Availability of resources is perhaps

one of the most important factors which

determines the success of any industry.

In the case of the fishing industry, at the

basic production level the availability

of fishable concentrations of fishes and

other marine life is the decisive factor

which controls the economy of the whole

system

Pillai, V N

Puthran, V A

CMFRI Digital Repository

INTERPRETATION OF FISH SHOAL INDICATIONS IN THE ARABIAN SEA V. A. Puthran & V. Narayana Pillai Central institute of Fisheries Operatives, Cachill. J ntroduction Availability of resources is perhaps one of the most il1)p~rt2nt factors which determines the success of any industry. In the case of the fishing industry , at the basic production level. the availability of fishable concentrations of fishes and other mHine life is the decisive factor which c~ntrols the economy of the whole system. Even when well e quippe d vessel, fishing gear an d trained person-nelare available , the success of the indu-stry is dependent on the availability of fishable concentrations of commercially important marine life at the appropriate time. By the word ' fishable concentra-tions ', the implication is availability of sizeable quantities of fishes which could be definitely caught using a particular type of craft and gear in a particular area at a particular time. Natur a lly, wi th the availabi lity of fishable concentrations of comnercially important marine life, the chances of getting a good ca tch are always high. He nce it assumes great significance at the basic productio n level of the industry, June, 1974 Some knowledge regardir.g the pre-sence of similar fishable concen trations of fish and other marine life is of great importance to the technical ski:] working on board fishing vess" ls. Their technical knowledoe in fishing cou ld be put to use more-eifectively .. ,>,hen pos it ive indi-cations with regard to the avai iability of shoals are known to them. The correct and quick interpre tation of thes e indi-cations would Dlace them in a better position from where they can use their judgment towards achieving great success in this endeavour. In India, only ve ry little werk seems to have been done along these lines especially to arrive at a positive corre-lation between certain natural indica-tions and presence of fishable concent-rations of com ITl"rciallv important marine life, especially the shoaling fisbes. The work done by Ba lan (1 961 ) is worth mention ing in th is context where the auth o r has ma de certain observa tions on the shoalir:g behaviour of the oil sardine of! Calicut along the Kerala coast. 9 Fish shoals " Fish shoals are naturally accumulated group of fishes which under nor mal con-ditions show a tendency to remain to-gether. The c ommercial feasibility of operating certain specific types of nets (Purse seines, Cast net, Ring net s , Ram-pani nets, Patta bala, Mar i bala, Colli va la etc.) is dependen t on the availabi-Iityof fishable concentrations of com-mercia lly important shoaling fishes. The necessary knowledge regarding the behaviour of similar shoaling fishes will enable us to use the specific type of fishi ng g9ar m~re effectively, thereby increasing the catches . Absence of such kno wledge can result in wastage of valuable time , labour and above all m oney. The behaviour of a fish shoal is cer tainly decided by the variations observed in the various environmental factors both mete orologica l and hydro-graphic , such as dire ction and force o f wind , amount of cloudiness, phase of the moon and tide, temperature, sali-nity, dissolved oxygen conte nt . currents, pre sence of organic and inorganic p ar-ticles and their intensity and the like. Eventhough, in general , fishes are known to swim against the prevailing curre nts during day time, a t night many of them are found to drift along with th e current, sometimes evidently in th e midst of food items on which they feed. Fish shoals common around Indian waters co uld be broad ly d ivided into two major groups, viz .. I) Surface shoals 2) Submerged shoals . In the case of the first cate gory, the shoal itself or rather individual fishes (of sufficiently large size) would be visible from the d eck of a fishing ve ssel. Among the second category there c an 10 be two types viz, those which are visi-ble and those which are not visible, The visibility is u sually governed by factors such as amount of I i ght and trans p arency of the water column and above all the depth at which the shoal moves. Based on their habits, shoaling fishes give diffe rent types of indica-tions suggesting their presence ip a 1 particular area/depth of the sea . The c orrect interpretation of these indica-tions is o f great importance as otherwise there is every possibility of losing the