INTERPRETATION OF FISH SHOAL INDICATIONS IN THE ARABIAN SEA

V. A. Puthran & V. Narayana Pillai Central institute of Fisheries Operatives, Cachill.

Jntroduction Some knowledge regardir.g the pre­ sence of similar fishable concentrations Availability of resources is perhaps of fish and other marine life is of great one of the most il1)p~rt2nt factors which importance to the technical ski:] working determines the success of any industry. on board fishing vess" ls. Their technical In the case of the fishing industry, at the knowledoe in fishing could be put to basic production level. the availability use more-eifectively ..,>,hen posit ive indi­ of fishable concentrations of fishes and cations with regard to the avaiiability of other mHine life is the decisive factor shoals are known to them. The correct which c~ntrols the economy of the whole and quick interpretation of these indi­ system. Even when well e quipped cations would Dlace them in a better vessel, fishing gear an d trained person­ position from where they can use their nelare available, the success of the indu­ judgment towards achieving great stry is dependent on the availability of success in this endeavour. fishable concentrations of commercially important marine life at the appropriate In India, only very little werk seems time. By the word' fishable concentra­ to have been done along these lines tions', the implication is availability of especially to arrive at a positive corre­ sizeable quantities of fishes which could lation between certain natural indica­ be definitely caught using a particular tions and presence of fishable concent­ type of craft and gear in a particular rations of comITl "rciallv important area at a particular time. Naturally, with marine life, especially the shoaling the availability of fishable concentrations fisbes. The work done by Ba lan (1 961 ) of comnercially important marine life, is worth mentioning in th is context the chances of getting a good ca tch are where the autho r has ma de certain always high. He nce it assumes great observations on the shoalir:g behaviour significance at the basic productio n of the oil sardine of! Calicut along the level of the industry, Kerala coast.

June, 1974 9 Fish shoals " be two types viz, those which are visi­ ble and those which are not visible, Fish shoals are naturally accumulated The visibility is u sually governed by group of fishes which under nor mal con­ factors such as amount of Ii ght and ditions show a tendency to remain to­ transp arency of the water column and gether. The c ommercial feasibility of above all the depth at which the shoal operating certain specific types of nets moves. Based on their habits, shoaling (Purse seines, Cast net, Ring nets , Ram­ fishes give diffe rent types of indica- pani nets, Patta bala, Mar i bala, Colli tions suggesting their presence ip a 1 vala etc.) is dependent on the availabi­ particular area/depth of the sea. The Iityof fishable concentrations of com­ c orrect interpretation of these indica­ mercially important shoaling fishes. The tions is of great importance as otherwise necessary knowledge regarding the there is every possibility of losing the behaviour of similar shoaling fishes will chances of a good catch, by employing enable us to use the specific type of a particular type of fishing method. fishi ng g9ar m~re effectively, thereby increasing the catches. Absence of such Indications and Interpretations knowledge can result in wastage of valuable time, labour and above all (A) Movement in the surJace waters: m oney. The behaviour of a fish shoal When the shoal moves up in the surface is cer tainly decided by the variations waters small sized r ipples are c reated observed in the various environmental depe nding upon the size of individual factors both mete orological and hydro­ fishes. It is presumed that the d istance graphic, such as dire ction and force of between two successive wave s is more wind, amount of cloudiness, phase of or less equal to d ouble the thickness of the moon and tide, temperature, sali­ individual fishes. They can be spotted nity, dissolved oxygen conte nt. currents, from a distance of 7 to 8 km, a nd are pre sence of organic and inorganic p ar­ usually dark in colour. Fishes which ticles and their intensity and the like. are found in similar shoals are mostly Eventhough, in general, fishes are cat fishes, sciaenids, seerlish, anchovies known to swim against the prevailing barracuda, carangids, macker e l, sar­ curre nts during day time, at night many d ines. gray mullets, etc. of them are found to drift along with the current, sometimes evidently in the (B) Taking water through the mouth midst of food items on which they feed. while moving: During winter when sea water temperature shows a sudden fall, Fish shoals common around Indian certain fishes move towards the sea .ur­ waters could be broadly d ivided into face and while moving they keep their two major groups, viz .. m ouths open half above the water sur­ I) Surface shoals face. Cat fishes, sciaenids , Otolithus sp., 2) Submerged shoals. gray mullets, carangids e tc. belong to this category and can be easily spotted. In the case of the first cate gory, the shoal itself or rather individual fishes (C) Showing the dorsal fin above the (of sufficiently large size) would be water level while moving: This is a typical visible from the d eck of a fishing ve ssel. characteristic of Cat lishes and caran­ Among the second category there c an g ids.

