5. Cuture Operations

Culture operations in pen culture have many silimarities with those of cage culture, i.e. in procurement of seed and stocking in nursery and growing phases, in feeding, disease control, maintenance etc. Since several of these aspects have already been discussed earlier under ‘Cage culture’ we shall refer here only to certain salinent points with reference to pen culture, from the known pen culture systems for milkfish, yellow-tail, red sea bream, salmonids etc. It is obvious that the operations would change with the fish species and also the local conditions prevailing. However, in developing new pen culture systems these might serve as general guidelines. A separate section (see section 7) on different aspect of pen culture and C. nigrodigitatus in Cote d'Ivoire results of experiments by Hem, 1979) is given at the end of this discussion.

5.1 Seed Procurement

Seeds of fish to be stocked in nurseries can be produced in a hatchery if sufficient control on the reproduction of the species is possible (e.g. salmonds, red sea bream or carps and catfish, if the latter species are to be stocked in fresh-water pens). Hem (1979) clearly showed that adequate quantity of seed of C. nigrodigitatus could be produced in captivity for stocking in pens. If the species to be stocked are not subject to breeding under controlled conditions, then the seeds or larvae have to be collected from the natural sources, as in the case of milkfish and yellowtail. The seeds are usually collected from shallow coastal areas (milkfish) and transported to the pens from the source.

The milkfish seeds (fingerlings) are transported to the fish pen by means of a live fish boat or by buses (or “Jeepneys” as referred to in Philippines). The fingerlings transported by land are placed in oxygenated plastic bags and those by boat (‘pituya’) are gradually acclimated in fresh water, as the boat enters fresh water in the lake, fresh water gradually enters the fish holds of the boat.

The seed/larvae of yellow-tail in Japan is collected from natural sources under a licence from the Government. They usually abound in areas with floating weeds and are caught by encircling nets and kept in floating “net pens” (cages - as per our treatment) as they are called (Fujiya, 1979). The size caught are of 25 – 40mm. The red sea bream or red porgy (Pagrus major) (categerized in Japan as a symbol of auspice and fortune) has been successfully bred under natural and artificial conditions. The breeders are collected from the wild and nursed separately and the seeds produced are used for stocking in pens and cages and also for restocking natural waters (of extensive aquaculture). The fry, the day after hatching, are transferred to “green sea water” (fertilized with 500g/m³ of water and inoculated with phytoplankton) usually in plastic containers (40,000 per M³ (ton) of water). The fry are subsequently fed with oyster larvae, rotifers and nauplii of brine shrimp (Artemia salina). Good water conditions should be maintained and some biological treatment of water, such as placing small crab, hermit crab or other shelfish in the tank, may be effective in keeping the water clean.

Seed production in salmonids (trouts and salmons) are well known. In the case of Atlantic salmon, which as we have referred earlier are cultured in enclosures in Norway, the stocking in pens is with smolt-size fish (50 – 100g or larger). They are transferred (e.g. A/S Mowi in Norway) in large plastic containers and avoid excitment are not handled at all in the process of transfer, For new information see recent issues “Fish Farming International”.

5.2 Cleaning nursery and ‘fattening’ pens

Before stocking the seeds/fry or fingerlings, the pen has to be cleared of all avoidable fauna and unwanted elements, especially predators and trash fishes. The embankments, filters and net barriers have to be kept clean (see ‘predator control’ given separately).

5.3 Stocking of nursery and fattening (grow out) pens

Milkfish fingerings after transport from source are kept in nursery pens (Fig. 9) (fresh water) for 5 – 6 hours or for as long as 2 – 3 weeks to prevent further environmental stress (also acclimation to fresh water) and reduce changes of predation. Mortality in the nursery pens (see Fig. 9) may vary from 1 to 3% during fine weather to 20 – 30% in inclement weather (SEADEC/ 'DRC, 1979)

From the nursery pen, the fingerlings are stocked at 30,000/ha in the rearing pen (grow-out or fattening pens), where they are grown to a marketable size of 200g or more. Mortality in the case of milkfish can be as high as 20 – 40% on transfer from the nursery pen to rearing pen. The stocking density of milkfish is relatively less, for the reason that most pen operators do not resort to artificial feeding. In most other cases stocking rates are higher, though not as high as in the cages.

