PNG/85/001. Field Document Number 2 March, 1989

PAPUA NEW GUINBA

Summary of the geology, geomorphology, climate and vegetation o:r: the Sep:i.k and Ramu River catchments

with notes on their relevance to fisheries

A report prepared for project PNG/85/001: Sepik River Fish Stock Enhancement Project

Compiled by

D. C0.2\'l'ES (Chief Technical Adviser)

FOOD AND AGRICULTURE ORGANISATION OF THE UNITED NATIONS Rome, 1.989

Th~s report was prepa~ed during the course of the project identified on the title page. The conclusions and recommendations given in the report are those considered appropriate at the time of its preparation. They may be modified in the light of further knowledge gained at subsequent stages of the project.

The designations employed and the presentation of the material in this document do not imply the expression of any opinion whatsoever on ~he part of the United Nations or the Food and Agriculture Organisation of the United Nations concerning the legal or constitutional status of any country, territory or sea area, or concerning the delimitation of frontiers.

TABLE OF CONTENTS

page

1. INTRODUCTION. . . . . . . . . . . • . . . . . . . . . • . • • • . . • . . . . . . . . . 1

2 . GEOLOGY. . . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . • . • • • • . 1

3. ALTITUDINAL ZONES. . . . . . . . . . . . . . . . . . . . . . • . . . • . . . • . . 4

4. CLIMATE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . 5

5 . VEGETATION. • . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . 7

6. DISCUSSION AND CONCLUSIONS ........................ 10

7 . REFERENCES . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . • . . 16

FIGURES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . • 18

TABLES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . • • • • . . 3 6

1. INTRODUCTION

This document provides background information for project PNG/85/001. A synthesis of knowledge of various aspects of the Sepik and Ramu River catchments is provided. The information is presented in relation to project objectives and activities. Most sections have been summarised from existing books and other information (quoted where relevant) but have been placed within the context of the Sepik and Ramu rivers and project PNG/85/001. The subjects covered here are relevant to fisheries related matters and this is explained later in the Discussion section.

This document covers the whole Sepik and Ramu river catchments from Om to the highest peak 4509 m.a.s.l.

2. GEOLOGY

The general geomorphology of Papua New Guinea has been summarised by Loffler (1977).

New Guinea and its associated smaller islands are situated between and are part of two major crustal elements, the continental, relatively stable land mass of Australia to the south and the deep ocean basin of the Pacific. to the north (Fig. 1). According to plate tectonics concepts, the New Guinea area has lain in the zone of interaction between the northward moving Australian continental plate and the westward moving Pacific plate since about Cretaceous times. As the Australian plate is moving north and the Pacific plate moving west the area is structurally complex. Extreme forces involved with such plate movements have resulted in upthrusting along approximately the centre of the present island forming what is referred to as the "Central cordillera" forming the PNG "Highlands".

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The major structural regions are shown in Fig. 2. The Sepik-Markham depression essentially delimits the lowlands of the Sepik and Ramu River catchments. This depression extends through the entire island from Geelvink Bay in Irian Jaya to the Huon Gulf in Papua New Guinea. The depression, also known as the central intermontane trough, has been a zone of relative subsidence since the later Tertiary and its margins are locally marked by steep fault scarps. The trough is now filled with terrestrial elastic sediments forming extensive alluvial plains and fans. Draining into this depression, however, are rivers arising from the New Guinea Mobile Belt (the "Highlands"). the Toricelli-Bewani Ranges and the Finisterre-New Ireland Arc (in this region referred to as the Finisterre Ranges) (see Fig. 2). Together, the

latter two regions constitute the ''Northern Coastal Range". The New Guinea Mobile Belt consists mainly of low grade metamorphic rocks and acid to basic and ultrabasic plutons, most of which have been intruded along major fault planes by encroaching alluvial deposition from the Sepik-Markham depression at lower altitudes. The Torricelli-Bewani Ranges consist of granitic and metamorphic basement which is exposed in the central part of the unit, flanked by a succession of sediments, predominantly sandstone and siltstone, which become progressively younger towards the outer zones of the mountains. The Finisterre Range (part of the Finisterre-New Britain Volcanic Arc - "10a'' in Fig. 2) is older and comprised of Eocene basic to intermediate volcanic rocks and associated intrusives overlain by middle to upper Miocene limestone.

The eastern section of the Sepik-Ramu depression (= intermontane trough) forms the Markham-Ramu trough which is a narrow graben zone occupied by a series of low-angle coalescing alluvial fans. These fans are formed of coarse debris derived from the tectonically very active Finisterre and Saruwaged Ranges which rise steeply along a series of faults along the north-eastern margin of the trough. The rivers have gradients similar to the fans (1 to 3 %) and flow in highly unstable wide braided floodplains with constantly shifting sand bars and channels. In their lower courses the present floodplain and the older fan surfaces merge and form a continuous plain. Westwards, the narrow Markham-Ramu trough opens into the much more extensive sepik depression, which is predominantly formed of vast swamps, meandering floodplains, and a series of low-angle variably dissected fans along the northern flanks of the depression. Structurally, the Sepik depression differs considerably from the Markham-Ramu graben as it is generally not fault-bounded but constitutes a large basin, the remnant of the Northern New Guinea Basin which during the Pliocene extended from the central ranges to the present north coast. The area south of the meander belt of the Sepik River is formed by a series of nearly continuous swamps which extend right up to the foot of the abruptly rising central ranges. The valleys draining into this region have characteristics of drowned valleys with the swampy alluvium extending into the mountain front. A great number of valleys also seem to have been cut beneath the present base level of erosion. This drowning could have been the result of either the post-glacial rise in sea level or subsidence or both. North of the Sepik River, swamps are markedly rare or absent. Instead, variably dissected low­angle fans cover much of the area between river and mountain front, indicating that this has been an area of uplift rather than subsidence.

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The geology of the Sepik and Ramu catchments is extremely complex and a simplified generalised geology is shown in Fig. 3. Adjacent sub-catchments can, however, have quite different surface rock types arising from various processes of weathering, glaciation, uplifting, faulting, deposition and lateral shifting of structural elements.

In addition to the central cordillera, the northern ranges (primarily the Toricelli-Bewani and Finisterre mountains) have also been areas of considerable uplifting. Such areas are still rising. Many stream beds along higher altitudes in such ranges (protected from overlaying alluvial deposition) are composed of intact corals indicating their recent marine origin.

