Biology of fork-tailed catfishes from the Sepik River, Papua New Guinea

Environmental Biology of Fishes 31: 55-74, 1991. 0 1991 Kluwer Academic Publishers. Printed in the Netherlands.

Biology of fork-tailed catfishes from the Sepik River, Papua New Guinea

David Coates Fisheries Department, Food and Agriculture Organisation, clo United Nations Development Programme, P. 0. Box 1041, Port Moresby, Papua New Guinea, and the Department of Fisheries and Marine Resources, Papua New Guinea

Received 13.9.1988 Accepted 153.1990

Key words; Floodplain fisheries, Subsistence fisheries, Feeding, Condition, Reproduction, Ecology, Niches, Arius, Hemipimelodus, Pisces, Ariidae

Synopsis

Ariids accounted for approximately 25% of the weight of catches and landings from rivers and lakes in the floodplain region (= 50% weight of native species). Habitat preferences were: A. solidus, lakes and rivers; A. utarus, rivers and occasionally lakes; A. coatesi and A. velutinus, rivers only; A. nox, lakes and floodplain. A. solidus and A. utarus occasionally entered the floodplain but only A. nox exhibited any, albeit modest, affinity for this environment. In general, the fishes showed no marked seasonality relating to the flood cycle: in four species slight increases in feeding, condition, breeding and fat deposition occurred during the dry season whilst only A. nox showed modest increases in these parameters during the flood season due to its increased feeding on the floodplain at that time. All species are omnivorous but diets differed according to morphology and habitat preferences. Diet overlap was greatest amongst A. coatesi, A. solidus and A. utarus which fed mainly on prawns and a variety of other plant and animal matter. A. nox is a specialised filter-feeder consuming mainly small crustaceans and small insect larvae. A. ve1utinu.s fed mainly on large insect larvae and emergent and terrestrial insects and its diet excluded prawns. A. solidus and, especially, A. utarus also fed on fish scales. Feeding commenced immediately after hatching while free embryos were still in the male parent’s mouth. Large amounts of fat were stored prior to brooding during which time males fast. The reproductive style of ariid catfishes limits their colonisation of the floodplain and is a constraint to increased fisheries exploitation of the stocks. The importance of ariids to the local fishery and nutrition (fat) for the local people should be recognised.

Introduction

The native freshwater ichthyofauna of New Guinea (Fig. 1) is essentially devoid of primary freshwater groups and the catfishes are restricted to the fam- iles Ariidae and Plotosidae, both secondary in- vaders from the sea (Allen & Coates 1990). A notable feature of the fork-tailed or estuarine catfishes (ariids) from New Guinea is the degree to which they enter freshwater and their speciation

there. About twenty ariid taxa are restricted to freshwaters in New Guinea (Kailola 1990). The Sepik River in northern Papua New Guinea (P.N.G.) supports a large and important subsistence fishery of which the ariid catfishes form a significant part (Coates 1985, 1987).

The vast majority of the literature on ariids refers to marine/estuarine species that only briefly enter freshwater, if at all. Much of the literature on Australian ariids is of limited use due to consid-

56

Study r I

3 Put&i River vLdL2&4

0 Fly River

-0 0.

t 200 km N.

’ ’

n - . - I I

AUSTRALIA

Fig. 1. Sketch map of Papua New Guinea, the Sepik River and the study site (inset). The stippled line represents the central range of mountains that separates northern and southern P.N.G. into different zoogeographic zones.

erable taxonomic confusion surrounding the family (Kailola 1983) and all Australian species are different to those from northern New Guinea (Kailola 1990, Allen & Coates 1990). Some work has been done on the biology of freshwater ariids from northern Australia but the majority remains un- published. Rimmer (1985a, 1985b) reported the biology of Arius grueffei which is affected by the temperature seasonality of the Clarence River, New South Wales. In general, even the smaller New Guinea rivers are larger than the largest Aus- tralian systems and are less seasonal in nature. Some information on the freshwater ariids from southern P.N.G. has been provided by Roberts (1978), Haines (1979), and Maunsell & Partners (1982), but these are all different species from those occurring in the Sepik (Allen & Coates 1990).

Ariids are important food fishes in all of the larger rivers in New Guinea, and this is probably the only region where they are so important in freshwater fisheries. This paper and that of Coates (1988) are the first on the biology of any of the ariids from northern New Guinea and represent the first detailed study published on any of the New Guinea species.

Tuxonomic note

In this paper I refer to all five known species of ariids from the Sepik River (Kailola 1990): Arius solidus Herre, A. uturus Kailola (referred to as Arius c.f. leptuspis by Coates 1987,1988), A. coatesi Kailola (referred to as Arius species 3 by Coates 1987,1988), Arius nox (Herre), andArius velutinus Weber. The Arius species 1 and 2 and A. kungunu- munensis Herre, referred to by Coates (1987) are thought by Kailola to be synonymous with A. solidus. Arius nox and A. velutinus were previously assigned to the genera Brustiurius (regarded by Kailola as a subgenus) and Hemipimelodus, respectively. The history of generic classification of the latter two species is relevant to comparisons with arilds from southern New Guinea undertaken later.

The study area

One area of the river system close to Angoram (Fig. 1) was studied intensively. Three ox-bow lakes (‘Imbuando’, ‘Magendo 1’ and ‘Magendo 2’)

57

were studied in addition to an area of floodplain (‘Pitpit’) which was adjacent to Magendo 2. The three lake sites are areas of permanent water which varies in depth but is generally greater than 16m. Pitpit site, being floodplain, was dry during the dry season and inundated to a maximum depth of 2 m during the flood. Comparisons of fish catches between Magendo 2 and Pitpit are particularly in- teresting because differences may reflect fish movements onto and off the floodplain from areas of permanent water. The study area was in a totally freshwater section of the river, the lowest site, Imbuando, being about 70km from the river mouth. Estuarine zones inland are non-existent in the Sepik due to the high discharge and absence of a delta.

In this paper I refer to ‘floodplain regions’ which are the lowland areas of the river system including areas of permanent water such as lakes and higher- order river channels. The term ‘floodplain’ refers to areas of land within this region that are seasonally inundated with water as the river periodically floods, and drying-out again as river level falls.