10 &aJood Ezport Journal (D) Flying movemenls above Ihe waler thereby splashing the water surface . If level: Certain fishes when they get the movement is s low, the water move­ frightened or when chase d by other ment caus ed by this splashing wil l r e ­ fishes exhibit flying movements above semble the moveme nt caused by rainfall the water surface, covering distances of at the sea surface. Similar indications 20 to 30 m. at a time. The typical exam­ are characteristic of shoals of anchovies, ples are the flying fish (E xocoetus sp.) carangids, and Ambassis sp. (near fr e sh and fishes like Hemiramphus sp. etc. water outle ts into the sea)

(E) Keeping Ihe gills open while moving: (I) Individual fi,ltes CUlling Ihe water While moving in a shoal, fishes like surface and moving back: Sometimes from mackerel and sardines are known to a moving shoal a few of the fishes move keep their gills wide open and move up and cut the water surface by their sideways. t,il and move down to join the shoal. If the sheal is m wing nearer to the sea (P) Splashing nlO vem' nls : According surface, the shoal would b e visible. to the law of nature, bigger carnivorous Fishes like Tuna, seer fish , leather skins fishes feed on smaller fishes. These and carangids give similar indications. carnivorous fishes usually take advant­ age of fish shoals. When the shoal is (J) Shoals which release a thin film of suddenly attacked the smaller fishes oily secretion: O ily secretions are chao show a t9nj9noy to mJ'/e faster to racteristic of waters where shoals of cat escape the predators and while doing fish, sharks rays (espe cially Ae lobates so cause a certain amount of splashing sp.), mackerel, sardine and caran g ids ~ of water at the sea surface. Eg. sar­ are common . Ce rtain varieties also g ive dines, m,ckerel, silver bellies, prawns a peculiar sme ll. One canno t a lways etc. The common predators encount­ expect [he shoal directly underneath ered are Sharks, seer, fish dolphins, these oily patches as these patches are saw fish, leather skins etc. found to move along with the prevailing wind and water currents. (G) Jumping movements: Some fishes are in the habit of jumping above the (K ) Changes observed in the colour of water line, at certain intervals. Some­ surface and bOllom walers : Generally, tim"!s it so happens. that they push the fishes which exhibit coloured indica­ anterior part of their bodies up and tions do not make other visible signs. before the tail portion leaves the water The dorsal surface of a·large number of surface, the head once again plunges fishes in a shoal when seen from the back into the water. Sometimes similar surface reveals specific color Blions movements are repeated .t longer inter­ depending upon the species . In the vals of 5 to 6 minutes. Cat fishes, sar­ case of fishes like pomfrets w hich exhi­ dines, silver bellies, anchovies etc. be­ bit sidew~s motion, the colou rauc n long to this category. . exhibited is that of the lateral side. Bottom fish shoals like those of g olden (H) Individual fishes moving up and anchovy reveal a reddish tinge, proba­ splashing the waler surface and then going bly brought about by a comb inatio n of down: In a fish shoal, sometimes indivi­ the dorsal colouration of some of these dual fishes come up and go down quickly fishes and also the reflected-radiation