With reference to yellow-tail, the recent trend is to raise the fish in floating or submerged cages, in which the No/M³ varies from 90 (10g) to 10 (1kg) (Fujiya, 1979) but we shall look at the yellow-tail enclosures in Japan for the pen culture details. At Adoike (27 ha) (see earlier description) 1/4 million yellow-tail fry (2.5cm; 3g) were stocked, which works out to 9260/ha, but here the depth is only 8m. In Tanoura enclosures (18.4 ha; 15m depth), the stocking density followed was 400,000 yellow-tail i.e. 21,740 yellow-tail per hectare. In 1965 they stocked 460,000 yellow-tail, a very good harvest for a large and deep enclosure Milne, 1979s). The marketable weight of yellow-tail is 1kg. The fry (3g) stocked in pen in May -July, grow to 2) - 700g by August end, 600 – 1600g by October and 700 – 2000g by December end. The pen and cage raised yellow-tail fetch better price that wild fish, due to the formers' better taste and flesh quality.

The stocking rates at Atlantic salmon in A/S Mowis pens are not available but from the catch potentials indicated (600 tons), the stocking density must be quite high and they undertake special efforts (pumping, feed loss check, anaerobic patch development checks by diving etc) to maintain good water conditions and fish health. In Cote d'Ivoire Hem (1979) stocked 8 – 10 catfish/M² (5000 fish in 25 × 25M² pens) C. nigrodigitatus. C. walkeri stocked were of 80g size on average.

As pointed out earlier the stocking density limits of any species are to be decided by the total biomass and its environmental requirements, the most important of which are oxygen availability determined by the requirement of the species, rate of O₂ depletion and replenishment and also other biological and physicochemical characteristics of the water.

5.4 Fertilization and feeding

In rich waters (of Laguna Bay, Philippines) the natural productivity is sufficiently high that fertilization is not needed. Even in other cases, since the pen enclosures are open to flux of waters with the tide, fertilization if pracitised, may not give full benefits. But in certain cases having solid embankments where flushing of water during certain periods season will be less, some fertilization can perhaps be adopted, the levels of nutrients in natural water could decide the fertilizer needs, but any large level fertilization appears inadvisable.

More advantageous would be techniques such as lights hung over pens and enclosures, to attract the forage organisms to the pen by the phototaxes and photo kineses.

Considerable information on feeding rates and conversion efficiences has been given under “Cage Culture” (of Coche, 1979), and much of this information is applicable in pen culture also.

In milk fish pens most operators do not feed fish. Some operators feed milkfish with bread crumbs, rice bran, broken ice cream cones, fish meal, egg yolk in small quantities, ipil-ipil (Leucaena leucocephala) leaves or Ipomea reptan leaves.

Other fish are given dry or moist pellets, if relevant and costs reasonable. The feed can be offered either in feeding trays and/or with demand feeders. Two important items to be considered in feeding are initial stocking rates of species in relation to the final anticipated weight of fishes confined within the enclosure and secondly the thinning out of population in the enclosure during the culture period, this should be undertaken to ensure the most efficient use of space available. The controlled quantity of feed supply to the fish should be as much as they will take or as a percentage of body weight (watch out conversion efficiency) (see also earlier notes on feeding). Species - wise information is to be obtained on the time of the day for optimum utilization of feed given. The best feed times for the grouper (Epinephelus tauvina) are 6 – 7 A.M and 6 – 7 P.M (Malaysia). The frequency of feeding would depend on the growth stage. Newly hatched fry are to be fed more frequently since they have not developed regular feeding patterns. In salmonids (trout) the amount of daily feed should be slightly less than the satiation level.

It is said that occasional manual feeding provides a useful opportunity to observe the behaviour and conditions of the fish. SEAFDEC/IDRC (1979) further recommends that “the fish may be conditioned to respond to noise before feeding. The use of lights above to attract insects and, (as mentioned earlier) even zooplankton near the surface, is stated to be specially useful in fingerling nursery enclosure”. The use of fish of uniform size reduces competition for feed provided.