North of the island is the deep Pacific basin which accounts for the deep waters immediately off the mouths of the Sepik and Ramu rivers. This is thought to account for easier discharge of water from these rivers and their single exits to the sea. In contrast, rivers in southern New Guinea drain onto shallow and extensive continental shelf. They have very extensive estuarine areas and deltaic systems. This, in addition to the continual coastal uplifting, is thought to explain the relative lack of extensive areas of mangroves along the northern coast.

2.1 Geological history

The present land mass of Papua New Guinea evolved as the result of a series of complex events which are all basically linked with the relative movements of the two interacting plates. The present land mass started to develop in the lower Miocene with the emergence of the New Guinea mobile belt (the present Highlands), which has remained land ever since and has been subject to intense erosion. However, it was not until the upper Pliocene, which is only a few million years ago, that the framework of the present landscape became visible with the emergence of the Torricelli-Bewani Mountains to the north. Raised coral platforms ranging in age from Pleistocene to Recent give evidence of continuing uplift in these areas, with maximum rates in the order of 3 mm per year, which is a quite spectacular rate in geological terms. The reason for recent rapid uplifting along the northern coast is the shift of interaction between the two plates from the New Guinea mobile belt to the Toricelli-Bewani ranges and the island arcs where most active volcanoes now occur.

Of particular note during recent geological events is the formation of the Sepik-Ramu floodplains. The whole of the lower altitudes on the Sepik-Ramu depression was

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originally occupied by an inland sea up until only 6,000 years ago (Fig. 4). Subsidence by the trough, accompanied by rapid uplifting along the northern coast and recent rises in sea level allowed the basin to fill with alluvium from the high sediment load rivers draining the recently formed, weather-beaten and highly unstable surrounding mountains. During this process the area developed from a deep inland sea to its present form of extensive, shallow alluvial floodplains. Fossil finds from the early Pleistocene (Ice­age) indicate that the "Sepik-Ramu Sea" was at least 200 m deep (Swadling et al. 1989). As this sea became shallower, it began to become freshwater and a habitat for colonising freshwater fish and shellfish. This is witnessed by more recent archeological finds in sediments from recent times (<3,000 years ago) (Swadling et al. 1989). During the more recent Ice Ages the sea level rose and fell and the shore­line changed accordingly. However, it is only since the sea level stabilised to its present limit (6,000 years ago) that the extensive floodplain and backswamps seen today have formed.

3. ALTITUDINAL ZONES

Altitudinal zones within Papua New Guinea are indicated in Fig. 5. Within the Sepik-Ramu catchment (and excluding coastal drainages draining the northern coastal range into the sea) the region is calculated to be divided into the following altitudinal zones (by approximate percentage area) :

3.1 O to 120 m - 10%

- primarily the Sepik-Ramu floodplains and poorly drained backswamps. Floodplains and backswamps are more extensive in the Sepik Basin due to its reduced gradient compared with the Ramu which is subject to greater seasonal height changes in water level. Floodplains generally extend to an elevation of about 50 m, but occasionally higher depending on location and local topography. The altitude of the Sepik River near the Irian Jaya border (October River area) is still less than 50 m at about 1000 km from the sea. Floodplains are developed along almost the whole length of the Sepik system but become extensive in the middle and lower sections. Ramu floodplains extend almost up to the origin of the main river system at the foot of the highlands. However, above this the river arises from a significant highlands catchment which discharges through the very steep "Ramu Gorge" (literally "falling off" the side of the central cordillera) . The Ramu Gorge has formed a considerable barrier to upstream colonisation by fishes within the Ramu system. Only eels and one obscure species of

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goby occur above the gorge (Allen, Parenti and Coates unpublished) . Such fish barriers do not occur throughout most of the Sepik system.

3.2 121 to 300 m - 30%

- lower hill streams areas draining primarily from the base of the northern coastal ranges and the central cordillera immediately into floodplain regions. Floodplains and swamps are generally absent and rivers vary in size and gradient according to the area of local sub-catchments and topography.

3.3 301 to 1500 m - 37 %

- mid altitude regions along the northern coastal range and central cordillera. Swamps are absent due to the generally steep gradient. Rivers vary widely in character.

3.4 1501 - 3000 m - 19%

- higher altitude regions primarily along the central cordillera and to a limited extent the Finisterre Range (absent from the Toricelli-Bewani range) . Swamps are generally absent although a few depressions in highland valleys have moderate areas of variously inundated land.

3.5 Greater than 3000 m - 4%

- highest altitudes representing the peaks of mountains primarily along the central cordillera but also in the Finisterre Range. Characterised by fast flowing streams and rivers, variously torrential with rock/boulder beds. Cascades are frequent but waterfalls rare along rivers and streams above first or second order.

4. CLIMATE

The climate of Papua New Guinea has been reviewed by McAlpine et al (1983).

The larger part of the country experiences relatively high annual rainfall of 2500 to 3500 mm per year. A few lowland areas of limited extent are drier, but annual falls of less than 1000 mm are unknown except for the national capital, Port Moresby (due to peculiarities of local winds and the proximity of mountain ranges) . Large areas of uplands in the Sepik catchment have average rainfalls in excess of 4000 mm and in some locations these can rise to over 10,000 mm per year. Rainfall varies seasonally in most areas, but the degree of seasonality is not great. This

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seasonality js most evident in the limited drier areas, but even here there is no record of nil or near nil monthly rainfall, as found in true "monsoon" climates. The seasonal variation of rainfall over most of Papua New Guinea can be described best as a change from "fairly wet" to "very wet".

Temperature regimes are equable, showing little seasonal variation. Daily mean maximum temperatures on the coast are around 30 to 32 °c, with minima around 23 °c. The most marked characteristic of temperature is the drop associated with increasing altitude. A large proportion of the population lives in highland valleys and mountains at altitudes between 1500 and 2000 m (Coates ang Mys 1989) , where mean daily maxima are around 22 to 25 C and night minima between 11 and 15 °c. Above 2200 m frosts can occur and snow may fall and settle above 4000 m.