June. 1974 11 from ths .ea bottom . S'Hface fish shoals, resulting water motion at the in general, are bl.ckish in colour. surface can b e so high th a t it can capsize small boats' which The ab~ve said .feature is noticed in the following · varities of fishes go nesr the shoal. viz. Crescent perches, ~harks, sales, (4) When a sufficiently fast moving prawns, white fish. rays, kilirneen, rib- fish shoal moves at the sea bot- bon fishes, polynemids etc. ' to m, particularly in shallow areas, muddy water usually (L) Bubbles appear;l'g at 'he sea surface: comes up towards the s urface . 4 When certain fish shoa ls move under­ layers. This is especially the neath we are likely to come across cas e in areas where the sea bot- either individual or large number of tom is composed of loose mud. bubbles in the surface layers. In (Mackerel, sardines, carangids, general, bubble forming shoals are anchovies, cat fishes, sales & found to move rat~er s lowly. Fishes rays). which exhibit this characteristic are sardines, engraulids, cat fishes, caran­ (N) Shoals which are visible at night due gids, white fish etc. to phosphorescence: Certain fish shoals are bioluminiscent. Fishes like Silver eM) SlolV movrml?llf of surface wafer bellies (Leiognathids), Bombay duck above the shoal: It is rather difficult to (Harpodon sp .) e tc. will illuminate a differentiate between a similar s low large area of the sea and at times one motion El f water caused by a moving may find it d ifficult to differentiate tbese s hoal (submerged ) and an ordinary low fi sbes from the bright light reflected intensity water current caused by a back from the sea. favourable wind. If the water column is s ufficie ntly cl"! ar, P3rt of the shoal itself would be visible. FALSE INDICATIONS: Some of the ab ove said natural indications can also The following major type s of water become fa lse indications as tbe parti­ motion are observed in the case oC cular indication may not be due to the fishes noted against each: presence of a fish shoal. For ego small (1) Formation of a small wave caus­ waves which are caused by a favour­ ed by the movement of a shoal able wind, current boundaries where from one place to another very two adjoining current streams form a near the surface. (Mackerel, mixed layer, places of diverging or s ardines etc.) converging currents, outlets of indu­ strialor'any type of naturally o r artifi­ (2) Surface water moving with low cially contaminated sewage flow, fl oat­ intensity splashing noise caused ing oil which has leaked out into the sea by a moving s hoal. (Carangid.) Cram oil tankers and the like, shado w of (3) Formatio n of a whirling moti",n low clouds especially when they are cause d by the movement of a moving, presence of swarms of other fish shoal which moves in cir-­ marine liCe (eg. Noctiluca sp.) which eles. (Mackerel, sardines. caran­ gives colour to the sea, under water gids & cat fishes) Sometimes r ocks, flock of birds, bubbles r e leased the speed of the shoal and the from the bottom mud in areas where

J2 Seafcod Export Journal chemical decom position is in progress III. Effect of variations in temperature: etc. c.n act as false indications. Special Every individual species of fish has care has to be taken whIle differentiat­ an optimum temperature eventhough' ing between the above said false indi­ eurythermal fishes can survive within cations and the real indications mention­ certain variations of temperature. It is ed under the items A to N., before con­ generally observ ed that fishes show a firming the presence of a fish shoal and tendency to aggregate together and . taking further measures fo r exploiting form shoals when the water temperature the same. falls below a certain optimum limit. When the water temperature rises, the EFFECT OF ENVIRONMENTAL fishes in a shoal are likely to spread out. FACTORS ON FISH SAOALS : This m ight be the reason why we come across more number of fish shoals I. Effect of Wind: around Indian waters during the months Along the west coast of India, in the September to December when the sea presence of light easterly wind, the water temperature is likely 10 fall. Dur­ general tendency of a fish shoal is to ing the months March to July, when the move down into deeper water. This in sea is comparatively warm in these turn can produce one of the indications areas. the number of fish shoals obse r­ listed under item M. I to 4 mentioned ved are less. Certain shoali ng fishes earlier. show a tendency to float at the sea sur­ face during the pe.k oi winter. During When the wind is northerly the such time, these fis hes are found to be general tendency noticed in these areas comparatively s luggish. Eg. Rays, Cra bs is the shoal moving up towards surface and the like. waters. When the wind is southerly the effect is just the opposite. With a west­ IV. Effect of currents: erly wind the shoal shows a tendency to In general shoaling fishes are found move shorewards . In rough weather to move against currents during the day fish shoals are generally found to m ove time. During the night time, especially down into deeper water. in the presence of large quantities of II. Effect of Light: food materials, fishes are known to drift a long with a particular water body In general, sho aling fishes show a which contains the food item. When the tendency to move towards light. Because curre nt speed increases beyond a cer­ of this, during morning hours shoals are tain optimum limit, e ven during day likely to face the east and towards time, shoaling fishes are found to drift evening, the west. A more or less along with the currents. But while drift­ similar tendency is also noticed with the ing it is observed that individual fishes rising and setting moon. Most of the in ·a.shoal always face the current even­ ffshes, especially Hemiramphus sp. and though they drift along with the stream. certain varieties of prawns are attracted towards artificial light at night. In Ram­ V. Effect of variations in salinity: pani fishing, artificial lights are com­ In areas where rivers discharge monly used to lure the shoal into smaller sufficiently large quantities of fresh nets to be transported to the shore. water into the sea, the tendency noticed

