The fishery, growth rates and notes on reproduction of the snakehead Channa striata (Bloch) (Perciformes: Channidae) in some irrigation reservoirs of Sri Lanka

Fisheries Research, 19 ( 1 9 9 4 ) 2 5 7 - 2 6 8

257

0 1 6 5 - 7 8 3 6 / 9 4 / $ 0 7 . 0 0 © 1994 - Elsevier Science B.V. All rights reserved

The fishery, growth rates and notes on reproduction of the snakehead Channa striata (Bloch) (Perciformes: Channidae) in some irrigation reservoirs of Sri Lanka K.H.G.M. de Silva Department of Zoology, University of Peradeniya, Peradeniya, Sri Lanka (Accepted 19 October 1993 )

Abstract The fishery for snakehead (Channa striata) in Sri Lanka was studied by monitoring the monthly catch from a large reservoir of 972 ha surface area and that of five closely situated smaller reservoirs of surface areas ranging from 37 to 265 ha for 18 months. The annual snakehead yield in the large reservoir was 1.76 kg h a - ~ while that of smaller reservoirs was 7.08 kg h a - ~. In all reservoirs, snakehead was only a by-catch of the tilapia fishery. The total annual C. striata yield in Sri Lanka is estimated to be over 317 t which is only about 0.8% of the total reservoir fish production. The growth parameters asymptotic length ( L ~ ) and growth constant (K) of the snakehead populations, as determined by monthly size-frequency data, were respectively 60 cm total length and 0.40 for the large reservoir and 53 cm total length and 0.35 for the smaller reservoirs. The mean length of the ten largest individuals (Lma~), the mean length of the ten smallest individuals (Lmin), mean lenglh at recruitment to the fishery ( L ' ) and mean size at first capture (Lc) of the two populations were respectively 51.6 cm, 26.6 cm, 37.5 cm, 42.5 cm for the large reservoir and 46.4 cm, 2.2 cm, 36.0 cm and 35.4 cm for the smaller reservoirs. The respective total, natural and fishing mortalities of the populations were 1.53, 0.84 and 0.69 in the large reservoir and 1.19, 0.77 and 0.42 in the smaller reservoirs. In all reservoirs, over 90% of the catch consisted of third and fourth year fish. Spawning was observed in all months, but it was much more enhanced during and immediately following the major rainy season.

Introduction The air-breathing snakeheads inhabit lakes, reservoirs, and marshes of tropical Asia and Africa. Although cultured in ponds on a small scale, the main supply of these high-priced fish still comes from natural sources. Among the several snakehead species, Channa striata is the most important in fisheries and aquaculture, but, being a top carnivore, it is expensive to culture. It could also be over-fished easily in its natural habitat because of its smaller population size in comparison with herbivorous and omnivorous species. There are five species of snakehead in Sri Lanka, but only C. striata is of $5~I0165-7836(93)00265-K

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K.H.G.M. de Silva/Fisheries Research 19 (1994) 257-268

commercial importance. This species, which usually grows to about 55 cm ( 1.4 kg) in Sri Lanka, is found in irrigation reservoirs, which are usually situated less than 200 m above sea level (a.s.1.) and cover a total surface area of about 162 000 ha (De Silva, 1988 ). Most of these are small (less than 10 ha) seasonal reservoirs that dry up completely or almost completely during the height of the dry season. The perennial reservoirs can broadly be categorised into medium-scale ( 10-300 ha surface area) and major reservoirs (over 300 ha surface area). There is no separate snakehead fishery in Sri Lanka and the species is usually obtained as a by-catch of the tilapia gillnet fishery of lowland reservoirs. The few existing records of snakehead fishery indicate that the catch has diminished greatly within the past few decades (Deraniyagala, 1952), probably because of over-fishing. There appears to be no previous detailed study on a snakehead fishery, although the growth rates of several species have been determined using lengthfrequency data and scale markings (Qasim and Bhatt, 1966; Bhatt, 1970; Devaraj, 1973; Kilambi, 1986; De Silva, 1991 ). The present study compares the growth rates, fishery and reproductive biology of two snakehead populations, one in a large reservoir and one in smaller reservoirs. Materials and methods