The combination of relatively high rainfall and temperatures is associated with high humidity and cloudiness and moderate rates of evaporation, which range on the coast from 1500 to 2000 mm per annum. Hence, much rainfall runs off the generally steep gradient land into large river systems with extensive floodplains in lower regions. Discharges from New Guinea major rivers (the Sepik and Fly) are amongst the highest in the world.

The rainfall distribution pattern for the SepiJc--Ramu catchment is shown in Fig. 6. The bulk of the area lies in the 2000 to 4000 mm per year zone. Areas with less than 2000 mm per year are restricted to lowlands, and especially the alluvial plains which are subject to seasonal inundation from the main rivers. Seasonal variations in rainfall are modest (F'ig. 7). Much of the area experiences little seasonal variation in rainfall with most variation occurring only along the northern slopes of the central cordillera. Seasonality of rainfall is shown in Fig. 8. Most of the catchment has moderate to low seasonality with high seasonality experienced only along the eastern section of the Finisterre ranges and north-western slopes of Mount Wilhelm (central cordillera), both draining into the upper Ramu and Markham Rivers. Variations between years in rainfall are shown in Fig. 9. The area possesses a remarkably reliable rainfall with the central highlands and Sepik. plains having a very low variabi1i ty of less than 15% whilst variability along the coast is higher, but still rarely exceeds 20%.

Rainfalls of over 100 mm per day do not occur in the main body of the central ranges but can rarely occur on coastal ranges. This lack of heavy daily rainfall ("flash floods") in the highland areas results from the fact that

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the diurnal cycle of convectional storms is the chief source of rains in these areas. Lowland areas can experience more frequent and higher daily rainfalls of over 150 mm but these average only once in between 1 to 3 years. Fig. 10 summarises this information.

Seasonal minimum air temperatures for January and July are shown in Fig. 11 and mean maxima in Fig. 12. Seasonal changes are modest. A greater relationship is evident with air temperature changes with altitude (Fig. 13).

The relatively stable high humidity in most areas combined with consistently high rainfall results in soil moistures within the Sepik-Ramu catchment remaining consistently high (Fig. 14). Significant soil moisture storage depletion is rare throughout most of the catchment. Mean annual surplus (runoff) for the region is shown in Fig. 15 and is high for most areas with more modest run-off along sections of the northern coastal ranges.

A summarised climatic classification of the region is provided in Fig. 16.

5. VEGETATION

5.1 Terrestrial/emergent vegetation

The vegetation of Papua New Guinea has been described by Paijmans (1976) and in larger works (Paijmans 1976a, 1982). Handbooks of the flora of Papua New Guinea are available (Womersley 1978, Henty 1981).

Papua New Guinea has a richly diverse flora, with extensive areas still being covered with primary forest. About 20,000 plant species are known or expected to occur. This diversity is largely attributable to the altitude range and geographical position. Papua New Guinea is considered to be in the area of interchange of the Indo-Malesian and Australian zones of flora. Significant elements of the flora are strongly Asian in their origins.

Major vegetation types are shown in Pig. 17. Within the Sepik-Ramu catchment the major vegetation types by percentage area are as follows (excluding coastal drainages along the northern coastal ranges):

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VEGETATION TYPE AREA (km2 ) PERCENTAGE AREA

Lowland forest 62,712 42.0

Lower montane forest 32,448 21.0

Upper montane forest 8,112 5.5

Grassland 6,084 4.0

Savana 0 0

Herbaceous freshwater swamp 6,864 4.5

Wooded freshwater swamp 20,124 14.0

Mangroves 488 <0.5

Gardens & grassland 12,012 8.0

(These classifications of vegetation types differ somewhat to those derived from topographic maps of the region as listed in Coates and Mys 1989).

Lowland forest is predominant and extends to approximately 1400 m in areas where rainfall exceeds 1800 mm per year. Lowland forest develops best on relatively well drained alluvial plains, where its canopy height is 35 to 40 m. Impeded drainage will cause the canopy to become more open and irregular in height. In very poorly drained to swampy conditions, sago and pandans are found in the understory. A considerable amount of the lowland forest in the Sepik basin is in the transitional zone to woody and herbaceous freshwater swamp and is seasonally inundated. Such areas can also be referred to as "floodplain forest". Floodplain forest is even more marked at low altitudes in the Ramu basin. In Fig. 17 it will be seen that most of the Ramu floodplain is marked as "lowland forest". The different flood regimes of the two rivers probably account for their differing vegetation types in lowland areas. The Ramu floodplains, with greater seasonal changes in water levels, are, presumably, better drained in the dry season allowing formation of more dense forest which is seasonally inundated in appropriate areas. Sepik floodplains are less well drained with better development of herbaceous freshwater swamps leading to wooded freshwater swamps and eventually lowland forest seasonally inundated in outlying areas.

Lower montane forest occurs at higher altitudes (1500 to 3000 m) principally along the foothills of the central cordillera and the Finisterre Range. Low cloud cover, common

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to this environment, gives an impression of wetness, and vegetation reflects damp conditions with mosses covering trees and fallen branches. The forest canopy is lower (generally 20-30 m), more regular and denser than the average rainforest. The number of tree species is relatively small, with oaks (Castanopsis, Lithocarpus), beech (Nothofagus) and species o~ laurel being most common, together with conifers and trees of the myrtle family above 2000 m.

Upper montane forest occurs above 3000 m in increasingly cold conditions. This is characterised by stunted, often gnarled, trees, approxima~ely 10-15 m in height, belonging to the conifer, myrtle, heath and rose families. Usually this forest forms a mosaic with grasslands, becoming more frequent with altitude. Near the limit of the tree line (3800 to 3900 m) the forest degrades into scrub and grasses. such areas are restricted to the higher elevations of the central cordillera and the peaks of the Finisterre Range.

Grassland, which is usually open with limited forest cover, is common at altitudes from sea level to more than 4000 m. Tussock grasses, alpine herbs and mosses are found only above 4000 m. Because of the cold climate and frequent frosts these include many southern temperate species. The tussock grasses Danthonia and Poa are dominant, together with ferns, lichens and mosses, the latter two being found in conjunction with bare rock exposures at the highest alti~udes. Between 2500 and 4000 m the grasslands are dominated by Danthonia together with Deschampsia. In the lowlands, in hilly and low mountainous terrains that have a moderate to high rainfall seasonality, kangaroo grass (Themeda australisJ, kunai (Imperata cylindrica) and speargrass (Heteropogon contortus) are the most abundant species. In the lowland plains, pit-pit (Sachharum spontaneum) and kunai are dominant. Most grasslands below 2500 m are thought to be anthropogenic and developed as a result of shifting cultivation and the practice of burning. such areas tend to be close to regions of high population density and within the Sepik-Ramu are restricted to the highlands and areas of the Torice~li-Bewani Ranges in the vicinity o= Maprik to Dreikikir.