Jun., 1974 . 13 is the spreading of fresh water at the the S. W. monsoon period off Cochin, surface (due to a low density) in the the decrease in the oxygen content of form of a tongue into the sea. It is the nearbottom waters results in the generally ob served that fish s hoals disappearance of demersal fi shes espe­ always try to avoid the tongue of fresh cially the d ominant variety, Nemep­ water either by diving into deeper water terus japonicus. or b y occupying the two s ides of the tongue where the effect of fresh water Conclusions is neg ligible. The same tendency is The importance of knowing some of also noticed in the presence of a sudden the above said indicatio ns a long with shower especially in localised areas their correct and speedy interpretation eventhough it is quite possible that some need not be stressed again. A fair of the euryhaline .fishes may be able to knowledge regarding the various envi­ withstand a certain variation in salinity. r onmental factors and their effect o n the behaviour of shoaling fishes will cer­ V/' Effect of variations in the dissoh'ed tainly enable the skipper of a fi shing oxygen content: vesse l to exploit the resources in a Majority of the commercially import­ better way. This will also help him to ant fishes require an optimum concent­ use the right type of gear and other r ation of dissolved oxygen in,) he sur­ a ccessories to catch a particular variety r ounding waters. If d ue to so me r e ason of fish under cer tain specific conditions. the quantity of dissolved oxygen falls below a certain minimum, they show a AcknolVledgements tendency to migrate toward s favourable environments. It has' been observed The authors wish to exxpress their that at certain times o xygen depletion sincere thanks to Mr. M. C. Perumal, can result in more or less complete Director, Central Institute of Fisheries absenr.e of shoaling fishes in a particu­ Operatives, Cochin fo r his e ncourage­ lar region of )he sea. Oxygen depletion ment and keen interest in the p resent is chuacteristic of enclosed or semi­ study and als o permitting them to enclos ed water bodies where it is publish this note. caused by poor vertical/lateral circula­ RE F ER ENCES tion processes. In the open s ~a the m3.in cause seems to be the putrification Balao, V. 1961 Some observations on th e ihoaling behaviour of the" oil sard ine, of large quantities of dead organic Sardinella /o llgiceps Val. Ind. J. F ish. matter coupled with poor circulation Vol. VlIl, No. 1 : 207-221. proce sses. An earlier record of a simi­ Banse, K. 1959 On upwelling and bottom trawling lar phenomenon reported from the shelf off the S. W. coast of India. J. Mar. waters off Bombay (B anse, 1968) is worth Bio I. Ass. India t (1) : )3 49. mentioning in this context. This is espe­ Banse, K. 1968 Hydrography of tbe Arabian sea cially true d ur ing the S. W. monsoon shelf of India an d Pakistan and period in areas where the p r ocess of effects on demersal fis hes. Deep sea upwelling is active thereby bringing Res., 15 (1) : 45-80. bottom waters with low dissolved oxy­ Jlmo Hela & Taivn Laevastu 19 70 Fisheries Hydro- gen concentrations to shallower depths. graphy. Fishing News (Books) Banse (1959) has a lso shown that d uring LondoD . • •

14 Seafood Export Journal