Habitat The snakehead catch was studied in one large and five smaller (mediumscale) reservoirs. Mahavilachchiya reservoir (8°28'N, 80 ° 1 I ' E ) , which has a surface area of 972 ha at its full supply level (FSL) and is situated at 54 m a.s.l., is one of the major irrigation reservoirs in Sri Lanka. The five smaller irrigation reservoirs have a total area of 492 ha at FSL and individual areas of 265, 107, 44, 39 and 37 ha. They were all situated within an area of 250 km 2 in the Nikaweratiya region ( 7 ° 3 8 ' - 7 ° 4 8 ' N , 8 0 ° 0 7 ' - 8 0 ° 17'E) and at an elevation of about 60 m a.s.1. These reservoirs retained water throughout the year except during prolonged dry periods, which occur only rarely. Channa striata inhabiting the five smaller reservoirs could be considered as belonging to a single population for the following reasons. There are more than 500 smaller seasonal reservoirs (less than 10 ha) within this area of 250 km 2 and the five reservoirs form a complex of interconnected waterways with these seasonal reservoirs through irrigation canals and paddy fields. Since all these reservoirs are situated within an elevation range of about 30 m, fish can move from one reservoir to another, especially in the rainy season when the reservoirs are full and overflow the adjacent low-lying areas. Snakeheads, being air-breathing fish, could move overland for short distances from one water body to another ( Day, 1875-1878; Deraniyagala, 1952 ).

K.H.G.M. de Silva / Fisheries Research 19 (1994) 25 7-268

259

Sampling The main fishing gear in the large and smaller reservoirs was the gillnet. About 100 fishermen operated from 40 small craft (fibre-glass canoes with an outrigger) in the large reservoir. Each craft operated for about 20 days per month, fishermen setting the nets (each about 100 m in length and with a knot-to-knot mesh size of 3-4 in. (7.5-10 cm ) ) in the late evening and hauling them early next morning. In smaller reservoirs, only a few dug-out, outrigger canoes (up to five) operated and the gillnets employed were smaller, each net only about 50 m in length and with mesh sizes as small as 2.5 in. (6 cm). Cast nets were also employed, often in the smaller reservoir. The smaller reservoirs were very shallow and some fishermen used cast nets during daytime.The entire daily catch of C. striata in the Mahavilachchiya reservoir was recorded on 2 consecutive days at the end of each month during the period of March 1986 to August 1987, and that of smaller reservoirs was monitored for 1 day in the middle and 1 day at the end of each month during the period January 1984 to June 1985. The total length and weight of each fish was recorded to the nearest 5 mm and 10 g in the field. The maturity stages of the gonads of each fish were determined according to the criteria given by Nikolsky ( 1963 ).

Analysis Because the peaks of the monthly size-frequency histograms could not be followed with reasonable accuracy for more than 2 or 3 months, the "Compleat ELEFAN" computer program (ICLARM, Manila) was used to analyse the monthly size-frequency data in order to estimate various population parameters, including growth parameters of the Von Bertalanffy growth equation of the two snakehead populations

Lt =Lo~{1-exp[-K( t-to) ]} This program uses the running averages taken over five size classes to identify peaks and troughs in size-frequency histograms, and determines the parameters of the Von Bertalanffy growth equation of the growth curve that connects the maximum number of peaks (Pauly, 1987). Crude estimates of Loo and K were first made using the program and then the probabilities of capture of various size classes by the gear used were estimated by constructing a size-converted catch curve and extrapolating to the left the descending right arm of the catch curve (Pauly, 1987). Final values of L~ and K were estimated using the length-frequency data corrected by taking into account the probabilities of capture. Total mortality (Z) was estimated from the catch curve obtained by using the original length-frequency data and the final values of Loo and K.

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I~ H. G.M. de Silva / Fisheries Researeh 19 (1994) 257-268

Results Monthly catches of the reservoirs are shown in Figs. 1 and 2. The monthly catch (per hectare) fluctuated more widely in the five smaller reservoirs, where the catch of snakeheads was about four times that of the larger reservoir per unit area (Table 1 ). The asymptotic length ( L ~ ) and the growth constant K, estimated using monthly length-frequency data, as well as the sizes of the smallest and the largest individuals and the mean length of the ten largest individuals (Zmax) -300

["] R a i n f a l t -6

.200 =

"T

ne

f-~ -100

2.

M A 1986

M J

J

A

S

0

N

D J F 1987

M A

M J

J

A

Months

Fig. 1. Monthly catch size of C. striata in Mahavilachchiya reservoir during the study period. Monthly rainfall of the area is also shown.

