Sri Lanka is a tropical island of mild climate without extremes. Its freshwater fisheries will be considered in the context of physical, climatological and sociological factors which have either a direct or an indirect influence on these fisheries.
Sri Lanka is thought to have separated from mainland India in the Miocene, which is geologically a relatively recent separation. It is thought to be an extension of the peninsular India and forms a part of the Indian Shield, which in turn is considered to be one of the oldest and most stable parts of the earth's crust (Cooray, 1984). The detailed geology of the island has been dealt with by Adams (1929), Coates (1935), Wadia (1945) and Cooray (1967). More recently, Dissanayake (1984) reviewed certain geochemical aspects.
More than four-fifths of the island is made up of Precambrian crystalline rock with two small basins of Mesozoic (Jurassic) deposits faulted into the Precambrian basement, the rest being of rudimentary Miocene limestone confined to the island's extreme north and the northwest. Cooray (1984) sub-divided the Precambrian deposits into major units viz.
(i) Highland series of meta sediments; metamorphosed for the most part under granulite facies conditions,
(ii) The Southwestern Group, lithologically somewhat dissimilar from (i), and
(iii) the Vijayan Complex of migmatites.
Of the Precambrian deposits the Highland series and the Southwestern group fall within the wet and intermediate climatic zones and the Vijayan complex almost entirely into the dry zone. The manner in which Precambrian deposits have reacted to the climatic conditions in the different zones are thought to be reflected in the soil pattern and groundwater conditions. Cooray (1984) has pointed out that over the island as a whole, and therefore in the Precambrian generally, climatic factors have the most influence on the ecology, and that lithological factors are subordinate.
Sri Lanka is characterised by a south central mountain range which rises up to 2524 m above sea level at its peak (Fig.1). The island is characterized by a variety of land forms, ranging from flat erosion or peneplain, to a very complex assemblage of mountains, ridges, plateaux, and valleys. Erb (1984) considered the island to be made up of seven major land form units, viz. (i) coastal plain, (ii) continental shelf, (iii) circum-island peneplain, (iv) central massif, (v) Sabaragamuwa hills, (vi) Galoya hills and (vii) Elahera ridges.
The landforms which make up the complex terrain of the island are thought to be a result of a variety of geomorphologic processes (Erb, 1984) acting upon a diverse geological foundation under a relatively pure equatorial climatic regimes. Erb (1984) discussed the importance of the landforms in relation to the drainage pattern.
Vitanage (1970) recognized three main morphological zones:
(a) the coastal lowlands: elevations from sea level to 270 m; slopes small to flat;
Fig. 1. Topography of Sri Lanka (Survey Department, 1972).
(b) uplands: elevations from 270 m to 1060 m, ridges and valley topography, 30% of the island, average slope varies from 10° to 35°;
(c) highlands: elevation from 1060 to 2420 m, well defined high plains and plateaux, characterizes the central part of the island.
There are four major works on the soils of the island i.e. those of Joachim (1955), Moorman and Panabokke (1961), Panabokke (1967) and UNDP/FAO (1969). At present 17 great soil groups are recognized, the reddish brown earths being the most dominant. The characteristics of the 14 major soil groups recognized by Panabokke (1967) are summarized in Table 1, and the distribution of the 17 great soil groups is shown in Fig.2. In general the soils are considered to be poor in mineral nutrients, have a low humus content and a pH ranging from 4.6–8.4, most being acidic (Joachim, 1955).
In very generalized terms Sri Lanka's climate is tropical humid. This generalization, however, masks the variety and the range of climatic patterns found on the island.
Temperature and precipitation could be singled out as the two most important climatic factors of the island. In the same token the most dominant factor determining the climatic patterns can be singled out as the two monsoons, the south-west active from June through September and the north-east active from December to February, and the related intermonsoonal effects. Climate maps and climatic zones have been compiled by a number of authors by integrating these two parameters (Thambyahpillay, 1952; Koelmeyer, 1958; Gaussen et al., 1964; Mueller-Dombois, 1968, 1968a). Gaussen et al. (1964) recognised 22 climatic zones based on two basic climatic parameters, whilst Leith (1960, in Mueller-Dombois, 1968a) recognized only four zones. The compilation of different climatic zones, their limitations and acceptability were evaluated by Mueller-Dombois (1968a).
The most accepted zonal differentiation, however, is based on the 75 inch (195 mm) mean annual rainfall isohyet. Mueller-Dombois (1968a) divided the wet zone into 2 sub-zones and the dry zone into 4 sub-zones The principal and sub-climatic zones which are currently in broad usage are given in Table 2. The rainfall pattern and the temperature variations at selected places, thought to be of relevance to this and included two other sub-zones which did not fit into either division.