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Herbaceous freshwater swamps and wooded freshwater swamps are extensively developed in floodplain regions of the alluvial plains. Usually the vegetation type is strongly related to depth and seasonality of flooding. In areas subject to deeper flooding "floating" swamp grasses are extensive, often interspersed with herbaceous growths. These often break loose forming floating ("sudd") islands. These are particularly prevalent along the edges of ox-bow lakes

extending out from floodplain forest across the lake surface and along the edges of depression lakes in floodplain areas (e.g. Chambri Lake). Such grasses in ox-bow lakes are responsible for the limited areas of accessible shallow waters (along the edges) of these lakes. In less deeply flooded areas tall cane grasses (e.g. Sachharum robustum and Phragmites karka) are most common. Permanent herbaceous swamps are dominated by herbs, sedges and ferns and wooded freshwater swamps by Campnosperma. The sago palm (Metroxylon sagu) forms groves in poorly drained areas of swamp where they are used as an important staple food by local people. such areas are extensive in the lower Sepik and Ramu basins and where major tributaries enter the main rivers in areas of low-lying ground.

Mangroves are limited in the Sepik and Ramu for reasons explained above. They are restricted to small areas in limited brackishwater zones at either side of the river mouths. There are no brackishwater areas at the sites of the mouths themselves and mangroves are absent there.

Gardens and Grasslands are found between sea level and 2700 m, but dominate in the highlands. They are similar to the grasslands already mentioned but differ in respect to the extent of more permanent cultivation. There is often intensive sweet potato cultivation between 1500 and 2500 m.

5.2 Freshwater aquatic plants

The diversity and distribution of freshwater plants in Papua New Guinea has been reviewed by Leach and Osborne (1985). Their list is incomplete, especially regarding algae and the Bryophyta (liverworts and mosses) and does not include plants growing on river banks and submerged during floods ("rheophytes"). Despite this it can be seen from Table 1 that the diversity of aquatic flora within Papua New Guinea is high. Most families and genera have a wide distribution within freshwaters worldwide and are "typical" of those to be expected to occur. Although minor regional variations may occur within Papua New Guinea (field collections are too incomplete to quantify this) for the present purposes it can be stated that Sepik-Ramu aquatic flora are typical of the rest of New Guinea and fairly represented by Table 1.

6. DISCUSSION AND CONCLUSIONS

The relevance of the geology of the area to sediments minerals, temperatures, nutrient loads and chemistry of Sepik-Ramu waters is to be discussed elsewhere.

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The geological history of New Guinea is fundamental to an understanding of the present distribution of native fishes within the region and fishery-related problems. This can be simplified as follows:

(1) New Guinea as a region is part of the greater Australasian landmass and, therefore, split from "Gondwanaland" at the earliest stage of continental separation. This whole region is devoid of primary and secondary freshwater fishes (with one exception -Scleropages spp) - all presently existing native fishes in freshwater areas are either diadromous or freshwater representatives of marine families (e.g. Roberts 1978, McDowall 1981, Allen and Coates 1990).

(2) Southern New Guinea (formed by the uplifting of the New Guinea Mobile Belt) was attached to Australia by a land bridge until recent rises in sea level formed the shallow Torres Strait. There is a close similarity between the freshwater fauna of southern New Guinea and northern Australia (Allen and Coates 1990).

(3) Northern New Guinea and the Sepik-Markham depression (= intermontane trough) developed later and have been relatively isolated from the southern sections via the already existing central cordillera. The northern section of New Guinea is a quite separate zoogeographic zone to that of southern New Guinea and nor~hern Australia. Almost all species of fish restricted to Sepik-Markham depression freshwaters are endemic to the region, whereas species with estuarine/marine s~ages in their life cycle are widely distributed throughout the region (Allen and Coates 1990). Diversity in this region is also much lower due to its younger age (Allen and Coates 1990).

(4) The Sepik-Markham depression traverses into Irian Jaya and includes the major river system in northern Irian Jaya, the Mamberamo River. The two great systems are presently separated by a modest mountain range but there is a great similarity in their freshwater ichthyofaunas (Allen and Coates 1990).

(5) The Sepik and Ramu basins are closely connected and actually adjoin at periods of high water levels. Their ichthyofaunas are almost identical apart from limited within region endemism (Allen, Parenti and Coates, unpublished, Van Zwieten 1989).

(6) The Markham and Ramu (and hence Sepik) basins are separated by a very modest altitudinal difference. Along the

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valley the two depressions are visually indistinct and may, in fact, interconnect at places. Reasoning would suggest that the Markham system is occupied essentially by Sepik­Ramu ichthyofauna although the Markham River has yet to be sampled intensively.

(7) The recent development of Sepik-Ramu floodplains is witnessed by the lack of native freshwater species adapted to floodplain conditions. This is discussed at length by Coates (1989) and is a major reason why the fisheries yield of Sepik floodplains is considered to be low (Coates 1985).

(8) Differences in coastal topography account for the lack of a deltaic system at the mouth of the Sepik and Ramu whose estuarine zones (inland) are considerably reduced (if not non-existent) compared with rivers in southern New Guinea. This factor may explain the absence from the Sepik/Ramu of certain families of fish reliant upon estuarine/mangrove/brackishwater environments for their life-cycle (Allen and Coates 1990). Several of these are important food fishes in southern rivers and further reduce fisheries potential in the Sepik/Ramu (Coates 1989).

(9) Reduction in species diversity due to the recent formation of northern New Guinea also applies to fishes inhabiting higher altitude rivers and streams draining the central cordillera and northern coastal ranges (Allen and Coates 1990, Van Zwieten 1989). Sepik/Ramu high altitude rivers are even more impoverished than counterpart environments in southern New Guinea.