40

300

-] Rainfalt

~

30

I

.2oo ~"

/

w

20~

.c .100

c.) 10

J 1984 I

M A

1it

M J I

Intlrmons~n

J

sw monloon

A S

0

N

Months 1 I

J 1985

M

A M

!

NW monsoon in termonloon 2rid

Fig. 2. Monthly catch size of C. striata in smaller reservoirs and the monthly rainfall of the Nikaweratiya area during the study period. The inter-monsoonal and monsoonal periods are also indicated.

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K.H.G.M. de Silva / Fisheries Research 19 (1994) 257-268

Table 1 Catch, lengths and growth parameters of C. striata in Mahavilachchiya reservoir and smaller reservoirs in the Nikaweratiya area

Daily catch ( means + SE ) kg d a y - ' kg ha- 1 day- 1 Daily catch range (kg day -~ ) Asymptotic length L~ (cm) Growth constant K Smallest length (cm) Largest length (cm) Lmax (cm) Lmin (cm) Mean length (cm)

0-20

5

5 A~.

B~

25 25~

50

I

,

Mahavilachchiya

Smaller reservoirs

4.69+0.219 0.0048 +-0.0002 3.3-7.8 60.0 0.40 24.5 54.5 51.6 26.6 38.6

9.55 +_ 1.024 0.0194 _+0.002 I 3.3-15.5 53.0 0.35 20.0 52.1 46.4 22.2 35.4

90 95 999,5 ~ .J, percentiles

%0

,

>, 0.15 g O.lO

20

"'o., ~

~,, , "

0.05

24

28

32

36

Z,O

44

Z~8

52

Total length (c m)

Fig. 3. Overall size-frequency distribution of the catch of C. striata in (A) Mahavilachchiya and (B) Nikaweratiya (smaller) reservoirs during the respective study period. Various percentiles of the two size-frequency distributions are indicated on top of the figure.

and the mean length of the ten smallest individuals (Lmi n) of the populations in the large and smaller reservoirs are also shown in Table 1. The overall size-frequency distribution of all monthly samples (Fig. 3) showed that 50% and 90% of the catch were, respectively, under 37 cm and 45 cm (or under 29 months and 42 months of age) in the Mahavilachchiya reservoir and under 36 cm and 42 cm (or under 34 months and 47 months of age) in the smaller reservoirs. The growth constants of the populations indicate that tmin, Lmax, and L~ will be reached, respectively, in 17 months, 4.7 years and 9 years in the Mahavilachchiya reservoir, and in 16 months, 5 years and 7 years in the smaller reservoirs. The relationship between weight ( W, g) and total length (L, cm ) o f W = 0 . 0 2 5 L 2"726 ( r 2 = 0 . 8 8 , n=302, length range 24-50 cm) of the popu-

262

K,H. G.M. de Silva / Fisheries Research 19 (1994) 257-268

lation indicates that the respective weights of fish at these lengths are 194 g, 1180 g, and 1780 g in the Mahavilachchiya reservoir, and 118 g, 883 g, and 1083 g in the smaller reservoirs. Only 10% of the catch was under 27 cm and over 44 cm in the smaller reservoirs and under 30 cm and over 48 cm in the Mahavilachchiya reservoirs (Fig. 3 ). Therefore, according to the respective growth constants, over 90% of the catches of both Mahavilachchiya and the smaller reservoirs consisted of third and fourth year fish. If m a x i m u m length is taken as 0.95 L~ in the Von Bertalanffy growth equation, the longevity (~) and growth constant K are related as ~ ~ 3/K. According to this relationship, the longevities of C. striata in the Mahavilachchiya and the smaller reservoirs were 7.5 years and 8.6 years, respectively. These estimates are in close agreement with longevities of 7 years and 9 years calculated from the respective growth curves. The mean length at recruitment to the fishery ( L ' ) , as estimated by the knife-edge m e t h o d using monthly size-frequency data (Pauly, 1983 ), is 37.5 cm in the Mahavilachchiya reservoir and 31.6 cm in the smaller reservoirs. According to the respective growth constants, these lengths will be attained in about 30 months in the Mahavilachchiya reservoir and 31 months in the smaller reservoirs. The mean size at first capture (Lc) of the fish that are fully recruited to the fishery was 42.5 cm in the Mahavilachchiya reservoir and 35.4 in the smaller reservoirs. Total mortality (Z) can be estimated from the relationship