chances of a good catch, by employing a particular type of fishing me thod. Indications and Interpretations (A) Movement in the surJace waters: When th e shoal moves up in the surface waters small sized r ipples are c reated depe nding upon the size of indiv idual fishes. It is presumed that the d ista nce between two successive wave s is more or less equal to d ouble the thickness of individual fishes. They can be spotted from a distance of 7 to 8 km, a nd are usually dark in colour. Fishes which are found in similar shoals are mostly cat fishes , sciaenids, seerlish , anchovies barracuda, carangids , macker e l, sar-d ines . gray mullets, etc. (B) Taking water through the mouth while moving: During winter when sea water temperature shows a sudden fall, certain fishes move towards the sea .ur-face and while moving they kee p their m ouths open half above the water sur-face. Cat fishes, sciaenids , Otolithus sp., gray mullets , carang ids e tc. belong to this category and can be easily spotted. (C) Showing the dorsal fin above the water level while moving: This is a typical characteristic of Cat lishes and caran-g ids. &aJood Ezport Journal (D) Flying movemenls above Ihe waler level: Certain fishes when they get frightened or when chase d by other fishes exhibit flying movements above the water surface, covering distances of 20 to 30 m. at a time. The typical exam-ples are the flying fish (E xocoetus sp.) and fishes like Hemiramphus sp. etc. (E) Keeping Ihe gills open while moving: While moving in a shoal , fishes like mackerel and sardines are known to keep their gills wide open and move sideways. (P) Splashing nlOvem' nls : Accord ing to the law of nature , bigge r carnivorous fishes feed on smaller fishes. These carnivorous fishes usually take advant-age of fish shoals. When the shoal is suddenly attacked the smaller fishes show a t9nj9noy to mJ '/e faster to escape the predators and while doing so cause a certain amount of sp lashing of wa ter at the sea surface. Eg . sar-dines, m,ckere l, silver bellies, prawns etc . The common predators encount-ered are Sharks , seer , fish dolphins, saw fish, leather skins etc. (G) Jumping movements: Some fishes are in the habit of jumping above the water line, at certain intervals. Some-tim"!s it so happens. that they push the anterior part of their bodies up and before the tail portion leaves the water surface, the head once again plunges back into the water. Sometimes similar movements are repeated .t longer inter-vals of 5 to 6 minutes. Cat fishes, sar-dines, silver bellies , anchovies etc. be-long to this category. (H) Individual fishes moving up and splashing the waler surface and then going down: In a fish shoal, sometimes indivi-dual fishes come up and go down quickly June. 1974 thereby splashing the water surface . If the movement is s low, the water move-ment caused by this sp lash ing wil l r e -semble the moveme nt caused by rainfall at the sea surface . Similar indications are characteristic of shoals of anchovies, carangids, and Ambassis sp. (near fr e sh water outle ts into the sea) (I ) Individual fi,ltes CUlling Ihe water surface and moving back: Sometimes from a moving shoal a few of the fishes move up and cut the water surface by their t,il and move down to join the shoal. If the sheal is m wing nearer to the sea surface , the shoal wou ld b e visible. Fishes like Tuna, seer fish , leather skins and carangids give similar indications. (J) Shoals which release a thin film of oily secretion: O ily secretions are chao racteristic of waters where shoals of cat fish, sharks rays (espe cially Ae lobates sp. ) , mackerel, sardine and caran g ids ~ are common . Ce rtain varieties also g ive a peculiar sme ll. One canno t a lways expect [he shoal directly underneath these oily patches as these patches are found to move along with the prevai ling wind and water currents. (K ) Changes observed in the colour of surface and bOllom walers : Generally, fishes which exhibit coloured indica-tions do not make other visible signs. The dorsal surface of a· large number of fishes in a shoal when seen from the surface reveals specific color Blions depending upon the species . In the case of fishes like pomfrets w hich exhi-bit sidew~s motion, the colou rauc n . exhibited is that of the latera l side. Bottom fish shoals like those of g o lden anchovy reveal a reddish tinge, proba-bly brought about by a comb inatio n of the dorsal colouration of some of these fishes and also the reflected-radiation 11 from ths .ea bottom . S'Hface fis h shoals, in genera l, are bl.ckish in colour. The ab~ve said .feature is noticed in the following · varities of fishes viz. Crescent perches, ~harks, sales, prawns, white fish. rays, kilirneen, rib-bon fishes , polynemids etc. ' (L) Bubbles