In the case of yellow-tail, 40 – 70% of the total expenditure is spent for food, the most suitable food for yellow-tail being white-meated fish, such as sand eel, horse mackerel, or anchovy. But the “continuous feeding of anchovy causes high mortality because of oxidation of the unsaturated fatty acids in anchovy flesh. The oxide and peroxide components are harmful, but mixed feeding with other fish reduces the problem. Thiaminase in anchovy flesh destroys Vitamin B₁ in yellow-tail and results in high mortality. Artificial diets, consisting of 70% white fish meal with 5 – 10% gluten as binder, vitamin mix and minerals, especially iron and cobalt, to prevent anaemia, are also in vogue”. Eventually the cost of feed and optimum utilization would decide the type and quantity of feed. The recommended feeding rates for hellow-tail are very high relatively i.e. ranging from 40 – 60% of body wight per day for 10 – 50g fish to 6 – 8% in 100 – 1200g fish.

The red sea bream also are fed frozen sand eel and anchovy and also certain artificial diets.

The feed scheduling of Atlantic salmon in Norwegian pens have reached near perfection - the feed being moist pellets. With ingredients as in natural feed, the demand feeder, pelletising feed automatically at the feed times. We have already referred to the general aspect of feeding earlier.

5.5 Maintenance of Pens

Routine maintenance operations of sluices and gates and net barriers, rigid and non-rigid, are most important. In the case of sublittoral enclosures the maintenance of piled net barriers, quite often a double netting, by regular cleaning and scrubbing operation, often involving under water diving, is a major part of the expenditure, perhaps next to the cost of supplementary feeding. In certain Japanese pen enclosures about onethird of the staff employed are divers who clean the nets.

Also there is considerable need for predator and weed control (see information given elsewhere). Regular checks and repairs/modifications should be done to reduce losses in this connection.

In addition to these routine operations for monitoring water quality especially D.O., salinity, temperature, pollution indices etc, should be done.

In Norwegian salmon enclosures regular diving operations are done not only to see if there is accumulation of left over feeds in the pen bottom, but also to find if anaerobic patches develop on the pen floor to undertake necessary remedial measures.

5.6 Cropping (Harvest)

Cages and pens are convenient for cropping at any time because of the confined space and high density of fish stocked - cages are smaller and easier to handle in this case. The pens are larger and as obvious in large pens fishing efficiency may be less. Seines and gill nets or framed nets, are often employed for catching fish. Harvesting can be dependent on the size and also shape of the pens (see rounded and square angles earlier — square angles are easier to operate framed harvest nets).

In the case of milkfish, some pen operators fish once a year only, stocking in May or June and harvesting the next May or June. Others stock twice a year — first stocking in March and April and harvesting in July or August; and the second stocking in July or August and harvesting 8 months after in February or March. The number of croppings depends on the operators' capital and number of milkfish stocked. When high price rules (as often done in advanced aquaculture systems) partial harvests during rearing period is done. While seines are used for total harvest, gill nets are used in selective (partial) harvest. The sizes of fishes harvested have already been referred to earlier.

TABLE II: PRODUCTION OF 200,000 MILKFISH IN FENCED ENCLOSURES WITH TWO CROPS PER YEAR AND AVERAGE WEIGHT OF 460g AT HARVEST

TABLE. III: PRODUCTION COSTS OF 50 000 CHANNEL CATFISH IN FENCED ENCLOSURES (160 DAYS AND 800g WEIGHT AT HARVEST, NO COST FOR WATER)

TABLE. IV: ESTIMATED INCOME FROM INTENSIVE CULTURE OF CHANNEL CATFISH IN CAGES, RACEWAYS, AND ENCLOSURES, AND EXTENSIVE CULTURE OF MILKFISH IN ENCLOSURES

1. 160 day rearing period = Catfish based on Table II

2. one year rearing cycle = Milkfish based on Table I