Z=K(L~ -L)/(Lo~ -L') where L is the mean length of the catch and L' is the smallest length of fish that are fully represented in the catch samples (Beverton and Holt, 1956). Ssentengo and Larkin ( 1973 ) estimated Z as

Z=nK/{ ( n+ 1)ln[ (L~ - L ' ) / (L~ - L ) ] } where n is the number of fishes used for the estimation of L. Z can also be estimated from constructing an age-converted catch curve (Pauly, 1983). Values of Z of C. striata estimated from the above methods, and from the catch curve are, respectively, 1.382, 1.557 and 1.532 for the Mahavilachchiya population and 1.350, 1.519 and 1.185 for the population in the smaller reservoirs. The natural mortality (M) of many tropical fish populations is shown to be related to the mean environmental temperature ( T, ° C ) as (Pauly, 1980) logloM= - 0 . 0 0 6 6 - 0.2701OgloL~ + 0.65431OgloK+ 0.46341oglo T The annual mean temperatures in the Mahavilachchiya and smaller reservoirs were, respectively, 30.2 °C and 29.3 ° C. Therefore, the natural mortality

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K_H.G.M. de Silva / Fisheries Research 19 (1994) 257-268

of C. striata in the Mahavilachchiya reservoir was 0.84 and 0.77 in the smaller reservoirs. Thus, using Z as estimated from the catch curve, the fishing mortality F, which is equal to Z - M , can be estimated as 0.695 in the Mahavilachchiya population and 0.418 in the population of the smaller reservoirs. The exploitation rate (E), which is the ratio between fishing mortality and total mortality, was 0.454 for the Mahavilachchiya population, but 0.353 for the population of the smaller reservoirs. The respective catch curves of the Mahavilachchiya reservoir and the smaller reservoirs showed that the size classes fully vulnerable to capture were those that were greater than 37 cm and 30 cm, respectively. The variation of the mean size of C. striata in the monthly catch from the reservoirs is shown in Figs. 4 and 5. The mean length of the monthly catch of the larger reservoir

48 !

t tt ttttt t

~42£ -~38' o 34-

30

M A 1986

M

J

J

A

S

O

N

D

Months

J F 1987

M

A

M

J

J

A

Fig. 4. Monthly mean length of the catch of C. striata in Mahavilachchiya reservoir during the study period. Vertical bars indicate the 95% confidence limits of the mean. 42-

4Ou

~

tt

38-

~_

98-

34-

32"

J F 1984

M

A

M

J

J

A

5

O

Months

N

D

J F 1985

M

A

M

J

Fig. 5. Monthly mean length of the catch of C. striata in the smaller reservoirs during the study period. Vertical bars indicate the 95% confidence limits of the mean.

264

K.H.G.M. de Silva / Fisheries Research 19 (1994) 257-268

varied more widely than that of the smaller reservoirs. In the Mahavilachchiya reservoir, the mean size increased from April to June 1986, September 1986 to February 1987 and decreased from May to September 1986 and February to April and June to August 1987 (Fig. 4 ). In the smaller reservoirs, the mean size of the catch increased from April to July 1984 and September 1984 to January 1985 and decreased from February to March and July to September 1984 and February 1984 to April 1985 (Fig. 5). Some nests and broods of fry were noted throughout the year, but breeding activity markedly increased with the onset of inter-monsoonal rains (Fig. 2 ) in the period September-October. The reservoirs receive the major portion of the rains in the northwest monsoon (Fig. 2 ). The highest numbers of nests and broods were observed during January and February at the end of the northwest monsoonal rains, when the reservoirs were full and water covered the low-lying areas. Although rains continue through the first part of the intermonsoon from February to April, the release of water from the reservoirs for irrigation purposes decreases the water level. When the water level is very low during the dry season (May-August), much of the littoral area becomes exposed and, as the dry season proceeds, grasses grow in these areas. Grazing mammals and wading birds frequenting these areas add substantial amounts of excreta. Thus, during the rainy season when the exposed area is again covered with water, suitable shallow sites among the covered vegetation will be available to C. striata for nest building and more food will be available to larvae and juveniles of fish in the form of, for example, plankton and insect larvae. It was also observed that if the rains fail and the reservoir level remained low, most C. striata do not spawn and the eggs, which have already become mature, degenerate. Such degenerating gonads can easily be recognized by their jelly-like masses of large opaque eggs. Discussion