Methods

Fleets of both monofilament and multifilament gillnets of stretched mesh size usually in 12.7mm intervals ranging from 25.4 mm to 152.4 mm were set vertically from the surface at each of the four regular sample sites described above. Nets were set at

Table 1. Maximum sizes recorded for Sepik River ariids. TL = total, SL = standard length, both in mm.

Species TL SL N

A. solidus

A. utarus

A. coatesi

A. nox

A. velutinus

male 585 485 310 female 590 490 276 male 530 440 223 female 550 450 197 male 750 660 89 female 740 655 84 male 340 280 342 female 350 285 416 male 600 500 74 female 580 485 83

these locations at least once every month. A ‘standard fleet’ of gillnets refers to a set of nets of standard composition set for a standard time, 1500 h to 0800 h the next morning. Additional sampling was carried out further afield using the same gillnets and rotenone but not on a regular basis.

Landings of fish at Angoram market (Fig. 1) were recorded twice a week (August, 1981, to March, 1983). Recordings of species, length, weight, location caught, and incidental observations were made. A close watch was continually kept on the market, which was opposite the field laboratory. Specimens were purchased from the market by an independent observer who was in- structed to obtain them irrespective of condition or size. All A. solidus and A. nox examined were from fish caught in my gillnets. About 50% of the A. uturus were from this source and the remainder were purchased from the market or local fishermen. All A. coatesi and A. velutinw were purchased.

The percentage fullness of stomachs was estimated by eye in 10% intervals. A 100% full stomach was assumed to be one extended to its maximum under normal feeding conditions. The data included here were recorded only after at least 9 months experience dissecting Sepik River ariids. The method is similar to, but more flexible than, the method used by Rimmer (1985b). Stomach contents were placed in petridishes, mixed with water and observed under a stereo microscope with variable magnification up to x 200. The percentage volume that the various categories of food materials (see results) contributed to the total volume of food within the stomach was estimated by eye in steps of 10%. This is similar to the points system which has been commonly used (Hyslop 1980). The mean percentage compositions by food category were corrected for stomach fullness when mean compositions for samples were calculated (e.g. a food item, x, composing 50% of the contents of a stomach 60% full would be calculated as 30%x). How- ever, no corrections were made for fish length but the majority of fish sampled were adults. Ariid eggs in the stomachs of brooding males were excluded from the analysis as was mucus in stomachs of all individuals.

Diets were compared by calculating ‘Schoener index’ values from the food categories listed in the results. This system (Schoener 1970) calculates the index = 1 - 0.5 x (the sum of [pxi - pyi]); where pxi = the proportion of food category i in the diet of species x and pyi = the proportion of food category i in species y. The method is subjective in that it is dependent upon the level of classification of food items. The broader the classification system the higher the Schoener index (e.g. ‘plants’ as one category versus ‘plant species’ as an alternative). In the present work, with a number of categories being somewhat broad, low Schoener indices (indicating reduced dietary overlap) can, therefore, be considered as valid indicators of differences in diet.

Condition factors, K, were calculated using the formula of Daget (1957), i.e. K = 105 w/P, where w = gutted weight in grams and 1= standard length in mm. Visceral fat deposits were estimated using the six point system proposed by Prozorov- skaia (1952) and suggested by Nikolsky (1963). In- dices of 0 (no visible fat) to 5 (maximum fat) inclusive, represented the proportion of the intestine obscured by fat. Maturity stages of ovaries were recorded on the six stage scale recommended by Nikolsky (1963) and as used by Rimmer (1985a): i.e., ‘immature’, ‘resting’, ‘ripening’, ‘ripe’, ‘running-ripe’ and ‘spent’. Gonadosomatic index, GSI, was recorded using the formula of Durand & Lou- bens (1970), i.e. GSI = GW lOOfIW, where

GW = gonad weight and TW = total weight (both in grams). All references to fish length are standard length unless stated otherwise.

River levels were recorded throughout the study period by reference to a fixed point on the river bank at Angoram (Fig. 1). ‘Relative river height’ is included in certain of the figures and should not be taken as river depth which is much greater; it is the change in river height that is important here since this affects the degree of inundation of adjacent floodplain. The ‘flood season’ refers to the period November to April inclusive, and the ‘dry season’ May to October inclusive.

Results

Relative importance to the fishery

Ariids accounted for about 25% of both the weight of the total catch from my fleets of gillnets and the market landings. They are also the most commonly caught fish on hook and line and in basket traps set by local fishermen. Introduced tilapia, Oreochro- mis mossambicus (Peters) accounted for about 65% and 50% of the annual landings or catch, respectively. Ariids, therefore, are estimated to account for approximately 50% of the catch of native species in floodplain regions, hence their local importance. Amongst the native species in

Table 2. Morphometric data for Sepik River ariids. TL = total length, SL = standard length, both in mm; TW = total weight in g. SLlTL relationships between the sexes are non-significant (p > 0.05) in all species. TW/SL relationships are given for each sex when they are significantly different (p < 0.05) between sexes or combined when non-significant (p > 0.05).

Species

A. solidus

A. utarus

A. coatesi

A. nox

A. velutinus

males females

males females

both sexes

both sexes

both sexes

13 N

SL= 0.85TL- 9.7 0.99 400 TW = SL3.” x 9.6 x 1O-6 0.91 200 TW = SL3,39x 2.02x 1O-6 0.93 200 SL= 0.84TL- 2.07 0.98 400 ‘l-W = SL3.22 x 4.2 x 1O-6 0.90 174 Tw= SL3.5’X 6.6X 10-7 0.94 163 SL = 0.95TL - 50.82 0.99 173 TW= SL’.*‘x 3.72 x 1O-6 0.97 139 SL = 0.8OTL + 2.12 0.99 400 TW= SL3.3R x 1.67 x 1O-6 0.95 400 SL = 0.82TL + 5.72 0.97 157 TW= SL’.35 x 2.31 x 1O-6 0.96 119

A. solidus A. utsrus A.nox a1 201 201

NDJFMAMJJASONDJF NDJFMAMJJASONDJF

20

Pitpit 10 AL NDJ FMAYJ J ASON DJF

rl

NDJFMAMJJ ASONDJF

:“jL* N DJ FMAMJ JASONDJF

NDJ FMAMJ JASONDJF

NDJFMAMJJASONDJF

Fig. 2. Mean number of individuals caught per standard fleet of gillnets from the Sepik River at each of the four sample sites for 16 months, 1981-1983. Relative river height indicates the fluctuation in river level throughout the same period and is the same for all data.

the fishery, they are rivalled in importance only by two eleotridid gudgeons (Allen & Coates 1990).

