A Pen is defined as “a fixed enclosure in which the bottom is the bed of the water body” (SEAFDEC/IDRC International workshop on Cage and Pen culture, 1979 (Philippines) - Proceedings - Summary Report, P. 20). Pen is to be destinguished from the Cage which in turn is defined as “an enclosure with bottom and sides of netting or bamboo etc., whether floating at the surface or totally submerged.” The word ‘pen’ here is also used synonymous with ‘enclosure’ as it is used in enclosure culture.
Various aspects of “cage culture” has been dealt with separately and therefore will not be referred to here, except to mention common points of both cage and pen culture - in such cases details are to be looked under original explanations given, so that repetitions can be avoided. As would be obvious there are several common points between the two systems of culture especially in the environmental set-up of both systems, affecting site selection and culture operations. The definition given above should, however, clearly separate the two culture systems. By the very nature of the fixed enclosure walls of pens it is obvious that they cannot be moved about as in the case of cage. There is economy of material in the pen for the bottom material used is saved and therefore and for other reasons the pen can be and are much bigger. As we go by further differences and similarities will become obvious, especially when the different types of pens are described.
In spite of the distinctions made above, many references in literature, do not distinguish between cage and pen clearly - often using the words synonymously. Indeed owing to the common points of cage and pen culture, this could be justified in many cases. We have, however, taken the stand that the two cultures are to be distinguished as indicated above and hence the two separate courses of cage culture and pen culture in our programme.
Of the 6 possible zones, i) shore, ii) interdidal, iii) sublittoral, iv) surface floating v) mid-water and vi) sea bed in the coastal waters (Milne, 1979) (Fig. 1) -also see under ‘Cage Culture’ - pen culture is possible only in the three zones, namely, intertidal, sublittoral and seabed -all having natural bottom as the limit of the lower side of the enclosure.
In the case of freshwater bodies, except for the very large lakes - evenhere tidal influence is little compared with the sea, the intertidal zone is non-existent. Largely the enclosure of a pen is restricted to shallow area adjacent to the shore. The pen or enclosure may be (a) completely enclosed on all four sides in the middle of a bay, with no foreshore or (b) a shore enclosure with a foreshore extending to deep water surrounded by a net structure or (c) a bay or loch enclosure with an embankment or net structure only at the entrance, (Fig. 2).
We shall look into the details of these later.
The pen and enclosure culture is a recent development (see Milne, 1979a; 1979b; Moller, 1979; Fujiya, 1979).
Perhaps the capture of fish in impoundments in the shore area with permanent earthen/stone embankments with sluices, might have induced interest in enclosure culture. To cite Milne (1979b) “development of sea enclosures in Britain and Europe in both intertidal and sublittoral zones has mainly stemmed from the large sea enclosures they were constructed in Japan for the farming of finfish, yellow tail (Seriola quinqueradiata).”
In Japan the 27ha yellow tail enclosure of Adoike near Takamatsu in the Inland sea, is the oldest, being over half a century old. This is a pond enclosure with shore embankments and two sluice gates. There are several other fish pens concentrated in the Inland sea area (South-west Coast of Japan) where the sea temperature is warmer and more conducive. At Hitsuishi, a 7.2ha area between two islands has been enclosed as a pen, with a 100m embankment at one end and a 350m piled net-barrier on the other.
Fig. 1. Six possible zones for cage culture in coastal waters (after Milne, 1979 1-shore; 2-intertidal; 3-publittoral; 4-surface floating; 5-mid-water; and 6-seabed
Fig. 2. Types of enclosures (see text)
At Matsumigauru (Japan) is the largest area (120ha) enclosed by a net barrier of 300m length, in Hamana lake. This enclosed bay is referred to as a ranch and not farm. Yellow tail, puffer fish and sea bream are cultured here. There are other fish enclosures in Ieshima, Megisima and Tanowia in Japan.
Marine aquaculture production in Japan in 1973 is 83,800 tons, much of which comes from the pen culture systems. Yellow tail (Seriola quinqueradiata), red sea bream (Pagrus major), file fish (Monacanthus cinhifer) and rock fish (Sebastes marmoratus) are the main species cultured, but yellow tail forms about 95% of production and yellow tail culture is considered to be the most representative type of coastal fish culture in Japan (Fujiya, 1979).