appear;l'g at 'he sea surface: When cer tain fish shoa ls move under-neat h we are likely to come across either individual or large number of bubbles in the surface layers. In genera l, bubble forming shoals are found to move rat~er s lowly. Fishes which exhibit thi s characteris tic are sardines, engraulids , cat fishes, caran-gids, whit e fish etc. eM) SlolV movrml?llf of surface wafer above the shoal: It is rather difficult to differen tiate be tween a similar s low motion El f water caused by a moving s hoal (su bmerged ) and an ordinary low intensity water current caused by a favourable wind. If the water column is s ufficie ntl y cl "! ar, P3rt of the shoal itself would be visible. The following major type s of water motion a re observed in the case oC fishes noted against each: J2 (1) Formation of a small wave caus-ed by the movement of a shoal from one place to another very near the surface. (Mackerel, s ardines etc.) (2) Surface water moving with low in tensity sp lashing noise caused by a moving s hoal. (Carangid.) (3) Formatio n of a whirling moti",n cause d by the movement of a fish shoal which moves in cir--eles. (Mackerel, sardines. caran-gids & cat fishes) Sometimes th e speed of the shoal and the result ing water motion at the surface can b e so high th a t it can capsize small boa ts' which go nesr th e shoal. (4) Whe n a sufficiently fast moving fish sho al moves at the sea bot-to m, particularly in shallow areas, muddy wate r usually comes up towards the s urface . 4 layers. This is especially the cas e in areas where the sea bot-tom is composed of loose mud. (Mackerel, sardines, carangids, anchovies , cat fishes, sa les & rays). (N) Shoals which are visible at night due to phosphorescence: Certain fish shoals are bioluminiscent. Fishes like Silve r bellies (Leiognathids) , Bombay duck (Harpodon sp .) e tc. will illumin a te a large area of the sea and at times one may find it d ifficult to differentiate tbese fi sbes fro m the bright light reflected back from th e sea. FALSE INDICATIONS: Some of the a b ove said natural indications can also become fa lse indications as tbe parti-cular indication may not be due to the presence of a fish shoal. For ego small waves which are caused by a favo u r-able wind, current boundaries where two adjoining current streams form a mixed layer, places of diverging or converging cu rrents, outlets of indu-strialor'any type of naturally o r artifi-cially contaminated se wage flow, fl oat-ing oil which has leaked out into the sea Cram oil tankers and the like , shado w of low clouds especially when they are moving , presence of swarms of other marine liCe (eg. Noctiluca sp.) which gives colour to the sea, under water r ocks, flock of birds , bubbles r e leased from the bottom mud in areas where Seafcod Export Journal chemical decom position is in progress etc. c.n act as false indications. Special care has to be taken whIle differentiat-ing between the above said false indi-cations and the real indications mention-ed under the items A to N. , before con-firming the presence of a fish shoal and . taking further measures fo r exploiting the same. EFFECT OF ENVIRONMEN TAL FACTORS ON FISH SAOALS : I. Effect of Wind: Along the west coast of India, in the presence of light eas te rly wind, the general tendency of a fish shoal is to move down into deeper water. This in turn can produce one of the indications listed under item M. I to 4 mentioned earlier. When the wind is northerly the genera l tendency noticed in these areas is the shoal moving up toward s surface waters. When the wind is southerly the effect is just the opposite. With a west-erly wind the shoal shows a tendency to move shorewards . In rough weather fish shoals are generally foun d to m ove down into deeper water. II. Effect of Light: In general , sho aling fishes show a tendency to move towards light. Because of this, during morning hours shoals are likely to face the east and towards evening, the west. A more or less similar tendency is also noticed with the rising and setting moon. Most of the ffshes, especially Hemiramphus sp. and certain varieties of prawns are attracted towards artificial light at night . In Ram-pani fishing, artificial lights are com-monly used to lure the shoal into smaller nets to be transported to the shore. Jun., 1974 . III. Effect of variations in temperature: Every individual species of fish has an optimum temperature eventhough ' eurythermal fishes can survive wi thin certai n variations of temperature. It is generally observed that fishes show a tendency to aggregate together and form shoals when the water tempera ture falls below a certain optimum limit. When the water temperature rises, the fishes in a shoal are likely to spread out. This m ight be the reason why we come