Maximum sizes and morphometric data

Arius coatesi is by far the largest species, followed by Ark velutinus, A. solidus, and A. utarus, whereas Aria nox is the smallest and, although

common, it is less important to the fishery (Table 1). There appears to be little difference between the maximum sizes the two sexes attain in any of the species.

Morphometric data are provided in Table 2. Fe- males are slightly, but significantly, heavier than males of the same length in A. solidus and Ark utarus.

60

2. A. solidus

1

A. uterus 201

2. A.coatesi

1

2. A.velutinus

1

I

0 1**.1.11,,,1,,* ,,r, ASONOJFMAMJJASONDJFM

1981 1982 1983

relative river heig Mm laorn

Fig. 3. Recordings of landings of ariid catfishes at Angoram market. Relative river height indicates the fluctuation in river level throughout the same period and is the same for all data.

Distribution, habitat preferences, and variations in catches

All the ariids referred to here are freshwater species. None have been recorded from Sepik estuarine or coastal regions. Comprehensive sampling throughout the river basin has shown that Sepik ariids inhabit larger rivers and lakes in the floodplain region and do not ascend lower-order tributary streams to any extent; they are replaced there by plotosid catfishes. Juvenile A. velutinw have occasionally been found in quiet pools in tributary rivers and streams and this is the only ariid to occur

in non-floodplain regions at higher altitudes (Allen & Coates 1990).

Variations in catches and landings for some species occurred (Fig. 2, 3), but the most striking feature is the lack of marked seasonality, with the possible exception of A. nox, that can be attributed to the flood cycle. Floodplain dwelling species enter floodplain and retreat again in response to changing flood conditions, resulting in marked seasonality in catches (see discussion). The lack of affinity of ariids for the floodplain is also supported by a comparison of catches between the lake site Magendo 2 and the adjacent floodplain site Pitpit.

61

Arius solidus, A. utarus and A. nox were all caught on the floodplain (Pitpit) but only at similar catch- rates to those occurring in lakes.

There are differences in habitat preference and other behaviour between the species. Catches of A. solidus were variable at all locations (Fig. 2) and market landings were fairly regular throughout the year (Fig. 3). Fishermen reported (in the landings census data) that A. sofidus is ubiquitous in the region but exhibits some preference for lakes. Catches of A. utarus were more consistent (Fig. 2), but it was less commonly caught in lakes and on the floodplain than either A. solidus or A. LOX. It is the most common species in landings (Fig. 3) and fishermen confirmed that A. utarus is more commonly caught from river channels.

Arias lzox is the most abundant ariid in the study area; its small size explains its absence from the market (Table 3). Catches from gillnets showed much variation temporally and between locations (Fig. 2). Arius nox was commonly caught on the floodplain (Pitpit) and the decline in catches there after May could correspond with increased catches in Magendo 2 in the succeeding months. Arias nox was caught in river channels but in low numbers. This species probably prefers lakes and enters the floodplain to a greater extent than any of the other ariids; as indicated by its modest seasonal change in abundance at the various sites.

Arks coatesi and A. velutinus were never taken from lakes or on the floodplain, with the exception of one large brooding male of A. coatesi caught close to the Sepik River at Magendo 2. Market landing records confirm that these two species are restricted to river channels and both species were

landed in the market all year round (Fig. 3). Peaks in landings of A. velutinus in April and October do not appear to relate to changes in river level nor are they consistent between the two years. Arius vefuti- nus may have a migratory habit and villagers report that juveniles enter lower-order rivers seasonally, but the timing of this migration appears to vary from region to region.

Feeding

The majority of fish sampled were adults (i.e. above minimum breeding size - see later) with the exception of references to feeding by juveniles in broods.

All male Sepik River ariids that were brooding had empty stomachs, unless they had swallowed eggs; they fast during this time. Eggs were always undigested within stomachs suggesting that they were swallowed accidentally on capture. Some authors, e.g. Lee (1937), have noted ariid eggs in the stomachs of females, occasionally in a partially di- gested state, suggesting that they had fed on them. This was not recorded in any Sepik ariid.

All Sepik River ariids are omnivorous, although the species differ in their diet preferences (Fig. 4). Dietary overlap is generally considered to be bi- ologically significant when Schoener index values exceed 0.60 (Zaret & Rand 1971, Mather 1977). Even with the broad categorisation of food items adopted here, Schoener indices are less than 0.60 in all comparisons between species (Table 4), indicating much ecological separation in feeding. Dietary overlap is closest between A. coatesi, A. solidus

Table 3. Approximate percentage composition of ariids by species in total ariid landings at Angoram market and in catches from standard fleets of gillnets (all regular sample sites combined).

Gillnet catches Market landings

% number % weight % number % weight

A. solidus 48 71 29 19 A. utarus 7 10 43 24 A. coatesi 0 0 18 38 A. nox 45 19 0 0 A. velutinus 0 0 10 19

62

100.

I A.utaw I1261

ol-------l--lL-----lx bcdefghijklmnopqr

A.co.shi (571

ii i 5o

o.~~--m~l-x-~l~~-~ x x

Food utagorier

Fig. 4. Contents of stomachs of five ariid species from the Sepik River. Percentages are based on the mean percentage contribu- tion of each food item to the total volume of food within each stomach, all samples combined. Numbers in parentheses refer to the number of individuals. X denotes no items of that category present. Food categories are: a - filamentous algae, b - aquatic macrophytes, c - Salvinia root hairs, d - other plant material, e - fine detritus, f - coarse detritus, g - small Crusta- tea, h - caridinid prawns, i - Macrubruchium prawns, j - gastropod molluscs, k - large insect larvae, 1 - small insect larvae, m - insects from terrestrial sources, n - Hirudinae, o- oligochaetes, p - unidentified eggs, q - fish scales, and r - whole fish.

and A. utarus. The diets of A. velutinus and especially that of A. lzox are distinctly different from each other and the other four species in each case. Opportunism in feeding is reflected in the raw data by the high frequency of occurrence of specimens of all five species having fed almost exclusively on one or another type of food at various times.