Limitation of fish production from the capture fisheries and also the increasing devastation caused by pollution, especially in rivers and estuaries near heavily populated areas, have turned the attention of many countries to marine fish farming in relatively less polluted coastal areas. It was predicted in seventies that mariculture would succeed in the Pacific Coasts north of Seattle in North America, West Coast of Scotland and Norwegian Coast. (It would appear that this prediction has certainly not taken many parts of the world into consideration). In any case much of the new developments in Pen culture started with the researches in enclosure culture in Scotland by biologists and engineers together, under the White Fish Authority in early sixties. Hydrographic and engineering surveys were made in the relatively pollution-free inter-tidal and sheltered sea lochs of west coast of Scotland in association with the Dept. of Civil Engineering, University of Strathclyde, Scotland, and Ardtoe was chosen in 1965 as a site for a marine cultivation unit. Here a 2ha inter-tidal area (bay) was enclosed by constructing two sea walls (with low level sluices) on rock foundation on either side of a central island (Fig. 3). Quarter million artificially hatched plaice (Pleuronectes platessa) from White Fish Authority hatchery in Port Erin (sle of Man) were stocked at Artdoe intertidal enclosure in 1965. While this was not a complete success, further improvements have been made at Artdoe itself and subsequently several other mariculture farms have been developed in the West Coast of Scotland by about 40 farming organizations (Milne, 1979a). The enclosure culture experiments of White Fish Authority have stimulated interest in the culture practice in other countries.
In France discarded salt pans have been converted as sea enclosures for culture of sea bream (Chrysophrys aurata) in Sete by a private company, Compagine des Salines du Midi. Mullet farms in the intertidal area with high mud embankments have been in operation in Awdenge France from early days but these are in the fashion of brackish water culture ponds as the “tambaks” of Indonesia, and fall in a different category, and therefore would not be considered further here.
In Norway private concerns have begun Salmon (Salmo salar) culture in sub-littoral enclosures (Milne, 1979a, b; Moller, 1979). The first enclosure (1.2ha) was completed in 1969 in Floyoykgolpo (on the island of Sotra) by A/S lowi and has a production capacity of 150,000kg. The area is enclosed by two sets of barrier nets at the mouth and across subsidiary channels of a bay (fiord). Another enclosure of 3.5 ha, built by the same company in 1970, is at Veloyjolp (Sotra island), with a production capacity of 600,000kg. The “pen” lies between two islands and has been sealed by two concrete barriers (concrete and aluminium screens) at either end. At Bjordal in Sognefjord (North of Bergen) Eros Laks A/S has constructed many small 30m square, not enclosures round the fiords. Each enclosure has nets on three sides and access from the shore on the fourth. The 1971 production of salmon here was about 30 tons. Moller (1979) describes the Osland enclosure, named after its constructor, fish farmer Erling Osland. The enclosure has on the long side, the short, with walls supported by piles on the three sides, having an area of 25 × 35m more in the shape of a horse shoe
The construction details will be discussed separately.
In United States of America Pompano (Trachynotus carolinus) cultivation is begun recently in pen enclosures, which are less expensive to build than score tanks. the first experiment was conducted in a 2.4ha enclosure (Long and narrow) in a small inlet enclosed by a short length of netting, in Tampa Bay, Florida.
Here they have also used two small square contiguous sublittoral enclosures (12.2 × 12.2 × 3.0m), constructed using 6mm galvanized mesh screen supported by timber framework. These pens were used for diet testing, storing 2,000 fish in each pen.
We have not taken into consideration those aspects of trout and salmon culture both in U.S.A. and Europe, mostly because even though they have been referred to at times as enclosure or pen culture, they are strictly falling under the category of ‘cage culture’ according to our definition, pointed out at the beginning. There are certain aspects of pen/enclosure culture in the sublittoral zone, reported as under research -these are not included here but will be taken up later as pertinent. Many recent innovations which have been reported often as news items also do not find a place here, but will have to be considered later.
In the workshop on cage and pen culture held in Philippines (1979), the following countries (among those represented) have been reported as having pen culture in an operational stage:
Freshwater - Philippines and Thailand
Marine - India and Thailand
Egypt and Indonesia have been reported to have initiated pen culture in freshwater and Philippines in marine areas. As a participating European country Hungary has reported having cage culture in fresh water.
As obvious most of our earlier references have dealt with pen/enclosure culture in the marine coastal area. Pen culture in fresh water is relatively less popular, but for the single exception of Laguna Lake milkfish culture of Philippines. Other pen culture activities are less important and so we shall first refer to pen culture of milkfish.
