Hydrographic, tidal and rainfall observations at a selected station situated near the mouth of Korapuzha

esluary during the period 1989-1993 revealed that the main causative factor which brings in variations in

salinity is rainfall and the associated river runoff rather than the tide eventhough the estuary is connected

with the Arabian sea throughout the year permitting a free flow of sea water into the estuary and a

counterflow of freh water in the opposite direction

Mathew, C V

Pillai, V N

Swarnalatha, P

Viswanathan, K P

CMFRI Digital Repository

NOTES 143 The chaetotary of legs as follows - Trochanter 1-1-1-0, Basifemur 2-2-2-2, Telofemur 5-5-2-2, Patella 4-4-3-4, Tibia 7-7-5-5; chaetotaxy of tarsi discussed in the text. Tarsus I bears 3 dorsal fossary setae, 1 solenidion, 1 profamulus besides 1 ventral filiform seta, 2 ventral distal singlest eupathidia and 4 PAS (Parambulacral setae) (Fig. 2a). Tarsus II bears 3 dorsal fossary setae, 1 solenidion 4 PAS (Fig. 2b) Tarsi I n  and IV Dept. of Biology Indian School of Learning, (Indian School of Mines A n m e )  Dhanbad - 826 004 Bihar, India. with 3 dorsal fossary setae, 1 proximodorsal seta and 2 PAS (Fig. 2c, d). All legs bear a bidentate median claw and two lateral claws. All lateral claws with a dorsal accessory tooth. The lateral claws of 11-IV faintly pectinate under high magnification. Indian Ocean specimen (300-340 pm long Idiosorna) is little bigger than Pacific Ocean specimen. This species belongs to bardi group of Bartsch (1984). B ~ m c q  I. 1984. Bull. mar. sci. 35 : 200-210. - , 1995b. (In press) J. Mar. Biol. Ass. India 37. -, 1992. Senckenbergiana biol. 72 : 465-488. S a m ,  A. L. N. AND T. CHATIERIEE 1991. J. Bombay Nal Hist. SOC. 88 (2) : 300-302. C r u m  T. 1991. J. Bombay Nut. Hkt Soc. a 0)  : 88-92. AND - 1993a. Ibid 90 (1) : 117-119. -, 1992. Ibid 89 (i) : 106-109. AND - 1993b. Ibid 90 (3) : -, 1995a. Ibid 92 (i) : 282-286 417-422. SEASONAL VARIATIONS IN SALINITY OF KORAPUZHA ESTUARY (CALICUT, KERALA) IN RELATION TO TIDE AND RAINFALL Hydrographic, tidal and rainfall observations at a selected station situated near the mouth of Korapuzha esluary during the period 1989-1993 revealed that the main causative factor which brings in variations in salinity is rainfall and the associated river runoff rather than the tide eventhough the estuary is connected with the Arabian sea throughout the year permitting a free flow of sea water into the estuary and a counterflow of freh water in the opposite direction. ' I ~ E  PRESENT study is based on hence one can expect a free flow of sea water hydrographic, tidal and rainfall obse~vations at into the estuary and a counterflow of fresh a selected station situated near the mouth of water in the opposite direction during all the the Korapuzha estuary. The estuary is connected seasons with the intensity varying from time with the Arabian sea throughout the year and to time (Fig. 1). 144 NOTES E a d i e r ~ i n t h e F e g i o n b y R a o a t l d G e o ~  (1m f a d  an invem ~ehtiowhip M e e n  dni ty  and 0 8 S C R v 4 r 1 0 1  WIN1 A R A B I A N  S E A  rainfall. No other published work is available on the variations noticed in any one of the major hydrographic conditions in the area. The station where the observations were made in the Korapuzha estuary is located about 10 km north of Calicut city. The estuary is never dry. During the rainy season, the river water from Korapuzha river and land drainage bring in large quantities of fresh water into the estuary. Since the estuary is comparatively shallow with an average depth of less than 5 m, the effect of high saline sea water is very much felt in the lower reaches of the estuary nearer to the barmouth, especially during the high tide. The tide in this region are semi- diurnal characterised by two high waters and TABLE 1. Total Number o f  Observations n two low waters during a 24 hour period. Thee is always a difference in the intensity of the first higMow tide with the second higMow tide on the same day. The area where the observations wexe made is important in view of a flourishing prawn fishery and also an upcoming edible oyster fishery apart f ~ m  the regular fishery for the typical estuarine forms. Surendranatha Kurup et a1 (1991) brought out the importance of Korapuzha estuary which serve as a nursery ground for commercially important prawns. The aim of the present investigation is to find out a possible explanation for the large scale variations noticed in the salinity