Fig.2. Distribution of great soil groups (Panabokke, 1984)
|Reddish brown earths||moderately deep, well drained||o.m, N-low; high in Ca, Mg; good neutral|
|Noncalcic brown soils||shallow to moderately well drained||o.m, P,N-low; Ca,Mg-reasona- ble; c.f. moderate to good; slightly acidic|
|Red-yellow podzolic soils||deep to very deep, well drained may have a shore line||o.m, N,P,K,Ca, Mg-low; good cation exchange; moderate to strongly acidic|
|Red-yellow latosols||very deep, extremely well drained, uniform throughout their depth||o.m, N,P,K - very low; low cation exchange; poorest soils; moderately acidic|
|Reddish-brown latosolic||deep to very deep, well drained, may have stone line at some depth||o.m, N,P,-moderately low; Ca, Mg, K - moderate; moderately acidic|
|Immature brown loans||moderately deep, well drained, young soils, may contain undercom- posed primary minerals||o.m,N,P-low; K-medium to high; good c.f. Ca,Mg-good; good c.f., neutral|
|Solodized solonetz||divisible into two horizons of contrasting characters; A horizon no free salts; B horizon hard, high Na control||o.m,N,P-low; Ca,Mg-good; saline|
|Grumusols||do not have a B horizon in the natural state; have AC profiles.||o.m,N-medium to low; P-low; K-high; Ca,Mg-good; v.good c.f.; slightly alkaline|
|Regosols||two kinds - sandy rego- sols in young beaches and variable regosols||o.m,N,P-low; K-medium; Ca,Mg -good; good c.f.; neutral|
|Bog and half-bog soils||organic layer of a minimum 30cm and >25% organic matter||o.m,N-v. high; P,K; Ca,Mg-low adaptable; extremely acidic|
|Low-humic gley soils||A-horizon soft; B-horizon heavier texture||-|
|Alluvial soils||no profile development||-|
The principal and sub-climatic zones which are currently in broad usage are given in Table 2. The rainfall pattern and the temperature variations at selected places, thought to be of relevance to this study, are shown in Fig.3. The rainfall data map is based on the 25 inch isohyets which essentially divide the island into 8 zones (Survey Department, 1983).
|Zone||Avg.monthly rainfall(cm)||Temp(C)||Drought period||Vegetation|
|(a)||arid sub-zone||<187||>6 months||sparse/thorny shrub|
|(b)||semi-arid sub-zone||<125||June-Sept.||dry evergreen|
|(c)||lowland dry zone||125–187||-do-||dry evergreen/moist deciduous|
|(a)||intermediate lowland zone||187–250||26.6||moist semi-ever- green villus damana type grassland|
|(b)||upland sub-zone||28.3||moist evergreen/ tropical savanna|
|(a)||lowland sub-zone||>250 (100m)||26.6–38.0||not marked||tropical wet evergreen|
|(b)||upland sub-zone||-do- (1000–1500m)||18.3–24.0||-do-||sub-montane savanna|
|(c)||highland sub-zone||-do- (>1500m)||15.0–18.3||wet patana|
Thambyahpillay (1954,1965) recognized the following climatic seasons in the island;
(i) the vernal convergence - convectional season; March to April
(ii) the pre-southwest monsoonal season; May
(iii) the southwest monsoonal season; June to September
(iv) the convergence - convectional - cyclonic season; October to November
(v) the northeast monsoonal season; December to February.
The recognition of the above seasons was based primarily on a causative analysis of the recurring wind patterns. Mueller-Dombois (1968a) pointed out that a sixth season should be included - the dry month of February, when the dryness is manifested over most of the island. February ‘dryness’ was considered as the ‘tailend’ effect of the northeast monsoon.
Fig. 3. Rainfall pattern of Sri Lanka (Anonymous, 1983). Also shown are the mean monthly variations in temperature and rainfall of selected places (after Mueller-Dombois, 1968).
In view of the topography of the island and the overall rainfall pattern, a substantial water resource is to be expected. The natural water resources of the island take the form of extensive river and stream systems, and the associated flood plains and marshes. Sri Lanka does not have any natural lakes.
There are 103 perennial rivers in Sri Lanka, of which over 90% radiate from the central highlands into the western, eastern and southern coasts. There are 28 rivers with a basin area exceeding 500 km2 (Survey Department, 1983; Fig.4). The river Mahaweli is the biggest river, and the basin covers approximately one sixth of the island (De Silva, 1985). The total discharge to the sea from the 103 perennial rivers amounts to 39,032x106m3, which is approximately 31.4% of the annual precipitation received. The 28 major rivers discharge a total of 36,089x106m3 or 92.5% of the total discharge (Table 3).