Snakeheads form only a small component of the fishery of all reservoirs. The annual total fish catch of Mahavilachchiya is about 251 t (De Silva, 1988). The catch statistics of the five smaller reservoirs were not available, but the total catch observed during the sampling days indicate that the five reservoirs produce about 152 t annually. Channa striata forms only about 1.4% of the catch in the Mahavilachchiya reservoir and 2.3% of that in the five smaller reservoirs, the main catch (over 80%) of these reservoirs being tilapia (Oreochromis mossambicus ). If the snakehead catch of 0.0048 kg h a - 1 day- 1 of the Mahavilachchiya reservoir is taken as representative of major reservoirs, then the 103 410 ha of such reservoirs on the island (De Silva, 1988) would produce 182 t annually. Similarly, if the catch of 0.0194 kg h a - ~ day- 1 of the five smaller res-

K.tt. G.M de Silva / Fisheries Research 19 (1994) 257-268

265

ervoirs was representative of that of medium-sized reservoirs, then the 19 004 ha of such reservoirs (De Silva, 1988 ) would produce 135 t, annually. Channa striata is also caught in seasonal reservoirs. There are 39 271 ha of such minor reservoirs, but their snakehead catch varies widely and cannot be estimated with a reasonable degree of accuracy. The production of snakehead in Sri Lanka can thus be estimated as over 317 t year- ' or over 2.59 kg h a - t year- 1. In comparison with this, the snakehead fishery in southeastern Asian countries such as Thailand is quite high. For instance, the catch of snakehead landed at Central Market, Yannawa, Bangkok, alone was 946 t between September 1979 and August 1981 (Wee, 1983 ). Since the annual fish yield of reservoirs in Sri Lanka is about 307 kg ha -~ year -~ (De Silva, 1988), the contribution of C. striata to the reservoir fishery is only about 0.8%. Nevertheless, the fishery is an important one because of the high price of C. striata. Values of L~ and K o f C. striata of Sri Lanka and India have been estimated using different methods. These parameters of the Mahavilachchiya population are close to those estimated by Bhatt (1970) for an Indian population (Table 2). The L~ value of the population of the smaller reservoirs is close to that estimated for a population in the western province of Sri Lanka (Kilambi, 1986 ), but the growth constant K is about 1.7 times as higher. (L~ and K are known to differ widely among different populations of the same species of fish, even within the same country. For instance, in 12 lowland reservoirs Table 2 Growth parameters of snakehead species (Channidae) Species

C. gachua C. marulius

L~ (cm) 17.85 113.0

K

~'

Location

Method

Source t

0.50

2.09 3.29

Monthlylength-frequency data using ELEFAN I Inter-annulidistance of scales Length-frequency data (monthly data pooled at 3 month intervals )

2

0.23

SriLanka (irrigation canal ) India (river) India (river)

India (reservoir) SriLanka (marsh) Sri Lanka (large ressrvoir) SriLanka (small reservoir)

Pooledlength-frequency

1

lnter-annulidistance of scales Monthlylength-frequency data using ELEFAN I Monthlylength-frequency data using ELEFAN I

4

C. punctata

Male Female C. striata

32.35 21.28 56.5

0.20 0.45 0.42

2.20 2.19 2.96

52.0

0.21

2.61

60.0

0.40

2.99

53.0

0.35

2.84

3 5

6 6

tl, Bhatt (1970); 2, de Silva (1991); 3, Devaraj (1973); 4, Kilambi (1986); 5, Qasim and Bhatt ( 1966); 6, present study.

