OF FISHES OF THE FAMILY LEIOGNATHIDAE ( PISCES ) OF THE VEMBANAD LAKE , KERALA ( S . INDIA )

The fishes of the family Leiognathidae distinguished by the protrusible jaws, and differentiated from the Gerridae by the presence of bony ridges, nuchal crest, cleithral projections and naked head (except in L. eZtmgotUB). It contributes on an average (from 1958-1967) about 2.96 percent of the total marine fish catch in India (James, 1973). Extensive studies on the Leiognathidae of India are published by Arora (1951)~ Jones (1966), Balan (1967) and James (1973). The present paper is a systematic account of the leiognathid fishes of the Vembanad Lake, their geographical and seasonal distribution and abundance in relation to the environmental conditions. The depth of the lake does not exceed beyond 5m except in the ship channel and so the vertical distributionof fishes in relation to environnlental conditions was not attempted. The hydrology of the lake has been extensively studied by Balakrishnan (1957), Ramamritham and Jayaraman (1963), George and Kartha (1963), Cherian (1967), Qasim and Gopinath (1969), Josanto (1971) and Wallershaus (1973). No attempt has been made so far to give a detailed systematic account of Vembanad Lake fishes and to relate their occurrence and abundance to the varying hydrological conditions of the lake. The available literature on the fish and fisheries of Vembanad Lake is confined to a few papers (Shetty, 1963, 1965 ; Jhingran and Gopalakrishnan, 1971 ; Gopalakrishnaya, 1972). The systematics and distribution of the leiognathid fishes are inadequate. Systematics and fisheries of lacustrine fishes have been extensively studied both in India and abroad by a number of workers, notably Chaudhuri (1916a, 1917, 1923), Coutter (1965, 1966, 1968); David (1954); Devasundaram (1954); Greenwood (1973a) ; Greenwood and Gee (1969); Holden (1967); Jacob (1961);


INTRODUCTION
The fishes of the family Leiognathidae distinguished by the protrusible jaws, and differentiated from the Gerridae by the presence of bony ridges, nuchal crest, cleithral projections and naked head (except in L. eZtmgotUB).It contributes on an average (from 1958-1967) about 2.96 percent of the total marine fish catch in India (James, 1973).Extensive studies on the Leiognathidae of India are published by Arora (1951)~ Jones (1966), Balan (1967) and James (1973).The present paper is a systematic account of the leiognathid fishes of the Vembanad Lake, their geographical and seasonal distribution and abundance in relation to the environmental conditions.The depth of the lake does not exceed beyond 5m except in the ship channel and so the vertical distributionof fishes in relation to environnlental conditions was not attempted.The hydrology of the lake has been extensively studied by Balakrishnan (1957), Ramamritham and Jayaraman (1963), George and Kartha (1963), Cherian (1967), Qasim and Gopinath (1969), Josanto (1971) and Wallershaus (1973).No attempt has been made so far to give a detailed systematic account of Vembanad Lake fishes and to relate their occurrence and abundance to the varying hydrological conditions of the lake.The available literature on the fish and fisheries of Vembanad Lake is confined to a few papers (Shetty, 1963(Shetty, , 1965 ; ;Jhingran and Gopalakrishnan, 1971 ;Gopalakrishnaya, 1972).The systematics and distribution of the leiognathid fishes are inadequate.Systematics and fisheries of lacustrine fishes have been extensively studied both in India and abroad by a number of workers, notably Chaudhuri (1916a, 1917, 1923), Coutter (1965, 1966, 1968); David (1954); Devasundaram (1954); Greenwood (1973a) ; Greenwood and Gee (1969); Holden (1967); Jacob (1961); 1 U. G. O. J'l,nior Resea•rch Fellow, Department of Marine Sciences, University of Ooohin~ Oochin-682 016.'Pro/6SSOr ana Head of the Deparl1nent, Depart1nent of Ind 'Ztstriq,l

388
Records of the Zoological Survey of I ndiu ]hingran et al., (1963); Jhingran and Natarajan (1966,1969); Jones and Sujunsingani (1954); Menon (1961) ; Pillai (1967) and Visweswara Rao (1977).The specific identity of the available species in the lake is a pre-requisite for any study on the fish and fisheries of the Vembanad Lake.
Of the fishes of the family Leiognathidae, Day (1876) reported 14 species from Indian seas; \X1eber and de Beaufort (1931) listed 16 species from Indo-Australian Archipelago; Smith (1949) listed 4 species from S. Africa; Munro (1955) reported the occurrence of 12 species from Sri Lanka waters and recently he has described a new species from New Guinea region (Munro, 1964(Munro, , 1967)).Tiews et al., (1965) recorded the occurrence of 17 species from Philippines; Kuhlmorgen-Hille (1968) reported 14 species from Gulf of Thailand which include one undescribed new species and listed 31 species of leiogna thid fishes in F. A. o. fish identification sheets (Kuhlmorgen-Hille, 1974).Monkolprasit (1973) reported the fishes of the genus Secutor from Thailand waters which include one more new species.James (1975) revised the family Leiognathidae which include the description of 18 species, out of which 17 species were collected from Indian seas which also includes two new distributional records and one new species already reported by him (James 1968, 1969).Jayabalan and Ramamoorthi (1977) reported the occurrence of 11 species from Porto Novo waters on the east coast of India.