The pen culture of milkfish (Chanos chanos) is the most important fresh water pen culture in the world. It began in 1968 as an experimental venture of the Philippine Bureau of fisheries and Aquatic resources in the fresh water lake, Laguna de Bay -there is a slight balimo incursion into the ‘Bay’ but it is largely fresh water). In 1970 the Laguna Lake Development Authority introduced a 40 hectare pilot commercial scale fish pen project and demonstrated that yield per hectare in the pens was 4 – 10 times higher than the natural production of the lake and thus a new industry was borh. In two years private operators cultivated 200ha of Laguna Bay under pen and by 1973 the expanded 993 pens extending over 5,000ha but the further expanded fishery was affected by typhoon damage, a frequent problem in Philippines. Pen sizes vary from 1 to 100 hactares depending on the resources and interest of the operator, the majority being of 10 – 50 hectares. The pens are made using the locally abundant bamboo poles forming split bamboo screens (Pongkol). Depth of water in pens average 1.5m.
The pen culture of milkfish has yielded production of about 4,000kg/ha (average), without supplementary feeding. Total milkfish production from fish pen in 1975 was 25,000 tons. The pen culture in 1979 (after typhoon damage) was about 3,000ha and the yield about 16,000 tons annually i.e. 16–20 percent of the total milkfish production in the country. The Laguna Lake Development Authority has claimed that the lake fish pen industry can grow to 15,000 hectares, and a yield projection of 80,000 tons annually. (see Guerrero and Soesanto, 1982, for more details).
Successful pen culture operations exist now in Ivory Coast and in Benin Republic.
The advantages and disadvantages of pen culture are in some cases common as those for cage culture. Obviously the pens are much larger and are stationery as their walls are fixed. It also appears that in large pens the culture will be less intensive, eventhough small pens can vie with the cages in this respect. The towability (or mobility) of the cage is its most definite advantage over the pen, but the later has the benefit that there can be interchange between the organisms within, with the natural bottom - at times of an inclement condition in the bottom the pen is decidedly difficult. Let us now enumerate the advantages and disadvantages of pen culture.
Intensive utilization of space: As we have mentioned the requirement of a pen can be small (a few square metres) or large (over 100ha in the case of the largest milkfish pen), but in all cases the space given is intensively utilized. Even in the large milk fish pens utilize space intensively and their production is 4 – 10 times higher than the natural production. We have already referred to the high production (per unit area) above, that production even in the large pens are much higher, not only because of the intense culture practices adopted, but also due to other factors such as protection from predators.
We have referred to the millfish production of 4 tons/ha (average) without recourse to any feeding. In Norway in the 1.2ha enclosure in Floyoykgolpo, the production capacity is said to be 150 tons, with intense culture practice (feed) and maintenance of environmental quality. In any case per square area basis, the production will be less than for the cages, and the larger the pen generally the lesser the production.
Safety from predators: Within the enclosure the predators can be excluded. Before stocking the predators will have to be removed; in the larger pens this would be more difficult, but in smaller pens this can be done as efficiently as in the cages (see also discussion following).
Suitability for culturing many varied species: Under artificial culture provided suitable environmental conditions are maintained, with artificial feeds, many varieties of species can be cultured as in the cage.
Ease of harvest: Eventhough in the large pens the harvest may not be aseasy as in the cages, it would be definitely more controllable and easier than in the natural waters.
The flexibility of size and economy: When compared with the cage, pens can be made much larger and construction costs will be cheaper than that of the cages.
Availability of natural food and exchange of materials with the bottom: Since, as pointed out earlier, the bottom of the pen is the natural bottom, unlike the cage which kept either on the bottom or floating, has always a netting/ screen seperating the cage from bottom; the pen culture organisms are at an advantage that while enclosed they can procure food/exchange materials.
Pen culture as cage culture is economical multiple use of same water body (e.g. pen culture in an irrigation reservour) - see disadvantages.
High demand of oxygen and water flow: Since the fish cultured are stocked in high density they deplete oxygen very fast and a good flow of water through the pen either by natural means or artificially by pumping is demanded for healthy and fast growing fishes.
Dependence on artificial feed: Since high density (biomass) is to be sustained in a restricted area, for high production artificial feeding is necessary, increasing the cost of production.
Food losses: Part of the feed is likely to be lost uneaten, and drifted away in the current, but the loss here would be less than in floating cages.
Pollution: Since a large biomass of fish are cultured intensively a large quantity of excrements accumulate in the area and cause a high POD - also substances such as ammonia and other excreted materials, if not immediately removed/ recycled. They pollute the water and cause damages.
Rapid spread of diseases: For the same reason of high stocking density in an enclosed area, any disease beginning will spread very quickly and can cause immense mortality of stock and production decline.
Risk of theft: Since the fish are kept in an enclosed area, ‘poaching’ and thefts can take place more frequently than in natural waters, but perhaps less than those from cages.
Conflict with multiple use of natural waters: In locations where a pen is constructed to the requirement of higher water level for eg. in a lake/reservour, would be against the interest, for eg. for irrigation water supply; enclosures can interfere with navigational routes and also with recreational activities, such as swimming, boating etc.