160 µm for larger rotifers, nauplius and copepodite stages of copepods;
· 300 and 500 µm for small water fleas and smaller species of cyclopoid copepods;
· 700 µm for adult water fleas of the Daphnia genus, large species of cyclopoid and
calanoid copepods, larvae and pupae of Corethra sp., etc.
A multi-purpose plankton net for zooplankton collection is schematically shown in Fig.
5.1. The net is conical shaped, 3-3.5 m long, the inlet opening is 1-1.2 m in diameter and
the end hole has a diameter of 0.2-0.5 m. There is a strip of thicker cloth on both ends;

The Baleen harvesting system consists of a boat specifically designed for harvesting
zooplankton (Fig. 5.3.). This vessel can filter the surface water at rates up to 400 l.s
-1.
The
zooplankton is scooped onto a primary dewatering screen, after which the organisms are
graded through a series of sieves. The stainless-steel mesh of the sieves and primary
screen can be changed according to the requirements of the target species. The graded
and concentrated zooplankton is stored in wells in the floaters of the vessel and can be
unloaded by pumping. The boat can be operated by one person and is powered by an
outboard motor and auxiliary petrol engine to drive the pumps and hydraulic rams.
Figure 5.3. The Baleen zooplankton harvesting system (Frish Pty. Ltd., Australia).

5.1.3.4. Flow-through harvesting
· Lake outflows
In reservoirs with a high water flow, a plankton net of adequate size may be placed at the
outlet or overflow; in this way the zooplankton present in the water leaving the reservoir
can be concentrated. In the case of ponds, the frame of the plankton net may be fixed to
the pond gates. The amount of zooplankton collected depends on the zooplankton
concentration in the water flowing out of the reservoir and on the volume of the water
leaving the reservoir. Again, the nets should be emptied once or twice an hour, depending
on prevailing conditions.
This method can be used effectively only in the case where the flow rate of the water at
the outlet of the pond is at least 5 to 10 l.s
-1
. Optimum conditions for this method exist in
large eutrophic lakes where the flow rate at the outlet is > 1 m³.s
-1
and where several
hundred kilos of zooplankton biomass are discharged every day.
· Propeller-induced water flows

5) Electro motor to drive the rotating sieve (12 V, 24 W and 20 rpm; 6) Submerged
pump for the spray washing system (15 V and 60 W) with feed pipe to jets; 7)
Recovery trough for washing water and plankton; 8) Filter sack for storage of
concentrated plankton; 9) Water level; Floaters are not shown. B. Cross-section of
the apparatus. 1) Lateral floats; 2) Casing around the apparatus; 3) Microsieve; 4)
Recovery trough; 5) Spray bar offset from centre (Barnabé, 1990).

With these devices it is necessary to replace the batteries and to harvest the plankton once
or twice a day to reduce mechanical damage of the plankton. The transport of the
zooplankton can be carried out in water in a 50 l reservoir and must be carried out very
quickly, since the viability of the harvested plankton is low (1h after harvesting already
5% mortality is observed).
· Pump-induced water flows
Another method of collecting zooplankton is to use pumps to pump the water into a
plankton net. The plankton net may be located at some distance from the outlet of the
pump or may be tightened with a string or rubber band straight to the outlet pipe of the
pump. The latter method is better because no plankton can escape by back flushing from
the net, but needs more frequent emptying of the net as denser nets are prone to clogging.
Using an electric pump with a capacity of 5 l.s
-1
, as much as 0.5 to 5 kg of zooplankton
(depending on zooplankton biomass in the reservoir) may be collected in a net with a
mesh size of 160 µm in 1 h (Fig. 5.6.).
Figure 5.6. Zooplankton is removed from the lagoon by a wheel filter. The plankton
is retained on the belt-driven, rotating wheels of the plankton mesh. These wheels
are continuously cleaned from behind by a flushing arm. The harvested plankton is
collected in a box.
5.1.3.5. Plankton light trapping
A more elegant method for zooplankton collection takes advantage of the positive
phototactic behaviour of some zooplankton species. The effectiveness of light to attract

, zooplankton can be kept at 10°C without
oxygenation for only 15-20 min. At higher temperatures or if the zooplankton is to be
kept alive for longer periods, the concentration must be reduced substantially. At a
temperature of 18-20°C it can be kept at a concentration of 15-20 g.l
-1
without aeration
for as long as about 4 - 5 h, although the most sensitive organisms will die. This is
certainly the case for Bosmina, Daphnia and others, that are very sensitive to oxygen
depletion. Rotifers, cyclopoid copepods and their developmental stages are less sensitive,
and some species of the genus Moina, larvae of the genus Corethra, and Daphnia magna
are very resistant to low oxygen levels.
When the collected zooplankton is transferred from the net to the transport container, part
of the material stays in a layer just above the bottom. These organisms are either
mechanically damaged or immobilised and could be administered to the fry first.
However, when these organisms die, they will soon start to decay. It is useless to
administer these dead animals because the fish will refuse it and their decomposing
bodies will spoil the water quality of the rearing system. For this reason, dead
zooplankton should always be separated from live zooplankton by decantation.
Preservation of harvested material for long periods is difficult. At present, freezing is the
only method used on a large scale. But even at very low freezing temperatures, (i.e. -
198°C) one-third of the free and protein-bound amino acids are lost from the plankton
samples through sustained proteases activity and leaching. Dehydration has been used
successfully on a small scale, while salting causes mortality in fish. Ensilage, using
various acids has also been attempted, but needs further investigations. 5.2. Production of copepods

5.2.1. Introduction
5.2.2. Life cycle