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K.H.G.M. de Silva / Fisheries Research 19 (1994) 257-268

in Sri Lanka, the Loo and K values of O. mossambicus differed from 26.7 to 39.3 cm total length and 0.32 to 0.70, respectively (De Silva et al., 1988)). In general, larger snakehead species appear to have smaller growth rates (Table 2). Qasim and Bhatt (1966) showed that males and females of Channa punctata have different growth rates (Table 2 ), but whether this is true for C. striata is not known. The samples obtained in the present study were too small to be analysed separately for males and females. Moreau et al. (1986) showed that the best index for growth of various populations of tilapia species can be given as qo' = log~oK+ 21ogl oLoo where L~ and K are determined using standard length data. Using this equation for C. striata, a value of 2.99 was obtained for the Mahavilachchiya population and 2.84 for the Nikaweratiya population. It is not possible to calculate this index for the other populations given in Table 1, because Loo and K have been estimated by the respective authors using the total length (i.e. the straight distance from the most anterior part of the head to the tip of the caudal fin). However, in both the Mahavilachchiya and Nikaweratiya (smaller reservoirs) populations, these parameters estimated for total length are about 1.05 times higher than those estimated using the standard length (i.e. the straight distance from the most anterior part of the head to the hypural crease). Therefore, using this ratio, approximate qO' indices of the populations were estimated (Table 2). qo' shows that the growth performance is best in Channa marulius and that the range of the index in C. striata populations is 2.662.99. It is interesting to note that the estimated ~ ' of 2.48-2.89 of O. mossambicus (the species that accounts for more than 90% of the reservoir fishery) populations in the lowland reservoirs of Sri Lanka (De Silva and Amarasinghe, 1989 ) is also close to the range estimated for C. striata. It is not clear why the growth rate of the snakehead is different in the large reservoir and the smaller reservoirs. However, the mean temperature of the large reservoir is about 1 °C higher than that of the smaller reservoirs and this difference would have an effect on the growth rates. The availability of food sources of smaller fish was similar in the large and smaller reservoirs. The differences in Loo as well as Lmin, Lmax and Lc are probably due to the differences in the fishing gear employed in the different reservoirs. The gear differences could also result in selectivity effects that could affect the growth estimates. When the recruitment to a fishery is periodical, the mean length of the sample is smallest in the month of recruitment and largest in the month prior to the recruitment (Ebert, 1987). Although such clear-cut changes of the mean length within 2 consecutive months were not apparent, the decrease of the mean length from July to September 1986 and March to April and July and August 1987 (Fig. 4) in the Mahavilachchiya (large reservoir) population and in March and from August to September 1984 and February to April

K.H.G.M. de Silva/FisheriesResearch 19 (1994) 257-268

267

1985 in the Nikaweratiya population (Fig. 5 ) may be indicative of enhanced recruitment during these periods. Because the mean age at recruitment, according to the growth curve, was 30 months and 31 months respectively in the Mahavilachchiya and Nikaweratiya reservoirs, the periods of lower mean lengths of the catch may be an indication of enhanced spawning in JanuaryMarch and September-October in the larger reservoir and March-April and September-November in the smaller reservoirs. These periods generally coincide with the beginning and end of the rainy season (Figs. 1 and 2), in which enhanced breeding activity was observed. The spawning period of C. striata in India also appears to be related to the rainy seasons (Alikunhi, 1953; Parameswaran and Murugesan, 1976). In southern India, where two monsoons prevail, there appear to be two spawning seasons coinciding with the rains. Channa marulius in southern India breeds during January-March and October-December (Chacko, 1956), and breeding of C. striata was observed in November-January and in March (Alikunhi, 1953). In northern India, where the northeast monsoon rains are absent, there appears to be only one extended breeding period (Qasim and Qayyum, 1961 ). In the Punjab area, the breeding of snakeheads extends from mid-April to the end of July (Khan, 1924), whereas in Karnataka State, C. marulius and C. striata breed from February to October/November, C. punctata from April to August, and Channa gachua from May to August (Parameswaran and Murugesan, 1976).

Acknowledgements Thanks are due to W.W. Wettewe and S.B. Weerasekera for their help in collecting the catch data of Nikaweratiya and Mahavilachchiya reservoirs, respectively, and T.S.B. Alagoda for drawing the figures. This work was supported by Grant No. AQ 722 of the International Foundation for Science (IFS), Stockholm, Sweden.

References Alikunhi, K.H., 1953. Notes on the bionomics, breeding and growth of the murrel, Ophicephalus striata Bloch. Proc. Indian Acad. Sci. Sect. B, 38: 10-20. Beverton, R.J.H. and Holt, S.J., 1956. A reviewof methodsfor estimatingmaturityrates in fish populations, with special referenceto sources of bias in catch sampling. Rapp. P.V. Reun. Cons. Int. Explor. Mer, 140: 67-83. Bhatt, V.S., 1970. Studies on the growthof Ophicephalus striata Bloch.Hydrobiologia,36:165177. Chacko, P.I., 1956. Observationson the biologyof the inland water fishes of Madras with special referenceto their suitabilityfor culture.Governmentof Madras FisheriesStation Report and Year Book, 1954-1955, pp. 247-270.