The rivers have a flow characteristic dependent on their geographic position, the physical characteristics of the terrain and the regional climatic parameters. Erb (1984) categorised the rivers into four groups (see also Fig.4):
(a) those with headwaters in the mountains, plateaux and plains of the inner region of the Central Massif: flow outwards across the Circum-Island Peneplain to the sea e.g. Mahaweli Ganga; most complex rivers;
(b) those with headwaters in the marginal hills, plateaux and mountains of the Central Massif: flow outward across the Circum-Island Peneplain and coastal plain to sea e.g. Walawe Ganga, tend to have high gradient; actively eroding headwater, greater variability in flow;
(c) those with headwaters in the mountains hills, plateaux of the Sabaragamuwa Hills, Galoya Hills or Elahera ridges: traverses the Circum-Island Peneplain and coastal plain to sea e.g. Kalu Ganga, similar characteristics to (b), complexity variable;
(d) those with headwaters on the Circum-Island Peneplain and traverses it and the Coastal plain to the sea e.g. Mi Oya: flows along low gradient channels.
|Basin||Catchment km2||Pptn m2×106||Discharge m3×106||%|
|From All Rivers||59,233||124,297||39,032||31.4|
The flow characteristics of the first three categories of rivers enable them to be developed for generation of hydroelectric power as well as for irrigational purpose, the latter in their course along the coastal plain.
In view of the steep gradients, the floodplain area including floodplain lakes or their equivalents, is limited in extent and distribution. The floodplain lakes are locally known as “villus”, which are vegetated land areas seasonally saturated with water and retaining their connection to the main river most of the year. A villu generally consists of two portions: the ponding area which is permanently filled with water, and the floodplain which is inundated by water during the flood period (De Silva, 1985).
True floodplain lakes are restricted to the Mahaweli Ganga basin to the east and the Kala Oya and Moderagam Aru basins to the west (Fernando, 1971). The total estimated extent of flood lakes is 40,000 ha, of which 16,000 ha was reported to be in the Mahaweli Ganga basin (Hunting Surveys Corporation, 1962; Mendis, 1977). The more recent estimates indicate that there are only 12,800 ha of villus in this basin (TAMS, 1980). Some of these villus range in surface area from 400 to 900 ha. The possible impact of the Mahaweli Ganga development programme on the extent of the villus, and their impact on fish and fisheries have been discussed by De Silva (1985).
Cooray (1984) has argued that the presence or absence of groundwater is an important and possibly a critical factor influencing human settlements. He considered groundwater as a “mineral resource”, the availability of which, in his view, is determined by climate rather than the geology. The extent of imformation available on groundwater resources in Sri Lanka is minimal.
There is no apparent scarcity of groundwater resources in the wet zone of the island. Recent surveys have reported that in the dry zone, extensive groundwater resources are found in the Vijayan Complexes, which form zones of highly jointed, fissured and fractured rocks, both gneisses and metasediments, with moderate to low permeability (Cooray, 1984). Apart from these, large perennial springs are known to occur in marbles. In the eastern part of Sri Lanka, particularly around Maha Oya (see Fig.4) a number of hot springs, thought to be derived from thermally heated circulating groundwater and gaseous emanations, has been recorded (Dissanayake, 1984).
Fig.4 Major rivers, their basins and flood lakes in relation to the landforms (Erb, 1984). Also shown are major reservoirs of over 2000 ha in surface area.
Pemadasa (1984) considered that of the biotic resources of Sri Lanka, the vegetation was the most outstanding in view of its diversity, species-richness, high degree of endemism and its great economic potential. Most vegetation analyses have been carried out in relation to climate, with a view to compiling vegetation-climate maps of the island. These attempts were aptly reviewed by Mueller-Dombois (1968a). The major vegetational types described by various authors are summarised in Table 4.
|Chapman (1947) de Rosayro (1950)||forests, scrub, patana, mangrove dry mixed evergreen forest; wet and montane evergreen forest (sub-climax);intermediate ever- green forest (ecotone);montane grassland (sub-climax);savanna and mangrove||Vegetation map of 1: 2.4x106 scale vegetation descri- bed in terms of Clements' Climax hypothesis;map 1: 1.5 million scale|
|Koelmeyer (1958)||9 forest and 4 grassland types||1:2.6 million scale map|
|Andrews (1961)||4 use-classes of forests viz: high, medium, and low yield and non-productive.||1:506880 scale map drawn for forest management|
|Gaussen et al. (1964)||management use-classes translated into physiognomic terms of Andrews (1961) e.g. non-productive in the dry zone - scrub woodland, low yield - semi deciduous forest||1: 1 million scale|
Abeywickrema (1955), who did some pioneering work on the vegetation of Ceylon, and whose findings still stand to a very great extent, postulated that nearly 30% of the 3000 or so species of vascular plants in the island were exotic. He recognised five major types of floral communities:
a. Marine vegetation - consisting of salt marsh vegetation, mangrove vegetation, sandy shore vegetation, sand dune vegetation(occurring along the coast).