THE VEMBANAD LAKE SYSTEM
The Vembanad backwater system is the largest on the south west coast of India, and extends between, latitude 9°28' and 10°10' Nand longitude 76°13' and 76°30'E.It has a length of about 90 km. and extends from Alleppey in the south to Thuruthippuram (east of Azheekode) in the north.The total area is about 300 sq.km.The depth varies from 1.5 to 12 m and the width varies from a few 100 m to about 14.5 km.On the northern part, there ar~ two permanent openings to the Arabian Sea, one at Cochin, a 450 m wide channel which forms the main entrance to Cochin harbour and the other at Azheekode.These regions are subjected to regular tidal influences and have all the characteristics of a typical tropical estuary.Tides in the Cochin backwaters are of mixed semidiurnal type and two successive high and low waters occur each day with substantial difference in range and time.
'-rhe main source of fresh water for the backwater is two large rivers-Periyar in the north and Pamba in the south.Four other small rivers viz.Achankoil, Manimala, Meenachil and Moovattupuzha also empty into the backwater.

MATERIALS AND METHODS
Fortnightly cruises were made in the estuarine region and n10nthly cruises were made from AUeppey to Thuruthippuram in the research vessel "Sagitta" during the year 1978-'79.Fish sanlpling surveys were conducted from various fishing operations like stake nets, chinese 5 0, °2 H 0 Y B1 ~1 t , , .shrimp nets and drag nets with the emphasis laid on species diversity and catch composition.Water samples for analysis were collected from 20 fixed stations.
The present study is based on the material collected exclusively from Vembanad Lake and the locations are plotted in the maps (Text-fig.2, 3 & 4).The number of specimens examined differed for each species because of their varied occurrence.Measurements and Records of the Zoological ~11.ll'vey of India counts were made on preserved fishes.Considerable shrinking of specimens occurred during preservation.Details of live colouration are noted for most of the species.Except for total length, all measurements were taken fronl point to point (straight line) on the left side of the fish (with mouth closed).i\ pair of dividers and vernier calipers \vere used for measurements and they were rounded off to the nearest millimeter.All proportions are in per cent of standard length except the snout length, orbit diameter and inter orbital distance which are in per cent of head length (Table 1, 2 & 3).Measurements and counts are made according to William and Venkataramani (1978) and Lowe -Me Connell (1978).
The following abbreviations are used (all measurements in nl.m.) for the characters used in this study (Text-fig.1).
SL 2 Total length, from tip of snout to tip of longest caudal fin.SL 1 Standard length, from tip of snout to caudal base (hypural bone junction).

D J
Body depth, from origin of firat dorsal spine to dorsal point of insertion of pelvic fin.
SH Head length, from tip of snout to posterior edge of opercular bone.
SO 1 Snout length, tip of snout to the first margin of eye.0 1 0 2 Orbit diameter, horiZiontal distance between the free orbit rims.
XY Dorsal spine length, from origin of sec.;onddorsal spine to its tip.Fishes of the family Leiognathidae can be easily diagnosed due to the presence of the follOWing specific characters : compressed body with deciduous cycloid scales; head scaleless (except in L. elongatu8) with bony ridges ending in a nuchal c~est.The jaws may be oblique or horboDtal and extremely protrusible, thus forming a tube.Cleithral projections are present on opercle (Jayabalan & Ramamoorthi, 1977).

Elt'
Oill membranes united with isthumus; five branchiostegal rays.A long single dorsal with 8 spines and 16 rays with conspicuous basal sheath; anal fin with 3 spines and 14 rays.Ventrals short with one spine and 5 rays with an axillary scale-like structure.Caudal fin forked • . The family Leiognathidae consists of three genera, Leiognathu8 Lecepede, Gazza Rupell and Secutor Gistel (Smith 1949, Munro 1955, Kuhlmorgell-Hille 1968, 1974, James 1975, Nelson 1976).The direction of the protrusible jaws and the nature of jaw teeth are used for differentia-tiDg the genera.The body depth in relation to standard length, length of-the second dorsal spine, presence or absence of breast scales, shape of snout and mandible, position of nostril and body colourations are also useful in differentiating the species within a genus.