(10) Recent geological events would suggest that the ichthyofauna of the Toricelli-Bewani and Finisterre Ranges (the west and east, northern coastal ranges) might have ichthyof aunas somewhat different to each other and reduced in diversity in comparison with that of similar environments draining the Sepik/Ramu side of the New Guinea Mobile Belt {the highlands). The northern ranges are younger and have been geologically separated. Certainly, a high degree of inter-regional endemism would be expected. Whether this is the case is still being investigated and the situation might be complicated by the possible migration of species across the previous "Sepik-Ramu Sea" as it became freshwater. For example, the plotosid catfish Tandanus coatesi is endemic to the Toricelli-Bewani ranges only (Allen and Coates 1990).

(11) Sepik-Ramu freshwaters (in common with the rest of New Guinea) are dominated by river systems which arise as variously torrential streams in the higher lands, becoming torrential rivers quickly entering extensive floodplains in the lowlands. River fisheries are presently and potentially

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much more important than lake fisheries. Although there are about 5,000 lakes within Papua New Guinea (Chambers 1987) they are limited in extent and modest in area due to the steep gradient of much of the terrain. Especially within the Sepik-Ramu catchment, major lakes are restricted to those which are extensions of river floodplain at low elevations, although a number of isolated crater lakes and those occurring in small enclosed valleys occur at higher altitudes.

Obviously altitudinal variations affect climate, vegetation types, water temperatures etc. and present fish distributions (Allen and Coates 1990; Van Zwieten 1989) and, therefore, fish species suitable for stocking such regions. Such factors will be discussed elsewhere. Of particular note is the fact that 90% of the Sepik-Ramu catchment is not floodplain which is generally restricted to below 120 m. zones in the region 120 to 300 m (30%) and 301 to 1500 m (37%) are of particular importance, with regions in the zone 1501 to 3000 m also of note (19%). Altitudinal zonation of the catchment is also important when related to the population distribution of people. Population distributions in the Sepik-Ramu basin were studied by Coates and Mys (1989) who noted the higher population densities in the moderate to higher altitude regions making such areas important regions for fisheries improvements. Potential stocking of colder waters at higher altitudes is, therefore, of immense importance. Although rivers in such regions are not expected to be highly productive, the extent of their area related to population densities (Coates and Mys 1989} highlights their particular importance for subsistence fisheries. In addition, as altitude increases, fish diversity and biomass reduces considerably, and abruptly (Van Zwieten 1989). There is an abrupt drop in ichthyomass above about 400 m.a.s.l. and many areas above 1000 m have negligible or nil fish stocks. This poses particular difficulties for stocking because most knowledge of river fisheries, their faunas and stocking practices, worldwide, applies to floodplain regions where more productive and commercial fisheries are to be expected.

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Runoff throughout most of the Sepik-Ramu catchment is high and consistent, with little seasonality. Regular rainfall with consistently high soil moisture levels result in most ~ivers and streams being fairly stable in nature. Rivers or streams that dry out (at any time) are exceedingly rare within the region and have only been noted in areas with steep and negligible catchments. In general, rivers and streams are stable habitats for fishes with only occasional marked variations in flow due to increased discharges during moderate flash floods (predominantly along the northern

coastal ranges, to a lesser extent in the highlands). In short, Sepik-Ramu rivers, creeks and streams are suitable habitats for appropriate species. Destruction of stocks through drought is not a threat in any significant area.

Air temperatures vary little seasonally, as does rainfall. Marked changes in air temperature do, however, occur with altitude resulting in a range of climatic regions from tropical lowland habitats to perhumid montane habitats within the catchment. The effect of altitude on water temperatures has been studied under project PNG/85/001 and data will be presented elsewhere.

Although the seasonal variations in rainfall are modest in most areas they are, however, sufficient to produce marked seasonal impacts on flooding in the alluvial plains. River level recordings in the region of the Sepik floodplains are shown in Fig. 18. The flood cycle of the Sepik River is typical of floodplain systems. The floodplain dry season extends from approximately May to October, inclusive, and the flood season from November to April, inclusive (i.e. six months each season). Mean maximum to minimum height changes of the Sepik River within six 12 month cycles has been recorded as an average of 2.25 m with a maximum height change of 3.11 m and a minimum of 1.7 m (Coates et al. 1983). These are modest compared with river depth at this location (estimated at around 20 m average) and in comparison to seasonal changes in river systems in other continents at higher latitudes. Despite thi.s modest seasonal height change the impact on floodplain inundation is marked. Due to the extensive area of flat land within the Sepik meander belt (below 100 m elevation - much of it at 0 m elevation), flooding is still extensive. The Sepik floodplains are estimated to occupy approximately 10% of the total catchment and are seasonally inundated despite limited seasonality of inflow from higher altitudes. The Sepik has more extensive floodplains than is the norm for such rivers (Welcornme 1976 regards rivers with floodplains 1.8 to 2.4 % of catchment to be the "norm") . The relaU.vely high proportion of floodplain within the Sepik is due to the unusually high discharge of the river (high rainfall, high humidity, high run-off) and their somewhat unique development by the sedimentation of a previously large inland sea - resulting in a large area of low-lying "flat" land. Such factors are important because floodplain areas are a primary factor determining fisheries yields (Welcomme 1976) which are theoretically high for the Sepik but not in reality (Coates 1985).

The flood regime of the Ramu River has not been studied. It has, however, been noted that seasonal river level

14

changes are much greater than for the Sepik as witnessed by greater isolation o= ox-bow lakes in the Ramu meander belt and more prominent and deeper river banks in its middle and lower reaches. This is probably due to the most marked seasonali~y of rainfall within the region being prominent in the upper to mi.ddle Ramu catchment as noted above. During peak floods water flows from the Ramu to the Sepik lowlands and vice versa during periods of reduced water levels (which are comparatively lower at such times in the Ramu) .