of the waters.,near the mouth of the estuary. The authors wish to express their since* thanks to Dr. M. Devaraj, Director, CMFRI for his keen interest and encouragement. Salinity data pertaining to the period 1989-1993 both at surface and bottom (5 m) is made use of in the present study. Rain fall data was gathered from the IDWR charts issued by the Indian Meteorology Department. With the aid of tide tables for the respective year the tidal variations for the nearest harbour viz. Beypore (situated about 15 km away from Korapuzha estuary mouth) were noted down, It was possible to divide the observations for any given month into three different categories viz. those falling at the time of low water, those falling at the time of high water and those falling in between during the lull period. The sampling was then classified considering the period in which the observation time fell. flade during rainyldry seasons (1989-1993) Period Total Rainy Season Dry Season observations Low Salinity High Salinity Low Salinity High Salinity LW LULL HW LW LULL HW LW LULL HW LW LULL HW 1989-1990 32 8 4 NIL 2 1 1 10 3 3 NIL NIL NIL 1990-1991 34 6 4 3 NIL NIL NIL 11 5 5 NIL NIL NIL 1991-1992 31 6 3 NIL NIL NIL NIL 12 NIL 9 NIL 1 NIL 1992-1993 24 7 NIL 1 1 NIL NIL 8 2 4 1 NIL NIL During the period under study, salinity rainy season (June-September) revealed low varied between 0.19% and 35.08%. In general, salinity values clearly revealing the influence the maximum values were associated with the of rainfall and river runoff in bringing down period of high tide especially during those the salinity at the point of observation. months when rainfall was very little or nil. Low salinity was associated with rainfall and river mn off especially during the period of low tide. There were few cases where high salinity values were observed during rainy season but the tidal cycle indicated a high water or a lull period. h e r e  were also one r two cases where low salinity values were observed during the dry weather season but the tidal cycle indicated a low water or lull period. Out of a total of 121 observations made during the period 1989-1993, 47 were made during the rainy season (38.8%) and the balance 74 during dry season (61.2 %) when the rainfall was negligible or nil1 (Table 1). Among 47 observation made during rainy season, 30 were made during low water (63.8%), 12 during the lull period (25.5%) and the balance 5 during high water (10.6%). Among 74 observations made during dry season, 42(58%) were made during low .water, 11 (13.5%) during lull period and the baIance 21 (28.5%) during high water. Among a total of 73 observations made during low water, 27 observations made during Central Marine Fisheries Research blstitute Cochin - 682 014. Among a total of 43 observations made during low water, 42 observations made during the dry season (October-May) revealed high salinity values clearly revealing the inverse relationship between rainfall and salinity. From the above it may be infered that the main causative factor which brings down the salinity in the area under observation is rainfall and not the phase of the tide. Atnong 42 observations made durin$ low water refered to above, inspite of low tide conditions high salinity values prevailed in the area. Similarly among a total of 5 observations made during higli water in the rainy season, 4 observations indicated low salinity eventhough the high saline sea water was flowing into the estuary at the time of observation clearly revealing the inverse effect of rainfall and associated river runoff on salinity. During the dry season, high salinity values were always observed during high water (21 observations) clearly revealing the effect of high saline sea water in the total absence of rains. During dry 'season low salinity values were observed only rarely, on 2 occasions one during low tide and another during lull perio. * Calicut Research Centre of C.M.F.R.I., Calicut-5. ho, S. V. S. AND P. C. GEORGE 1959. J. Mar. AM) S. IAKSHMY 1991. Proc. National Workshop on low biol. Ass. India 1 (2) : 212-223. energy fislzing, Aug. 1991, Cochin, Fishery Technology KURUP, N. S., P. T. SARADA, K. KAUMUDI MENON issue) : 130-136 

Seasonal variations in salinity of Korapuzha estuary (Calicut, Kerala) in relation to tide and rainfall

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Seasonal variations in salinity of Korapuzha estuary (Calicut, Kerala) in relation to tide and rainfall

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