I
A KEY TO THE SPEOIES OF LEIOGNATHID FISHES OF VEl\fBANAD LAKE Jaws extremely protractile, canine teeth absent.
Jaws protractile, canine teeth present.L. equulus (Forskal) ...  Occurrence and Abundance in the Lake: L. splendens occurs from •October to May in the catches from those parts of the lake (Textfig.2) where the salinity ranges from _19.2-33.3%0and temp~rature 25.0-34.2°C.From October to January its occurrence was very sporadic and the distribution was more or less confined to the mouth and lower reaches of the estuary.The penetration of this species into the interior parts of the lake \vas coincided with gradual increase in salinity gradient.It's regular occurrence in small numbers was noticed from February to May in the estuary when the salinity (23.0-33.3(/~0)and temperature (30.4 -34.2°C) were high.Occurrence and Abundance in the Lalce: L. equulus occurs throughout the year in the catches from the lake.It is one of the most abundant species and contributes about 40% of the total silver-belly fishery of the lake.These are euryhaline fishes which are able to occupy all regions of the lake (Text-fig.2) where the" salinity (0.2 -33.3%) and temperature (23.0 ---34.2°C)were highly fluctuating.It was.relatively abundant in the mouth,• middle and lower reaches of the estuary where salinity was invariably high (18.0-33.3%0) during the pre-monsoon period.The catch composition of this species was relatively more ill the northern half (Cochin-Thuruthippuram) of Vembanad Lake than southern half.The occurrence of this species in the region east of the Thannirmukham barrage was scarce where salinity does not exceed 3.7%0.During monsoon season almost freshwater condition prevailed in the lake, so that its availability was reduced but during intermonsoon months fairly good catches were noticed especially from February to May when the salinity (9.2 -33.3%0) and temperature (29.3 -34.2CO) were comparatively high.Occurrence and Abundance in the Lake: L. bindus occurs in the months from January to May in the catches from the lake.It's occurrence was strictly confined to the mouth of the estuary (Text-fig.2) when the salinty (26.2-33.3%0)and temperature (25.0-34.2°C)were uniformly high.During January and February its occurrence was very sporadic.From March to May its regular occurrence in traces was noticed in the catches when the condition prevailed in the estuary was closely related to those of the nearby inshore areas.The salinity (28.5-33.3%0)and temperature C32.5-34.2°C)were remarkably high during the above months' in th~ estuarine mouth.By examining the catch composition it was clear that this species maintains a very low level of population in the head of Cochin barmouth and at Azheekode during the months from March to May.During these months the entire estuary has a marine habitat.High salinity (29.6-33.3~~o)and temperature (32.8-34.2°C)were noticed ftom its known region of occurrence (Text-fig.2)._By examining the catch composition it is clear that this species have a meagre population in the estuary during these months and constitute only a minor local fishery.]Jistribution: Indo-Australian Archipelago, Gulf of Thailand, Sea of Penang, South Arabia, Andanlan Islands, Sri Lanka, Siam, Queensland, India-East Coast: Palk Bay, Gulf of Mannar, Coromandel Coast, Chilka .lake~nd Porto Novo;-West Coast: Goa a~d Cochin.
Height of the body at D origin less than half of S.L. Snout length greater than orbit diameter.Gill rakers on first arch (3-4)+ (13-14)= 16•18.Lateral line with 35-52 sensory tubes, ceasing before end of soft dorsal.
Occurrence and Abundance -in the Lake: L. berbis was found only in very small numbers during the first week of May from the ship channel and the "mouth of the estuary (Text-fig.3) where high salinity (33.3%0) and temperature (32.8°C) were noticed.
Distribution: Indo-Australian Archipelago.Red Sea, Zanzibar Philippines, India-East Coast: Palk Bay, Gulf of M~nnar and Porto Novo, West Coast : Goa and Cochin.
Height of the body at D origin less than half or nearly half of S.L. Snout length more than orbit diameter.Gill rakers on first arch (4-6) + (18-19)=22-25.Lateral line with 55-60 sensory tubes.
Intensity of colour varies with localities.Diffuse 12-14 irregular bands on dorsal profile.Narrow bluish green strand from isthmus to caudal peduncle in fresh condition.Opercle yellow.Golden yellow on lateral line, bordered by dusky margins.The membrane between anal spines and rays fringed with orange red colour.
Occurrence and' Ab'undance in" the lAke: L. brevirostris was equally abundant as L. equulus and its occurrence was noticed throughout the year in the catches from the lake.These are also eury haline fishes which were able to occupi all regions of the lake (Text .. fig. 3) and forms l.ocal fisheries of considerable importance.The distribution pattern of this species was more or less similar to that of L. equulus.During monsoon its availability was reduced but during intermonsoon months fairly good catches were noticed, especially from December to May  Height of the body at D origin less than half or nearly equal to half of S.L. Snout length less than orbit diameter.Gill rak~rs on first arch (5..-5)+(10-24)=.25-30.., L~teral~ line ,with 44-58 sen~ory tubes, extends almost upto two thirds of soft dorsal.
Occurrence and Abundance in the Lake: S. insidiator occurs from Septemb'er ~o middle of June 'in the' lake (Text-fig.4) where' salinity ranges lrorn 14.6•33.3%0and temperature 24.0-34.2°C.DUling :the monsoon periods the lake is influxed by freshwater as a result of heavy rainfall, so that this species completely disappears in the catches.By examining the catch composition it is clear that from September to December it was very scarce and its occurrence was more or less confined to the mouth and lower reaches of the estuary.From January to May fairly good catches were noticed /rom northern half where salinity (13.8-33.3%0)and temperature '(24.5-34.2°C)were comparatively high.Only very stray catches were noticed from the southern half of Vernbanad Lake.In those parts of the lake where the salinity (25.8-33.3%0)and temperature (30.4-34.2°C)were invariably high in the above months and nearly ma•rine conditions were prevailing.Lateral line with 60-62 sensory tubes.
Occurrence and Abundance in the Lake: G. minuta occurs very sporadically in the catches from the lake.It's occurrence was noticed only in the later half of April from the mouth of the estuary and the ship channel and was in small numbers.During the above months the estuary had a marine habitat and high salinity (32.9%0) and temperature (32.8°C) were noticed from the reported region of occurrence (Text-fig.4).