Arius solidus feeds on larger food items (Fig. 4) with coarse detritus, Macrobrachium spp. prawns,

and larger insect larvae featuring conspicuously in the diet. It is the only species feeding to any extent on algae, aquatic macrophytes, Sulviniu root hairs and other plant material. The high incidence of Salvinia root hairs in the diet (compared with the other species) perhaps indicates that this species browses for invertebrates under mats of this plant. Small amounts of fish scales were also eaten. Arius solidus is the only Sepik River ariid feeding to any extent on whole fish (although A. coat& occasionally ate these). Fish consumed were usually Ophie- leotris uporos (Bleeker).

Seasonal differences in feeding were analysed by comparing consumption of each food category between the flood and dry seasons (t-test). The consumption of coarse detritus and larger insect larvae by A. solidus was significantly higher during the flood season (p < 0.001); feeding on Macrobrach- ium was significantly higher during the dry season (p < O.OOl), which coincides with an increased abundance of these prawns at this time (Robertson 1983); feeding on gastropod molluscs was minimal (Fig. 4) but significantly higher during the flood season (p < 0.05). Differences between the seasons in all other food categories are non-significant in A. solidus (p> 0.05). There was no marked seasonal change in mean stomach fullness in A. solidus except for the drop in feeding in March/ April (Fig. 5). During the dry season, there was slightly but significantly increased mean stomach fullness amongst those fish with stomach contents (t-test, p < 0.01) due mainly to increased feeding on prawns. This comparison, however, becomes non-significant when fish with empty stomachs are included in the analysis (p > 0.05). There was an increase in the occurrence of empty stomachs in the dry season, and this may be due to increased breeding in A. solidus at that time. The data suggest that A. solidus does not feed extensively on the floodplain (Pitpit) when compared with Magendo 2 (adjacent lake), but only when fish with empty stomachs are excluded (Table 5). Feeding is, however, significantly lower in Magendo 2 compared with Pitpit when fish with empty stomachs are included. This result may be erroneous in as much as significantly more male A. solidus with empty stomachs were encountered in Magendo 2 compared with

Pitpit (p < 0.001). It may be that brooding males prefer lakes to the floodplain.

Arius uturus prefers larger food items including coarse detritus, larger insect larvae and especially prawns (Fig. 4). Appendages from prawns of the genus Macrobrachium had frequently been eaten on their own. There was a high incidence of feeding on fish scales (22% volume - much higher than in A. solidus) and whole fish were never consumed. This species is apparently voracious and frequently rasps at the body surface of larger prey (prawns or fish). The diet of A. utarus differed from A. solidus especially in its preference for caridinid prawns and fish scales and its reduced feeding on plant material and absence of fish. Its diet differed from A. coatesi in these respects and others that are listed below. Differences in mean stomach fullness between the flood and dry seasons are not significant in A. utarus. Feeding on prawns increased significantly in the dry season and feeding on coarse detritus and fish scales increased significantly during the flood season (p < 0.0.5), perhaps in response to a reduced availability of prawns. Differences between the seasons for the other food categories are non- significant (p > 0.05).

Arius coatesi feeds predominantly on fine and coarse detritus, other plant material (particularly seeds and small pieces of bark), and especially prawns (Fig. 4). Occasionally, whole small fish were eaten, invariably Op. aporos. The fine detritus consumed was much finer than in the other Arius spp. (except A. velutinus) and represented fine river sediments (‘mud’). Caridinid prawns dominated the food (31% of volume) and featured in the diet more than Macrobrachium spp. The preference of this species for these smaller prawns

63

is a main feature distinguishing its diet from all other species. Differences in mean stomach fullness between the flood and the dry seasons are not significant in A. coatesi (p > 0.05), nor are seasonal differences in the amounts of the various food categories consumed (only 57 specimens account for this observation, however).

The diet of A. nox is clearly different from all the other species (Fig. 4), as indicated by very low Schoener values (Table 4). Arius nox feeds predominantly on small insect larvae and small crustaceans (excluding prawns) which together accounted for 82% of its diet. Arius lzox has an unusually high number of gill-rakers for a Sepik ariid (48 LGR, compared with 18 to 24 for the other species, Kailola 1990) and it probably filter feeds to a large extent. It also has long, forward pointing barbels which may be used to disturb small invertebrates which are then extracted from the water by the gill rakers. Mean stomach fullness in A. nox is significantly higher in Pitpit compared with Magendo 2 and all of the other sites (Table 5). The data suggest that A. nox feeds on the floodplain to a greater extent than does any other species. A significant decrease in feeding activity was noted in fish from Imbuando compared with all other sites (including or excluding empty stomachs) and in Magendo 1 compared with Magendo 2 (in this case, only when fish with empty stomachs are included, Table 5). Significantly more males with empty stomachs were encountered in Magendo 2 compared with Pitpit (p < O.OOl), and this again suggests brooding occurs in lakes and not on the floodplain. There was a slight decrease in feeding activity by A. nox during the dry season (Fig. 5). Mean stomach fullness is significantly lower in the dry compared with

Table 4. Schoener index values giving comparisons of dietary overlap among the five species of Sepik River ariids. Schocner index values calculated from the data on food categories in Figure 4.

A. solidus A. u&rus A. coutesi A. nox A. velutinus

A. solidw 0.563 0.540 0.171 0.326 A. utarus 0.536 - 0.535 0.117 0.205 A. coatesi 0.540 0.535 - 0.178 0.340 A. nox 0.171 0.117 0.178 - 0.183 A. velurinus 0.326 0.205 0.340 0.183 -

64

18 58 57 58 34 51 63 26

N= 27 67 69 58 54 43 74 32

A. nox

N JF MA MJ JA SO ND JF N= 5rl 66 63 78 36 39 78 67

1981 1

1982 1983

Fig. 5. Mean stomach fullness over the sampling period (vertical lines = 95% confidence limits) for A. solidus and A. nor, all sample sites combined. Closed circles and full lines = data for all fish; open circles and dotted lines = data for fish excluding those with empty stomachs, data points displaced slightly to the right to avoid overlap. Numbers immediately below the months refer to sample size including all fish, those above the figures to sample size excluding fish with empty stomachs. Relative river height indicates the fluctuation in river level throughout the same period and is the same for all data.

the flood season (p < 0.01). Increased flood season feeding may be accounted for by increased feeding on fine detritus, small crustaceans, and small insect larvae, all of which were significantly higher than in the dry season (p < 0.001). Comparisons between seasons for all other food categories are non-significant (p> 0.05).