The flora of Papua New Guinea has a high diversity by world standards. This reflects the great variety of altitudes and habitats within the country. A similar diversity applies to other types of fauna such as birds, insects and reptiles (Gressitt 1982) . This normally extremely high diversity within the region highlights just how depauperate freshwater fish diversity is by comparison. The great diversity of habitats (available to, for example, plants, insects and birds) is not reflected in the freshwater fish fauna. In simple terms, the geological history of Papua New Guinea (and particularly the Sepik­Markham area) has had zoogeographic consequences mainly on types of biota that are more restricted by isolating factors - pri·ncipally f:::'.'eshwater fishes (but also marllffials) . This factor is most important because it suggests that the "zoogeographic problem" only applies to fish (and marrunals). It could be fairly reasonably assumed that Sepik-Ramu freshwaters have a similar ecology to other tropical regions in all general respects other than the freshwater fishes found there (and possibly molluscs). In particular, processes of allochthonous inputs into rivers and streams and autoch~honous processes can be asswned, broadly, to be similar to other regions and not present a potential barrier to the rationale for increased fish stocks through stocking. This, o~ course, is not withstanding the fact that the region may have certain associated faunistic peculiarities. The point is that no gross differences in freshwater ecology are expected (apart from fish faunas) .

Without an impractical amount of research it is, of course, impossible to verify the above conclusion. Those activities undertaken by the project have so far lead to no information that would suggest that Sepik~Ra.mu waters have an unusual ecology. These include investigations of water chemistry and nutrient loads (sub project PNG/85/001/-07) and :'...nvestigations of potential fish food availability (sub~ project PNG/85/001/-12 - Dudgeon unpublished). When there is such an obvious ichthyologically based explanation for low fish diversities, biomasses and fisheries yields, in a practical situation, it becomes futile to look widely for alternative illumination of the problem.

15

The vegetation cover of the Sepik-Ramu catchment has a further important implication for fisheries. At altitudes at least up to 2500 m, most of the area is essentially forest covered. The major habitat for fishes outside the floodplain is, therefore, forest stream (or river). The degree of shading generally depends on stream width and altitude and water temperature depends on altitude, shading and gradient. Sepik-Ramu streams could be expected to have allochthonous inputs (e.g. leaf litter, seeds, fruits, terrestrial insects etc.) at least on a parr with similar habitats in other regions and possibly higher due to the large extent of undisturbed cover. Detrital and autochthonous processes would also be expected to be equable. Forest streams are the major habitat in terms of area within the catchment. A third major habitat, in addition to floodplain, would be more open rivers and streams in less densely forested areas above 2500 m where the availability of algal and aquatic insect fish food sources would be expected to be relatively high.

7. REFERENCES

Allen, G. R. and Coates, D. (1990). An ichthyological survey of the Sepik River, Papua New Guinea. Records of the western Australian Museum. In press.

Chambers, M. R. (1987). The freshwater lakes of Papua New Guinea: An inventory and limnological review. Journal of Tropical Ecology 3: 1-23.

Coates, D. (1985). Fish yield estimates for the Sepik River, Papua New Guinea, a large floodplain system east of "Wallace's Line''. Journal of Fish Biology 27: 431-443.

Coates, D. (1989). Fish fauna of the Sepik and Ramu River floodplain regions: Summary of information on fish ecology, identification of vacant niches and categories of fish species suitable for stocking. FAO Project PNG/85/001 Field Document No. 3. 59 p.

Coates, D., Osborne, P. L. and Redding T. A. (1983). The limnology of the lower Sepik River, roundwaters and floodplain. Department of Primary Industry, Fisheries Research and Surveys Branch, Report No. 1983-17. 31 p.

Gressitt, J. L. (ed.) (1982). "Biogeography and Ecology of New Guinea". Dr. w. Junk, The Hague.

Henty, E. E. (ed.) (1981). "Handbooks of the Flora of Papua New Guinea". Volume 2. Melbourne University Press. 276 p.

16

Leach, G. J. and P. L. Osborne (1985). "Freshwater Plants of Papua New Guinea". The University of Papua New Guinea Press. 254 p.

Loffler, E. (1977). "Geomorphology of Papua New Guinea". CSIRO and Australian National University Press, Canberra. :95 p.

McAlpine, J. R., Keig, G and R. Falls (1983). "Climate of Papua New Guinea". Commonwealth Scientific and Industrial Research Organisation and Australian National University Press, Canberra, Australia. 200 pp.

McDowall, R. M. (1981). The relationships of Australian freshwater fishes. In: "Ecological Biogeography of Australia" (ed. A. Keast), pp 1253-1273. Dr. W. Junk, The Hague.

Pai~mans, K. (ed.) (1976). "New Guinea Vegetation". CSIRO and Australian National University Press, Canberra.

Paijmans, K. (1976a). Vegetation.. In: "New Guinea Vegetation". Ed. K. Paijmans. CSIRO and Australian National University Press, Canberra. pp 23-104.

Paijmans, K. (1982). Vegetation. In: "An Atlas of Papua and. New Guinea". 2nd edit:ion. Robert Brown and Associates, Aust:::alia with the University of Papua New Guinea.

Roberts, T. R. (1978). An ichthyological study of the Fly· River ~n Papua New Guinea with descriptions of new species. Smithsonian Contributions to Zoology No. 281, 72 p.

Swadling, P., Hauser-Schaublin, B., Gorecki, P. and Tiesler, F. ( 1988). "The Sepik-Ramu: An Introduction". National Museum, Boroko, Papua New Guinea. 75 p.

Welcomme, R. L. (1976). Some general and theoretical considerations on the fish yield of African Rivers. Journal of Fish Biologv 8: 351-364.

Womersley, J. s. (ed.) (1978). "Handbooks of the flora of Papua New Guinea". Volume 1. Melbourne University Press. 278 p.

:t.7

,.

~-· . "' Bismarck

..

Solomon

Sea

• "<Q

Corsi

Fig. 1. The main crustal elements of Papua New Guinea. major faults, ---------- present fault boundary

(from Loffler 1977).

18

12 c;:::_y •• . "'

Fig. 2. Structural regions of Papua New Guinea (from Loffler 1977).