DISCUSSION
The Vembanad Lake is the largest estuarine system of Kerala and. it has a rich and diversified fish fauna.The silver-belly fishes constitute an important fishery in the lake.Shetty (1963) listed only six species of leiognathid fishes from Vembanad Lake.Among those six species, Leiognathus lineolatus (Valenciennes) and L. fasciatu8 (Lacepede) were not collected so far by the present authors.The authors collected 9 species of silver-belly fishes from the lake which are described in this paper.Menon (1961) listed 6 species of silver-belly fishes from Chilka Lake, Visweswara Rao (1976) listed 7 species from Godavari estuary.On comparison it is clear that Vembanad Lake which is smaller than the Chilka Lake has more species of leiognathid fishes.
The distribution of fishes within any water body shows a zonal pattern, both longitudinal and transverse, due to behavioral adjustments (physiological adaptation) for optimal water currents, temperatures and Records of the Zoological SU1'vey oj India chemical concentrations (John D. Hopkrik, 1973).The study on the distribution and availability of silver-belly fishes present in the Vembanad Lake shows that they evince different patterns of distribu-
Fig~ ..b!. .Leioyuathus berbi (Valeu<'ienne~) F. L, t'ioqnatlua~ bl'ct'i fo;c;tris n"alenciennesl Occurt'ence and Abdunaance in the -' Lake :: S•. ruconiu8 occurs from February to May in the catches from the lake.During, February and March only juveniles (39.0-65.0mm S. L) were present in the estuary.)was mainly confined to the northern half of Vembanad Lake and only' very stray occurrences were noticed upto lCultUP &.SAMUBL: Di8tribution of Leiognathidae 401 Thevara (South of Cochin).The occur~ence of this species was noticed Fig.V. Gazt<• f UHnuta• (Blot.:h) . 5. Comparison of body depth with standard length for Leiognathus splendens, L. equulus, L. bindus, L. da'Ura, L. berbis and L. bret:irostris.tion.The lake is permanently connected with the Arabian Sea.So there is' a regular ingress and egress of marine fishes into and from l<URUP & SAMUEL: Di8tribution oj Leiognathidae 405 the Lake, caused in part, atleast, by tidal flow.The seasonal influence on the occurrence and abundance of leiognathid fishes in the lake

TABLE L
Body pmportioDl of L • .,,.,,,4,,,,.. L. sguuZ,.and L. bift4w 8. percent of ltandam leugth and head I_Ph- Height of the body at D origin less than half of S.L. Snout length more or less equal to orbit diameter.Gill rakers on first arch (4-6)+ (15-16) = 19-22.Lateral line with 49-57 sensory tubes.Occurrence and Abundance in the Lake: L. aaura occurs from the middle of March to May in the catches from the mouth of the estuary.-iS ~ -. ~ \0 Ul Records of the Zoological Survey oj Ind,~