Arius velutinus feeds mainly on fine detritus (river mud), larger insect larvae, and insects from terrestrial sources (Fig. 4). Its diet differs from the other species (Table 4) mainly in the reduction in

coarse detritus and the complete absence of prawns (Fig. 5). The insect larvae and insects from ‘terrestrial’ sources consumed were predominantly adult mayflies (Ephemeroptera) that were emerg- ing en masse at the time most specimens were collected. Their emergence is periodic (personal observation). The absence in the diet of fish scales, fish, and prawns suggest that this species (together with A. nox) is much less voracious in its feeding habits than the other three species.

65

Feeding by embryos and juveniles in broods

Broods held within the mouths of male A. solidus and A. uturus were also investigated (samples were lacking for the other species). Observations showed that young fish start feeding at a remark- ably early state of development, probably immediately after hatching. Both free-embryos and juveniles of the two species had generally eaten very small fragments of plant material, detritus, and small crustaceans.

Sepik ariid eggs range in size from about 10 to 13 mm diameter, depending on species and parent size (Coates 1988). Embryos immediately post-

hatching still have enormous yolksacs and would be clearly incapable of locomotion. Feeding at this state of development probably commences within the mouth of the parent, although older juveniles with reduced yolksacs may well feed outside the parent’s mouth; this has been observed in the field in an Australian species by K.A. Bishop (personal communication). Parents presumably provide food to the young. This conclusion follows from the observation that in one brooding A. solidus, all of the young (12 individuals, mean total length of 42.8mm) had eaten only fish scales. These most likely had been provided by the rasping action of the parent. The male parent involved was not feed-

Table 5. Comparisons of mean stomach fullness between the four sample sites for A. solidus and A. nox.

p levels for comparisons between sites

Ark solidus Imbuando

Magendo 1 Magendo 2 Pitpit

-%VE - % VT

42.2 (27.4) [96] X 32.9 (34.6) [118] X

<o.ool(+) < o.ool(+)

X X

>0.05 >0.05

>0.05 <0.05(-)

<O.Ol(-) <O.Ol(-)

> 0.05 < 0.05(-)

X X

<O.OOl(-) < O.OOl(-)

> 0.05 <O.Ol(-)

<O.OOl(-) <O.OOl(-)

X X

Magendo 1 -%VE - % VT

26.9 (18.2) (241 13.2 (18.5) [49]

<0.05(-) < O.OOl( -)

Magendo 2 -%VE - % VT

39.7 (27.7) [87] 31.9 (29.4) [log]

X X

Pitpit - % VE -%VT

43.9 (21.6) [92] 38.8 (24.8) [104]

A rius nox Imbuando

- % VE -%VT

28.1 (18.9) [76] X 20.5 (20.4) [120] X

< O.Ol( -) < O.Ol( -)

X X

<O.OOl(-) <O.OOl(-)

Magendo 1 -%VE -%VT

45.5 (19.9) [15] 34.5 (26.2) [22]

20.05 > 0.05

Magendo 2 -%VE -%VT

36.8 (17.6) [ 1361 35.3 (18.7) [142]

X X

Pitpit -%VE -%VT

47.3 (24.2) [120] 45.8 (25.2) [124]

% VE = mean percentage stomach fullness excluding those fish with empty stomachs. % VT = mean percentage stomach fullness for all fish (including those with empty stomachs). Numbers in parentheses are standard deviations. Numbers in square brackets are total numbers of individuals. p levels were determined by t-test between sample sites in each case. + signifies that the sample site in the left (vertical) column has a significantly higher value than the comparison site in the horizontal row (- signifies that a significantly lower value is obtained).

66

bolidus 5l 51

A.nox

ObF 8 N: 63 48 73 28 31 78 38 8 N: 103 52 77’ 50 12 26 27 13

1981 1982 1983 1981 1982 1983

1. utarus

01 I , I I I I NbbMAMJ JASOND JF

N: 14 15 17 13 16 17 13 17 1981 1982 1983

Fig. 6. Mean condition factor (closed circles/continuous line) and mean fat deposit index (open circles/discontinuous line) for three species of Sepik River ariids. Vertical lines are 95% confidence limits for mean condition. rrh = relative river height indicates the fluctuation in river level for the same period.

ing itself, in as much the stomachs of brooding males are always empty of food. It is not known if females accompany males and assist with feeding the brood.

Body condition and visceral fat storage

Both A. solidus and A. utarus exhibited slightly increased fat deposition and body condition in the dry season, with modest decreases in the flood season (Fig. 6). In A. lzox the situation was re- versed; both factors increased during the flood sea-

son and decreased in the dry season. Comparisons between the data of the flood and dry season are significant in each case for all three species (t-test, p< 0.01). There are insufficient data to include either A. coatesi or A. vehtinus in Figure 6. The data for A. coatesi show significantly increased fat deposition and condition in the dry as compared with the flood seasons. This comparison in A. velutinus is non-significant (p> 0.05) but there are probably too few data for that species.

Reproduction

Development of the pelvic fins in female ariids that relate to the reproductive cycle have been described and illustrated by Rimmer (1985a). Devel- opment of these ‘claspers’ was observed in all Sepik species and they appear to be similar to what Rim- mer described.

Length at 50% maturity could not be determined for any of the species due to the low numbers of immature fish collected. Data on minimum breeding sizes are presented in Table 6. The data on brooding in males are limited. Where reasonable numbers of observations exist, the data suggest that minimum breeding size in both sexes is about 50 to 60% of the maximum size attained. Arius solidus of either sex may mature at the relatively smaller size of 40% of maximum length.

The sex-ratio (number males/females) for fish from gillnets (all sample sites combined) was: A. solidus = 1.6 (N = 299), A. uturus = 0.61 (58) and A. nox = 0.75 (319); the differences in numbers of fish of each sex are significant in each case (Chi- square, p< 0.001). There was no obvious difference between sample sites, nor were any seasonal effects on the sex-ratio evident. These ratios could either reflect relative abundance of the sexes or be due to sexual differences in behaviour that affect gillnet selectivity. There are too few data for A. coatesi and A. velutinus.