b. Fresh water aquatics and marsh communities.
c. Tropical wet evergreen forests -occurring in the southwestern lowlands: average temperature 25–27°C, rainfall exceeding 254 cm annum-1, lateritic soils.
d. Tropical semi-evergreen forests - the ecotone between the wet evergreens and the dry mixed evergreens: average temperature - 27°C, annual rainfall between 127 and 190 cm.
e. Tropical thorn forest found in the north-western and southwestern coastal areas: rainfall below 127 cm, average temperature 27– 28°C, soils variable.
General studies have been complemented by detailed studies on different floral communities, for example the tropical wet evergreen forests by De Rosayro (1945), grasslands by Pemadasa (1984), tropical thorn forests by Chapman (1947) etc.
The present population of the island is estimated at 16 million. Perera (1984) divided the long history of man's tenure in Sri Lanka into a number of periods, such as pre-historic, proto-historic, dry zone civilization (5 BC–12 AD), late medieval (1200–1500), western colonial (1500–1947) and the post-independent period from 1948. Perera (1984) conceded that the above divisions are somewhat arbitrary and each division is not clear cut.
The pre-historic and post-historic periods are of little relevance to the present study and for details the readers are directed to major works of history such as Paranawitana (1958, 1960) and Boisselier (1979).
All available evidence from legendary, linguistical and anthropometrical sources point out that the Sinhalese were people of Aryan origin and colonized the land about 500 BC, settling initially in the northwest coast and in the southeast. Apart from the west coast settlements the major Sinhalese settlements were established in the dry zone, perhaps riverine in character and the staple food was rice. The major settlements were sustained by irrigation. During the 1500 year-long period the civilization - popularly known as the dry zone civilization - was able to lay out paddy fields, construct reservoirs of varying proportion and complexity, terrace hill slopes and construct magnificient religious monuments. Controversy still exists as to the size of this great civilization. Estimates vary from 17 million (Tennent, 1861) to 14 million (Denham, 1912) to 2–4 million (Murphey, 1957), and the estimate of 70 million written in an old palm leaf seems unrealistic (Domros, 1976).
The dry-zone civilization began to decline around the middle of the 13th century. Reasons attributed to this are diverse: Nicholls (1921) attributed it to the spread of malaria, Codrington (1960) to the destruction of the large irrigation works due to wars, Murphey (1957) to malaria and foreign invasions, Liyanagamage (1968) to the cumulative result of foreign invasions, internal warfare, dissensions and spread of disease, Indrapala (1965) to the growth of the Tamil Kingdom to the north of Polonnaruwa and finally, Roberts (1971) to the icreasing infertility of the soils as a result of erosion and increasing salinity, which necessitated colonization of new lands.
The decline of the dry zone civilization had the obvious consequence of immigration into the wet zone, which in a broad sense has remained so to this date (Domros, 1976). The present population distribution in the different administrative districts is such that the wet zone at present contains nearly 65% of the island's population. Perera (1984) traces the settlement and land usage patterns and the underlying reasons for the success of these settlements since the decline of the dry zone civilization to the present times.
The increasing population in the wet zone and the increasing dependence on imports for the staple diet of the populace are thought to be two major reasons for the consideration of the resettlement in the dry zone, which received its greatest impetus from 1932 onwards.
The restoration of the Minneriya tank in 1935 could be considered as the starting point, when planned colonization began to be affected. The rapid restoration of the large irrigation schemes resulted in a migration from the wet zone to the dry zone. The new settlements were more dispersed and not the clustered type. This trend continues to this date, with further restorations taking place as well due to new irrigation schemes being built. For example the new irrigation schemes coming into being as a result of the development of the Mahaweli Ganga has been dealt with in detail (De Silva, 1985), inclusive of the settlement patterns, land usage etc. thereof.
Undoubtedly, the population changes resulted in consequent changes in land usage. The most significant change in the land use pattern occurred with the British occupation in 1796. This was the beginning of the growth of plantation agriculture: initially coffee, later to be replaced by tea in the highlands and midlands and rubber in the lowland wet zone. Although further opening of virgin land for these crops is now at a standstill, the opening up of land, on a large scale, for these crops in the 19th and early 20th century has reduced the total forest cover in the island to less than 20% of the land area.