1. The Fly Platform 2. The Papuan Fold Belt 3. The Aure Trough 4. The Kubor Anticline 5. The New Guinea Mobile Belt 6. The East Papua Ultrabasic Belt 7. The South-east Papua Volcanic Province 8. The Cape Vogel Basin 9. The Sepik Markham Depression

10. The Finisterre-New Britain Volcanic Arc a. The Palaeogene Volcanic Arc b. The Quaternary Volcanic Arc

11. The Toricelli-Bewani Ranges 12. The Bougainville-New Ireland Volcanic Arc

a. The Palaeogenic Volcanic Arc b. The Quaternary Volcanic Arc

19

CAINOZOIC

Quaternary

~ Alluvium, raised coral

Subaerial lavas and pyroclastics

Pliocene

~Marine and terrestrial fine­~ grained sediments

~Limestone

Miocene

~Limestone

~ Greywacke, siltstone and ~ conglomerate, volcanics and

some limestone

Intrusives, grano<lioritc, tonalite, diorite and gabbro, some andesite porphyry

MI;SDZOIC

Cretaceous and Jurassic (and some Tertiary)

~ Siltstone, shale, quartz, ~ sandstone, conglomerate

(unmetamorphosed)

~ Greywacke, siltstone, ~ conglomerate interbedded

with marine volcanics (metamorphosed)

o.~--.... : ,. .lb

~ Marine volcanics (slightly ~ metamorphosed in places) ~ Basic and ultrabasic igneous ~ rocks

Triassic

~ Dacite volcanics ~

PALAEOZOIC

Permian

~ Low grade metamorphosed ~ elastic sediments intruded by

granite and granodiorite

.·

Fig. 3. The generalised geology of Papua New Guinea (from Loffler 1977) .

20

21

The Sepik-Ramu coastline 6,000 years ago:

o~·---~50 km

The Sepik-Ramu coastline 2,000 years ago:

0 50 km

The Sepik-Ramu coastline today:

Fig. 4. The recent geological development of the Sepik-Ramu coastline and formation of the Sepik-Ramu floodplains by alluvial deposition in the intermontane trough (upper figure referred to as the "Sepik-Ramu sea"). (From Swadling et al. 19 89) .

D D . . . Em

0-10 Ill lmtt:::m lo l - 300 Ill

11-30 Ill > 300 Ill

31-100 111

' .

. .

.

. ' I ' ' ~~,;·

D II

0-300 111

301-1500 111

• 1501--3000 Ill

• >3000 m

'"D

.. ~Do•

, "'

.•

• "'\:::?

Fig. 5. Upper: Local relief classes, Lower: altitudinal zones, of Papua New Guinea (from Loffler 1977).

22

~o

30oo

~

' ~O'\,"i

\ ~ 300o ~ \

!'~~" ~\

6000 \.

"""' \ 4000 \ <a

2000 I ~ e>

Fig. 6. Mean annual rainfall over Papua New Guinea (from McAlpine et al. 1983).

23

m;;;o400mm

~ l00-399mm

LJ<1oomm

D >400rnm

§ 100·399mm

{ [ < 100mm

JANUARY

JULY

Fig. 7. Seasonal rainfall patterns in monthly rainfall for January and July McAlpine et al. 1983).

24

200f)~ 300

•oo '' 500

Papua New Guinea. Mean are shown (from

High

0·1

Moderate 0·5 -0·1

Low <0·5

. :c:. ·~:

25

Fig. 8. Distribution of the index of rainfall seasonality in Papua New Guinea. The seasonality index is the lowest mean monthly rainfall subtracted from the highest and divided by the annual (from McAlpine et al. 1983).

( ?

~ >20

[ill] 15-20

c=i <15

• C>

.. ·+·

• 0

Fig. 9. Coefficient of variation of annual rainfall in Papua New Guinea (from McAlpine et al. 1983). The variability of rainfall is lowest in the highlands and only slightly higher in most lowland and coastal regions of the Sepik-Ramu. No areas within the Sepik-Ramu catchment have moderately high variability.

26

mil >4

filllll 2 -4

~ 1-2

r::::::::l < 1 ~

CJ 0

-

Fig. 10. Average number of days per year with rainfall exceeding 100 mm in Papua New Guinea (from McAlpine et al. 1983).

27

1 • o~

January July oc oc

E3 ~ 0-

20-24 19 - 23 UJ] 14-20 [f]] 13 - 19 <:o D 6-14 D 5 -13 • < 6 • < 5

Fig. 11. Mean minimum air temperature for January and July in Papua New Guinea (from McAlpine et al. 1983).

28

G 0 28-30

ITID 24 -2a

D 15-24

• < 15

JANUARY

JULY

29

. ~

• =- .. ~

Fig. 12 Mean maximum air temperature for January and July in Papua New Guinea (from McAlpine et al. 1983).

35

..

..

10

•Lumi

•Annual maximum

• Armual minimum

.. ..

-s~----.so*"o----.1'°"000,.----~1~soo,,---2""'000~-,,,>,,o~o -.,.,!3000~~,,'=oo--='•oo-o

Altitude (m)

Fig. 13. The relationship between altitude and mean maximum and mean minimum air temperature in Papua New Guinea (from McAlpine et al. 1983).

30

OOlfilill Severe and regular

§ Moderate and irregular

LJ Low and infrequent

D Rare

c:::7 -· • a

Q

.()

Fig. 14. Frequency and intensity of soil-moisture depletion in Papua New Guinea (from McAlpine et al. 1983).

31

f:'.:'.:'.:j >4000 mm

D 1000-4000

- <1000

.. 3000~· ·;,

2000----~~

Port

Fig. 15. Mean annual water surplus Guinea (from McAlpine et al. 1983)

(runoff) (mm) •

... 0 -

0 0

for Papua New

32

LOWLAND CLIMATES (0-500m)

8 Lowland dry subhumid

0 Lowland subhumid

QJ Lowland h.umid

~ Lowland perhumid

PREMONTANE CLIMATES (500-1400m)

r '.S.: 'j Premontane subhumid

j: :~ :·1 Premontane humid

I."?:] Premontane perhumid

3

~

3 ~

0

LOWER MONTANE CLIMATES (1400-3000m)

illfill Lower montane subhumid

[ffiill Lower montane humid

Ell Lower montane µerhumid

UPPER MONTANE CLIMATES (>3000)

, , I Upper montane humid

• 0

Fig. 16. Climatic classification of Papua New Guinea based on altitude and mean annual rainfall (from McAlpine et al. 1983).

33

144" 147" ~ 150"

~--JJ

BISMARCK SEA

f22J lowland forest

lIIIIII1 Lower montano forest

.m Upper montane forest

~Grassland

~Savanna

~ Herbaceous freshwater swamp

~ Wooded freshwa1er swamp

• Mangroves

mJ Gardens and grassland

Fig. 17. Paijmans

Major vegetation types 1982).

SOLOMON SEA

r

· .. ··"-~

100

in Papua New Guinea

3•

..