Ariid testes usually remain small, presumably because few sperm are required to fertilise the extremely low number of eggs produced by females (Coates 1988). Attempts to assign testes maturity

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stages were abandoned. Very small and unquanti- fiable macroscopic changes in ariid testes were also noted by Rimmer (1985a). The breeding activity of males was best recorded by observations of mouthbrooding. Females with ovaries indicating reproductive activity (i.e. running-ripe, or spent) were present at all times of the year with little obvious seasonality in any of the species (Fig. 7). Brooding occurs all year round but the data are unreliable for assessing seasonality (data do not represent % of males brooding since males may lose their broods on capture).

In A. solidus there was a significant (t-test, p < 0.01) increase in mean gonadosomatic index of females in the dry season (Fig. 8). This suggests increased reproductive activity in the dry season in A. solidus and agrees with the possible increase in brooding in males (Fig. 7). The increase in gonadosomatic index in the dry season in A. solidus also coincides with the increased feeding at this time. Mean gonadosomatic index in female A. lzox during the flood season is not significantly different from the dry season mean (p> 0.05). Peaks of gonadosomatic index in A. nox were evident in March/April and January/February (1983) which is during the flood season. Mean gonadosomatic index in A. nox for these two bi-monthly periods is significantly higher than the mean for the remainder of the period in each case (p< 0.05). This suggests increases in breeding activity at the begin- ning and middle of the flood season in A. nox. Mean gonadosomatic index in female A. utarus during the flood season was 2.5 (& 3.1, N = 92) which is not significantly different from the dry

Table 6. Data for minimum breeding sizes in Sepik River ariids (SL, mm) related to the maximum size attained.

females males

min breeding size % max size min breeding size % max size

A. solidus 190 (294) 39 200 (38) 41 A. utarus 245 (98) 54 265 (12) 60 A. coatesi 360 (76) 57 370 (17) 73 A. nox 170 (347) 60 160 (29) 57 A. velutinus 280 (86) 58 N/A

Minimum breeding size in females is the smallest fish with ripe, running-ripe or spent ovaries. Minimum breeding size in males is the smallest male observed to be orally incubating (brooding) eggs. Numbers in brackets are the total numbers of observations.

~‘:iJ , , , , , , , , ND JFMAMJJASOND JF

Nr 34 48 30 22 18 42 43 19

= % ND JF MA MJ JA SO ND JF

STAGE 1

STAGE 2

STAGE 3

STAGE 4

STAGE 5

STAGE 6

A. utarus

ND JFMAMJ JA SOND JF

18 10 10 7

13 14 12 8

ND JF MA MJ JA SO ND JF

STAGE 1

STAGE 2

STAGE 3

STAGE 5

STAGE 6

A. nox

ND JF MA MJ JA SO ND JF

75 38 42 24

34 30 30 15

ND JFMAMJ JA SO ND JF

Fig. 7. Seasonality in reproduction of three species of Sepik River ariids. Upper figure: percentage of females above minimum breeding size with ovaries in each of the six stages of maturity. Lower figure: number of males observed orally brooding.

season value of 2.7 (k 2.9, N = 36, p> 0.05). Data are insufficient for this comparison in either A. coatesi or A. velutinus. In males of all species the gonadosomatic index was much lower than that in females (data for two species in Fig. 8). Differences in testes weight between the various stages of maturity are also minimal, and seasonality in reproduction is difficult to assess by this method in males.

Differences between the seasons in gonadosomatic index in males are non-significant for all species.

Very few juvenile ariids were caught in gillnets. Information on juvenile recruitment also does not help determine any seasonality in reproduction because juvenile ariids (except A. nox) are difficult to identify as to species. Villagers were employed to catch juvenile ariids with hand-nets and their catch-

69

A. solidus A.nox

h

N: 27 29 28 17 19 32 14 12 N: 17 32 23 21 31 29 12 18

Fig. 8. Mean gonadosomatic index for male and female A. solidus and A. nox from the Sepik River. Vertical lines are 95% confidence limits. rrh = relative river height and indicates the fluctuation in river level for the same period.

es indicated the possibility of peaks in juvenile abundance, but these were not regular between years. Peaks also did not occur at river draw-down, suggesting that juveniles do not enter the floodplain. Other information from villagers and rotenone sampling also indicate that juveniles occur in the rivers and not the lakes.

Males increase in condition and fat deposition prior to brooding, and body weight and fat deposits are slowly lost over the brooding period. To il- lustrate this, brooding males were divided into two groups: First, those brooding young embryos, in which the length of the germinal plate is less than 10% of the circumference of the yolksac; and sec- ond, those brooding older embryos or juveniles, i.e. those with little or no yolksac remaining. Mean condition factor and fat deposit index for males in each of these two groups were compared with the means for males without broods but with food in their stomachs (this definitely excludes brooding males that lost their brood on capture; the stomachs of brooding males are empty of food). Males with new broods had a much higher mean fat deposit index and condition factor, and they were

much lower in males with older broods, compared with the means for non-brooding males. These differences are significant in each species for which a reasonable number of observations exist (Table 7).

In all species, non-brooding males have significantly higher fat deposit indices than females (t- test, p < 0.001). This highlights the energy storage required in males prior to their fasting during brooding.

No significant correlation exists between condition and gonad stage in the females of any of the species (p > 0.05). Fat deposit indices are significantly negatively correlated with gonad stages in females of all species (p < 0.001). Presumably, fat deposits are used by females as an energy store for egg production.

Discussion

The striking lack of seasonality in much of the data for Sepik ariids is probably due to the lack of exploitation of floodplain habitats by most species. This contrasts sharply to floodplain dwelling spe-

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ties which tend to exhibit seasonality in catches and landings and increases in feeding, growth, condition, reproduction, and fat storage during the pro- ductive period on the floodplain during the flood (e.g., Welcomme 1979; also shown in a Sepik floodplain dwelling species by Coates 1990). Arius lzox is the only Sepik ariid exhibiting any such changes, although they are modest, during the flood. Arius solidus, A. utarus and A. coatesi all exhibited increases in condition and fat storage during the dry season. This reverse situation, to that normally expected with floodplain species, could be due to the increased abundance of major food items, particularly prawns, in rivers and lakes in the dry season. Reproduction also possibly increases in the dry season in A. solidus, but there was little seasonal difference in this respect in A. utarus. Seasonal changes in diet in A. nox and the lack of changes in diet in the other species, are also likely to be due to their relative degrees of exploitation of the floodplain. The increased feeding by A. nox on the floodplain (Pitpit) could reflect the greater abundance of smaller invertebrate life expected there during the flood season (Welcomme 1979).