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Day, F., 1875-1878. The fishes of India; being a natural history of the fishes known to inhabit the seas and fresh waters of India, Burma and Ceylon. William, Dawson & Sons, London, 778 pp. Deraniyagala, P.E.P., 1952. A Colored Atlas of some Vertebrates from Ceylon, Vol. 1. Ceylon Government Press, Colombo, 147 pp. De Silva, K.H.G.M., 1991. Population ecology of the paddy field-dwelling fish Channa gachua (Giinther) (Perciformes, Channidae) in Sri Lanka. J. Fish Biol., 38: 497-506. De Silva, S.S., 1988. Reservoirs of Sri Lanka and their fisheries. Fish. Tech. Pap. 298, FAO, Rome, 128 pp. De Silva, S.S. and Amarasinghe, U.S., 1989. Stunting in Oreochromis mossambicus (Peters) (Pisces, Cichlidae): An evaluation of past and recent data from Sri Lankan reservoir populations. J. Appl. Ichthyol., 5:203-210. De Silva, S.S., Moreau, J. and Senaratne, K.A.D.W., 1988. Growth of Oreochromis mossambicus (Pisces, Cichlidae) as evidence of its adaptability to Sri Lankan reservoirs. Asian Fish. Sci., 1: 147-156. Devaraj, M., 1973. Biology of the large snakehead Ophicephalus marulius (Ham.) in Bhavanisagar waters. Indian J. Fish., 20: 280-301. Ebert, T.A., 1987. Estimating growth and mortality parameters by non-linear regression using average size in catches. In: D. Pauly and G.R. Morgan (Editors), Length-based Methods in Fisheries Research. ICLARM, Manila, pp. 35-44. Khan, M.H., 1924. Observations on the breeding habits of some freshwater fishes in the Punjab. J. Bombay Nat. Hist. Soc., 29: 958-962. Kilambi, R.V., 1986. Age, growth and the reproductive strategy of the snakehead, Ophicephalus striata Bloch from Sri Lanka. J. Fish Biol., 29:13-22. Moreau, J., Bambino, C.A. and Pauly, D., 1986. Indices of overall growth performance of 100 Tilapia (Cichlidae) populations. In: J.L. Maclean, L.B. Dizon and L.V. Hosillos (Editors), The First Asian Fisheries Forum, 26-31 May 1986, Manila, Philippines. The Asian Fisheries Society, Manila, pp. 201-206. Nikolsky, G.V., 1963. The Ecology of Fishes. Academic Press, London, 352 pp. (Translated from Russian. ) Parameswaran, S. and Murugesan, V.K., 1976. Breeding season and seed resources of murrels in swamps of Karnataka State. J. Inland Fish. Soc. India, 8: 60-67. Pauly, D., 1980. On the interrelationships between natural mortality, growth parameters and mean environmental temperature in 175 fish stocks. J. Conserv., 39:175-192. Pauly, D., 1983. Some simple methods for the assessment of tropical fish stocks. Fish. Tech. Pap. 234, FAO, Rome, 52 pp. Pauly, D., 1987. A review of the ELEFAN system for analysis of length-frequency data in fish and aquatic invertebrates. In: D. Pauly and G.R. Morgan (Editors), Length-based Methods in Fisheries Research. ICLARM, Manila, pp. 7-34. Qasim, S.Z. and Bhatt, V.S., 1966. The growth of the freshwater murrel, Ophicephaluspunctata Bloch. Hydrobiologia, 36:165-177. Qasim, S.Z. and Qayyum, A., 1961. Spawning frequencies and breeding seasons of some freshwater fishes with special reference to those occurring in the plains of Northern India. Indian J. Fish., 8: 24-43. Ssentengo, G.W. and Larkin, P.A., 1973. Some simple methods of estimating mortality rates of exploited populations. J. Fish. Res. Board Can., 30: 695-698. Wee, K.L., 1983. Studies in intensive snakehead (Channa spp. ) culture with special reference to their nutrition. Ph.D. Dissertation, University of Sterling, 273 pp.