across more number of fish shoals around Indian waters during the months September to December when the sea water temperature is likely 10 fall. Dur-ing the months March to July, when the sea is comparatively warm in these areas. the number of fish shoals obse r-ved are less. Certain shoa ling fishes show a tendency to float at the sea sur-face during th e pe.k oi winter. During such time, these fis hes are found to be comparative ly s luggish . Eg. Rays, Cra bs and the like. IV. Effect of currents: In general shoaling fishes are found to move against currents during the day time. During the night time, especially in the presence of large quantities of food materials, fishes are known to drift a long with a particular water body which contains the food item. When the curre nt speed increases beyond a cer-tain optimum limit , e ven during day time, shoaling fishes are found to drift along with the currents. But while drift-ing it is observed that individual fishes in ·a.shoal always face the current even-though they drift along with the stream . V. Effect of variations in salinity: In areas where rivers discharge sufficiently large quantities of fresh water into the sea, the tendency noticed 13 is the spreading of fresh water at the surface (due to a low density) in the form of a to ngue into the sea . It is generally ob served tha t fish s hoals always try to avoid the tong ue of fresh water either by diving into deeper water or b y occupying the two s ides of the tongue where the effect of fresh wate r is neg ligible . The same tendency is also noticed in the presence of a sudden shower especially in localised areas eventhough it is quite possible that some of the euryhaline .fishes may be able to withstand a certain variation in salinity. V/' Effect of variations in the dissoh'ed oxygen content: Majo rity of the commercially import-ant fishes require an optimum concent-r ation of dissolved oxygen in ,) he sur-r ounding waters. If d ue to so me r e ason the quantity of dissolved oxygen falls below a certain minimum, they show a tendency to migrate toward s favourable environments. It has ' been observed tha t at certain times o xygen dep letion can result in more or less complete absenr.e of shoaling fishes in a particu-lar region of )he sea. Oxygen depletion is chuacteristic of enclosed or semi-enclosed water bodies where it is caused by poor vertical/ lateral circula-tion proce sses. In the open s ~a th e m3.in cause seems to be the putrification of large quantities of dead organic ma tter coupled with poor circulation proce sses. An earlier record of a simi-lar phenomenon reported from the shelf waters off Bombay (Banse, 1968) is worth mentioning in this context. This is espe-cially true d u r ing the S. W. monsoon period in areas where the p r oce ss of upwelling is active thereby bringing bottom waters with low disso lved o xy-gen concentrations to shallower depths. Banse (1959) has a lso shown tha t d ur ing 14 the S. W. monsoon per iod off Cochin, the decrease in the oxygen content of the nearbottom waters results in the disappearance of demersal fi shes espe-cially the d ominant variety, Nemep-terus japonicus. Conclusions The importance of knowing some of the above said indicatio ns a long with their correct and speedy interpretation need not be stressed again . A fair knowledge regarding the various envi-r onme ntal factors and their effect o n the behaviour of shoa ling fishes will cer-tai n ly enable the skipper of a fi sh ing vesse l to exploit the resources in a be tte r way. This will also help h im to use the right type of gear and othe r a ccessories to catch a particula r variety of fish under ce r tain specific conditions. AcknolVledgements The authors wish to exxpress their sincere thanks to Mr. M. C. Perumal , Director , Central Institute of Fishe r ie s Operatives, Cochin fo r his e ncourag e-ment and keen interest in the p resent study and a ls o permitting the m to publish this note . RE F ER ENCES Balao, V. 1961 Some observations on the ihoa ling behaviour of the" oil sard ine , Sardinella /o llgiceps Val. Ind. J. F ish. Vol. VlIl , No. 1 : 207-221. Banse, K. 1959 On upwelling and bottom trawling off the S. W. coast of India. J. Mar. Bio I. Ass. India t (1) : )3 49. Banse, K. 1968 Hydrography of tbe Arabian sea shelf of India and Pakistan and effects on demersal fishes . Deep sea Res., 15 (1) : 45-80. Jlmo Hela & Taivn Laevastu 1970 Fisheries Hydro-graphy. Fish ing News (Books) LondoD. • • Seafood Export Journal 

Interpretation of fish shoal indications in the Arabian Sea

http://eprints.cmfri.org.in/7449/1/462-SEAFOOD_EXPORT_JOURNAL___1974.pdf

Interpretation of fish shoal indications in the Arabian Sea

Abstract

Similar works

Full text

Available Versions

CMFRI Digital Repository