Lack of affinity for the floodplain and year- round breeding, involving marked associated variations in fat deposition and condition in both sexes,

may explain the high degree of variance in the data for Sepik River ariids. For example, standard deviations for means for condition and gonadosomatic index for Sepik species are much higher than the values given by Rimmer (1985a, 1985b) for the more seasonally influenced A. graeffei with its regular breeding cycle.

Differences in diet between the species may relate to their different morphology, and, in particular, Sepik ariids show marked variations among species in the arrangement of their palatine teeth and mouth size and shape (Kailola 1990 gives dia- grams). Arks solidus has the largest area of palatine teeth (in a single patch) and the largest mouth. Arius utarus also has sturdy palatine teeth (in four patches in a row), although reduced in area compared with A. solidus, and a large mouth. These two are the most voracious of all Sepik ariids, particularly in their ability to rasp the bodies of relatively large prawns of the genus Macrobrachium and fishes. Arius nox has a large area of palatine teeth but is a much smaller species than the others. It is more highly specialised, filter feeding on small invertebrates, which are swallowed whole, but the palatines may also aid in dealing with larger prey when the opportunity arises. Ark coatesi has a much smaller mouth relative to the above three species and reduced palatines (in two small patch-

Table 7. Comparisons of both mean fat deposit index and mean condition factor between male Sepik River ariids at two states of brooding.

Mean for males mean for males brooding mean for males brooding older above mbs young embryos embryos and juveniles

R s.d. N R s.d. N P ?i s.d. N P

Fat state: A. solidus 2.7 1.5 107 3.9 0.7 11 < 0.01 0.96 1.2 9 < 0.01 A. utarus 2.9 1.3 74 4.2 0.8 5 >o.os 1.0 1.0 3 > 0.05 A. coatesi 2.7 1.2 15 4.1 1.2 7 < 0.05 1.0 1.1 5 <0.05 A. nox 2.6 1.4 131 3.0 1.2 9 < 0.05 0.5 0.8 10 < 0.01

Condition factor: A. solidus 1.8 0.3 96 1.9 0.2 10 < 0.01 1.7 0.2 9 < 0.01 A. utarus 1.75 0.3 76 1.8 0.3 5 < 0.05 1.6 0.2 3 > 0.05 A. coatesi 1.89 0.3 15 2.0 0.2 7 <0.05 1.72 0.2 5 <0.05 A. nox 1.35 0.2 119 1.4 0.2 9 < 0.01 1.17 0.1 10 < 0.01

Mean values for males brooding young embryos and those brooding older embryos and juveniles are compared with the mean values for non-brooding males above minimum breeding size (mbs). For further details see text; p levels based on t-test.

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es). It is also much less voracious than either A. solidus or A. uturus. Artus coatesi also prefers the smaller caridinid prawns; the larger Macrobrach- ium prawns were also eaten but they were invariably small and were swallowed whole. Arius vefu- tinus has a smaller and more sub-terminal mouth lacking palatine teeth. This species occurs in the same environment as A. coatesi but their diets are notably different in that A. velutinus does not eat prawns of any kind, feeding on presumably ‘softer’ foods (e.g., Ephemeroptera). Overlap in diets is higher among A. coatesi, A. solidus, and A. utarus, and diets are greatly different in both A. lzox and A. velutinus, each of which previously was assigned to a separate genus (see note on taxonomy). Some differences in diet among the species could also reflect their habitat preferences. Arius nox feeds extensively on floodplain invertebrates but the diet and habitat preferences of A. solidus and A. utarus overlap; a similar situation exists in A. coatesi and A. velutinus, both being restricted to rivers.

Roberts (1978) and Haines (1979) reported on the feeding habits of Fly River ariids, but Roberts’ observations were sometimes based on low numbers of recordings. The largest Fly River species, A. augustus (Roberts), was reported to be exclusively piscivorous by Roberts (1978) but Haines (1979) suggests that it is less stenophagous than this, and, thus, indicating the need for larger sample sizes with opportunistic feeders. No Sepik ariid is a significant predator of other fishes but small fish are occasionally eaten by A. coatesi and to a greater extent by A. solidus. Other members of the genus Arius in the Fly have similar diets to A. coatesi, A. solidus, and A. utarus in the Sepik. The Fly River genera Cinetodus and Cochelfelis are both absent from the Sepik, and both have species with more stenophagous habits than occurs with Sepik ariids. Nedystoma duyi (Ramsay and Ogilby) from the Fly River filter feeds mainly on dipteran larvae and its feeding habits are similar to those of A. uox in the Sepik; both species have a large number of gill-rakers. Fly River Hemipimelodus spp. appear to have similar diets to A. velutinus (= H. velutinus) in the Sepik. Roberts (1978) reported feeding on fish scales only in A. cleptolepis (Roberts) from the Fly, but two species have this

habit in the Sepik: A. solidus and particularly A. utarus. Ariids may rasp at fish caught in nets, as also noted by Roberts (1978), and my samples are more reliant upon netting than those of Roberts. It is, however, probable that rasping at other fishes is a natural habit since several ariids were caught from the Sepik that had eaten scales from species not caught in my nets. Both A. sofidus and particularly A. uturus also rasp at prawns, frequently consuming their appendages. Fish scales in the diets of ariids have also been reported by Singh & Rege (1970), Morely (1981) and Bishop et al. (1989).

A considerable amount of literature is available on the feeding habits of marine and estuarine ariids in other regions. These ariids are also opportunistic and broadly omnivorous with prawns featuring conspicuously in their diets (Venkataraman 1960, Rao 1967, Tobor 1969,1978, Singh & Rege 1970). This might suggest that ariids are pre-adapted to occupy trophic niches in the major New Guinea rivers where prawns, in particular, are abundant. Only the Sepik A. lzox and the Fly River N. dayi are known to differ markedly in feeding habits from marineiestuarine species.