12•

200km

(from

34

..§

.µ ,c Ol

·.-< QJ ::i:

S-< v > .....

O::·

<IJ >

·.-< .µ co

.-I Cl)

a::

7.0

6.0

s.o

4.

1977 1978 1979 1980 1981 1982 1983

Fig. 18. Recordings of relative river height (height relative to a fixed point on the river bank) at Angoram on the Sepik River (from Coates et al. 1983). Vertical lines are maximum and minimum recorded relative heights around weekly means.

35

Table 1. Checklist of freshwater plants of Papua New Guinea (from Leach and Osborne 1985).

CHARACEAE Chara corallina Chara fibrosa Chara globularis Chara setosa Lychnothamnus barbatus Nitella cristata Nitella Jurcata Nitella pseudojlabellata

AZOLLACEAE Azolla pz"nnata

BLECHNACEAE Stenochlaena milnei Stenochlaena jJalustris

EQUISETACEAE Equisetum deMle .

ISOETACEAE Isoetes sp. nov. Isoetes habbemensis Isoetes neoguineensis Isoetes stevensii

MARSILEACEAE Marsz"lea crenata

OLEANDRACEAE Nephrolepis biserrata Nephrolepis radicans

PARKERIACEAE Ceratopteris thalictroides

POLYPODIACEAE .Microsorium brassii Microson'um jiteropus JHicrosorium schneideri

PTERIDACEAE Acrostichum aureum Acrostz"chum speciosum

SAL VINIACEAE Salvi"n£a molesta

TH:ELYPTERIDACEAE A mpelopteri.s prolifera

Cyclosorus interruptus T hclypteris confluens

ALISMAT ACEAE Caldusia oligococca

var. oligococca Caldesuz pamassif olia Caldcsia sp. aff. grandis Sagittarza platyphylla Sagittaria subulata

AMARANTHACEAE A ltemanthera sessilis

APONOGETONACEAE /1 jJonogcton loriae A ponogcton womersleyi

ARACEAE A corus calamus Color;asi'a esculenta Cryplocoryne dliata

var. ciliata C1yjJtocoryne dewitiz' Cryptocoryne versteegii Lasia spinosa Pistz'a stmtiotes

CABOMBACEAE Cabomba caroliniana

CALLITRICHACEAE Callitrt'che palustris

CAMPANULACEAE Lobelz'a alisnoides

CERATOPHYLLACEAE Ceratophyllum demersian Ceratophyllurn submersum

CONVOLVULACEAE I pomoea aquatica

CRUCIFERAE Nasturtium officinale·

CYPERACEAE Carex pseudocyperus

v<1 r. Jascicularis

36

Table 1 continued.

Carex sp. Cyperus cephalotes Cyperus imbricatus Cyperus platystyHs Eleocharis acutangula Eleocharis dulcis Eleocharis philippinensis Eleocharis retroflexa Eleocharis sphacelata Scirpus articulatus Scirpus crassiusculus Scirpus grossus Scirpus inundatus Scirpus litoralis Scirpu.s_ .mucronatus

ssp. mucronatus Scirpus mucronatus

ssp. clemensiz" ELATINACEAE

Elatine triandra ERIOCA ULACEAE

Eriocaulon setaceum GRAMINEAE

Brachiarz'a muti'ca Echinochloa praestans Hymenachne acutzglmna Ischaemum polystachyum Ischaemum timorense Leersi'a hexandra Oryza longiglumis Oryza mznuta Oryza ridleyi Oryza rufzpogon Oryza sativa Panicum auritum Panicum paludosum Phragmites karka Sacdolepis myusuTOides

HALORAGACEAE Myri·ophyllum coronatum Myriophyllum dicoccum Myrz'ophyllum pedunculatum Myriophylh1m pygrnaeum

HANGUANACEAE Hanguana malayana

HYDROCHARITACEAE Blyxa aubertii

var. aubertii' Blyxa aubertii

var. echinosperma Blyxa japonica

var. japonica Blyxa novoguineensis Blyxa octandra Hydrilla vertz'cillata Hydrocharis dubz'a Ottelia alismoides Vallisneria natans

JGNCAGINACEAE Triglochin procera

LABIATAE Pogostemon stellatus

var. rox burgz'anus Pogostemon stellatus

var. stellatus LEGUMINOSAE

A eschynomene indica Sesbania javanica

LEMNACEAE Lemna perpusilla Lemna trimlca Spfrodela polyrhiza Woljjia globosa

LENTIBULARIACEAE Utricularia aurea Utricularla australis Utrz"cularz'a bifida Utrz'culana exoleta Utricular£a minor Utr£cularia muelleri

LYTHRACEAE A mmannia baccifera Rotala mexicana

MENYANTHACEAE Nymphoz'des aurantiaca Nymphoides exiliflora Nymphoides geminata Nymphoides i'ndica Nymphoides parvzjolia

37

Table 1 continued.

NAJADACEAE Najas browniana Najas graminea

var. graminea Najas indica Najas malesiana Najas tenuif olia

ssp. pseudograminea var. pseudogramt'nea

NYMPHAEACEAE Hydrostemma motleyi Nelumbo nudfera Nymphaea dictyophlebia Nymphaea gigantea

· Nymphaea macrosperma Nymphaea nouchali Nymphaea pubescens Nymphaea violacea

ONAGRACEAE Ludwigia adscendens Ludwigia hyssopifolia Ludwi'gia octovalvis

PHIL YDRACEAE Philydrum lanuginosum

PODOSTEMACEAE Torrentz'cola queensland£ca

POLYGONACEAE Polygonum attenuatum Polygonum barbatum Polygonuni lapathif olium Polygonum minus Polygonum on·entale Polygonum strigosum

PONTEDERIACEAE Ei.chhornt'a crassipes Monochoria hastata Monochoria vaginalis

PORTULACACEAE Mantia Jontana

POTAMOGETONACEAE Potamogeton javanfrus Potamogeton malaianus Potamogeton puszUus Potamogeton sp. A Potamogeton sp. B

SCROPHULARIACEAE LimnapMla aromatica Limnaphila z'ndica

SPARGANIACEAE SfJarganimn sim.plex

TYPHACEAE 'f'.ypha orientalis

UMBELLIFERAE Hydrocotyle sibthorpiodes

38