The reproductive biology of ariids has been re- viewed by Rimmer & Merrick (1983). Fecundity and the sizes of eggs and brooded embryos in Sepik ariids have been studied by Coates (1988). All Se- pik ariids are confirmed freshwater spawners, except possibly A. velutinus, where observations on mouthbrooding are lacking.

Spawning probably occurs the year-round in all Sepik species, with modest seasonal peaks only in A. solidus (dry season increase) and A. nox (flood season increase). Reproduction in southern New Guinea species was mentioned only briefly by Ro- berts (1978), Haines (1979), and Maunsell & Part- ners (1982). All of these authors reported year- round breeding in those species mentioned, with seasonal peaks in the flood season (but based on more limited data). Arius leptuspis (Bleeker) from northern Australia was reported to breed seasonally by Bishop et al. (1989) and so was A. grueffei in New South Wales by Rimmer (1985a). Australian rivers are probably subject to much greater seasonal variations in temperature and flow in their main

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channels, particularly at lower latitudes. Marine species appear to spawn seasonally (Rimmer & Merrick 1983). Breeding migrations have been noted in several marine and estuarine ariids (Rim- mer & Merrick 1983). Haines (1979) also reports that even amongst ariids that breed in freshwater in southern P.N.G. rivers, there is a tendency for at least part of the population to move downstream to the estuary to spawn. Estuaries are, however, well developed in all the larger southern P.N.G. rivers whilst the northern rivers, including the Sepik, are characterised by the absence of deltas.

Visceral fat deposition in male Sepik River ariids appears to be in preparation for the period of fasting during oral incubation. Rimmer (1985b) reported that in A. grueffei incubation was associated with decreased liver weight and not with changes in fat deposition. Fat storage in females is associated with egg production in Sepik ariids, and a similar relationship was reported by Rimmer (1985b). En- ergy storage and usage in Sepik River ariids appear to be related more to breeding requirements than to seasonal fluctuations in food availability. Ariids have a long period of buccal incubation which Rim- mer & Merrick (1983) suggest could be about two to three months in most species. This would have a significant effect on their reproductive rate, given that males, at least, probably take quite some time to build up fat reserves between broods. A maximum of four or five spawnings per year might be expected and probably only one or two. This, together with their extremely low fecundities, explains why their reproductive rates are very low (Coates 1988).

Ariids have undergone a relatively high degree of speciation in the freshwaters of New Guinea. Despite this, they remain essentially higher-order river and lake dwellers. Only A. ylux has shown any tendency, although modest, to occupy the Sepik floodplains, the major environment in such river sytems (Welcomme 1979). Their reproductive strategy probably limits their ability to colonise the floodplain, Floodplain species generally require high reproductive rates in order to cope with mas- sive natural mortalities through fish trapped on the floodplains each season as the water recedes (e.g., Welcomme 1979). Male Sepik River ariids also

appear to avoid the floodplain when brooding; car- rying the brood in permanent waters presumably avoids the unnecessary risk of mortality from floodplain hazards. Juvenile ariids also avoid this environment.

Roberts (1978) records thirteen species of ariids from the Fly River in southern P.N.G., whilst there are only five recorded from the similar-sized Sepik River. Comparisons at the generic level are difficult to make due to different taxonomic treatments of ariids in the two regions. The Fly genera Cine- todw and Cochelfelis, both having specialised feeding species, are absent from the Sepik, and there are three species of Fly Hemipimelodus compared with a single one for the Sepik (= A. velutinus). There are six Fly species of Arius compared with only three (excluding A. aox and A. velutinus) from the Sepik. The Fly and Sepik rivers are clearly in different zoogeographic zones and there are no ariids endemic to both regions (Allen & Coates 1990). The reduced diversity in the Sepik may be accounted for by the absence of any extensive delta, the ariids are usually considered to be ‘estuarine catfishes’, and the Sepik being geologically much younger than the Fly (Loffler 1977).

Coates (1985) estimated the annual fish catch from Sepik floodplain regions to be between 3000 and 5000 t . Consequently, based on catch and landings data, approximately 1000 t yr-’ of ariids are caught and consumed locally. Their lack of seasonality also makes them available to fishermen at all times in rivers and lakes, particularly at times when other species are less accessible while on the floodplain. Ariids are large and relatively easily caught on hand lines and it is also significant locally that they store large amounts of fat. Ariids probably represent the only major source of fat in the natural diet of people inhabiting floodplain regions. Ariids are even more important in the local fishery than their present status in landings implies. The low reproductive rate of Sepik River ariids, however, suggests that they may be particularly vulnerable to fishing pressure (Coates 1988). This exacerbates problems with improving yield from this low yield- ing fishery (Coates 1985); in brief, ariids account for about 50% of the biomass of present landings of native species but they are an unsuitable group on

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which to promote significant increases in catches (Coates 1987,1988).

Roberts (1978) suggested that ariids may have been naturally eliminated from freshwater ichthyo- faunas in other regions by the evolution there of the more slowly dispersing primary freshwater catfishes. This suggests that New Guinea freshwater ariids may be particularly susceptible to the introduction of certain primary freshwater catfishes. The present paper has determined the significance and possible vulnerability of Sepik ariids. It has highlighted the vital importance of undertaking basic studies before any fish introductions occur, either to im- prove riverine stocks (Coates 1987) through aqua- culture or other means (see also Redding-Coates & Coates 1981, Coates 1984). Pollard & Burchmore (1986) incorrectly quoted Petr (see aforemen- tioned reference) that the introduction of the catfish Clarias sp. into the Sepik River was to be sponsored. However, Clarias sp. has already be- come established in western New Guinea, Irian Jaya (Fig. l), by transfer from Java for culture purposes. Fortunately, clariid catfishes are predominantly floodplain dwellers, whereas ariids avoid this habitat. The transfer to the Sepik, or to anywhere in the Australasian region, of certain freshwater catfishes that predominantly inhabited rivers would be an even greater cause for concern.

Acknowledgements

I thank K.A. Bishop and P. Kailola for useful comments on the manuscript, the latter also for extensive help with taxonomy; E. Winrow for drawing the figures.

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Biology of fork-tailed catfishes from the Sepik River, Papua New Guinea

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