Seasonal variation of mixed layer depth in the north Arabian Sea

The Arabian Sea is unique due to the extremes in atmospheric forcing that lead to the semi-annual seasonal changes. The reversing winds of summer and winter monsoon induce the variation in the characteristics of mixed layer depth. The importance of mixed layer depth is recognized in studying the biological productivity in the ocean. In this paper variability of mixed layer depth in the north Arabian Sea have been discussed. The study is based on the data collected under North Arabian Sea Environment and Ecosystem Research (NASEER) program. The results of the study indicate that there is a significant variation in the mixed layer depth from summer to winter monsoon as well as coast to offshore.

Mixed culture of fishes in seasonal ponds through fertilization and feeding

A study on mixed culture of mirror carp (Cyprinus carpio Lin.), tilapia (Oreochromis niloticus Lin.), silver carp (Hypophthalmicthys molitrix Val.) and Thai sharpunti (Puntius gonionotus Bleeker) in the ratio of 1:2:2:5 was conducted in 12 seasonal mini ponds (30 m² each) for 105 days. There were six treatments each with two replicates and each pond was stocked with a total of 100 fishes. Rice bran and mustard oil cake were used as supplemental feed either in combination or alone in presence or absence of fertilizer. Fertilizers were used in the form of organic, inorganic or both. The best growth performance of mirror carp, tilapia and Thai sharpunti was obtained in treatment III which received both fertilizer (organic+inorganic) and rice bran while the highest growth of silver carp was obtained in treatment VI receiving only inorganic fertilizer. However, the overall best production (2450 Kg/ha) and economic return for the culture period was obtained in treatment VI followed by treatment III. The results are discussed in the light of water quality parameters.

Effect of different rice brans on the growth of Thai silver barb (Puntius gonionotus Bleeker) in seasonal ponds

An experiment was conducted for a period of 110 days to study the effect of different rice brans on the growth of Thai silver barb (Puntius gonionotus, Bleeker) in rainfed seasonal ponds (30 m² each). Each of the ponds was stocked with 150 fish with the mean initial body weight of 2.03 ± 0.03 g. There were three treatments namely R1, R2 and R3 each having four replicates. Three types of rice bran namely coarse, auto (fine) and red were applied to the treatments R1, R2 and R3 respectively. Fish received different types of rice bran at a rate of 5% of body weight daily. The water quality parameters were found within the productive range. The results showed that treatment R1 produced significantly (p<0.05) highest growth and treatment R2 produced the lowest growth. The survival rates varied between 77 and 84% with treatment R3 producing the highest survival. However, the overall best production (1530 kg/ha) and economic return for the culture period was obtained in treatment R1 receiving coarse rice bran. The results of the present study demonstrated that the coarse rice bran resulted in better growth and production of P. gonionotus.

Seasonal variation in the occurrence of some zooplankton in a fish pond

Observations were made on the seasonal variation in the occurrence of some zooplankton in a fish pond. A total of 31 genera were recorded, out of these, 8 belongs to Rotifera, 5 to Copepoda, 112 to Cladocera, 3 to Protozoa, 2 to Nematoda and 1 to Oligochaeta. The role of temperature, dissolved oxygen, free Co2, pH, turbidity and alkalinity in the occurrence of different genera is discussed.

Studies on the seasonal cycle of sea surface temperatures, salinities and phytoplankton in Puttalam Lagoon, Dutch Bay and Portugal Bay along the west coast of Ceylon.

The monthly average temperatures at Puttalam Lagoon, Dutch Bay, Portugal Bay towards Kovilmunai and Portugal Bay towards Pallugaturai showed a distinct annual cycle. The peak was in April and values gradually fell till September. There was a further gradual fall in temperature from October to January. The highest temperatures in all four stations were in April. The highest salinities in all the stations were from May to October i.e., during the south-west monsoon. The salinities at Dutch Bay and Portugal Bay were high in March and April corresponding to the highest temperatures reached during these months. Two maxima have been observed in phytoplankton production. A primary maximum in May-June and a secondary maximum in October. The primary and secondary maxima are due to the influx of nutrient laden waters from the rivers Kal Aru and Pomparippu Aru. The phytoplankton producing blooms were Rhizosolenia alata. Rhizosolenia imbricata, Chaetoceros lascinosus, Chaetoceros pervianus, Ch,aetoceros diversus, Coscinodiscus gigas, Thallasionema nitzschioides, Thalassiosira subtilis, Thallassiothrix frauenfeldii, Asterionella japonica, Sceletonema costatum, Bacteriastrum varians and Biddulphia sinensis. Sudden outbursts of a single species were common. These diatoms were species of Chaetoceros and Rhizosolenia, and Thallassiothrix frauenfeldii. Wide fluctuations have been observed in the distribution of phytoplankton but no definite conclusions can be drawn as the period of observation was only one year.

Seasonal variation of alginic acid content in Sargassum cervicone Greville from Hikkaduwa

Sargassum cervicone Greville is one of the species of Sargassum occuring in abundance along the South Western Coast of Ceylon. Durairatnam 1966 carried out a survey from Ambalangoda to Hambantota, and estimated that approximately 130 tons of dried Sargassum can be harvested manually. The bulk of the Sargassum found in this area was Sargassum cervicone Greville.

Seasonal variation in the distribution and availability of Carangids ("Paraw" and "Parati") in the Wadge Bank

The Wadge Bank has been exploited regularly by Ceylon based trawlers since 1955 (Sivalingam & Medcof 1957). The bank is about 4,000 sq. miles in extent (Malpas 1926) and is located south of Cape Comorin within the Continental Shelf of India (Fig. 1). Carangids are taken in large numbers during the months of the south west monsoon (Sivalingam 1966). The quantity, in which the Carangids are captured as shown by the fishing records, is large enough to influence the total catch from the bank during these months (Fig. 2). Carangids are held in high esteem as excellent quality food fishes and accordingly constitute a commercially important group. This paper records the results of a detailed examination of the past fishing records in respect of this group, based on the analysis of the catch and fishing effort for the years 1957 to 1962. The distribution pattern of this group of fish in the Wadge Bank is discussed.

Studies on the seasonal cycle of sea surface temperatures, salinities, oxygen and phytoplankton in Koddiyar Bay, Trincomalee in relation to the shore seine fishery

In Sri Lanka waters, Durairatnam (1963, 1969) had reported on the seasonal variation of plankton at Puttalam lagoon, Dutch Bay, Portugal Bay and in the inshore and off shore waters off Colombo. Similar investigations were carried out at Koddiyar Bay especially in the estuarine waters where the three tributaries of the river Mahaveli empty their waters into the bay. The estuary is perennial. The fishes constituting the estuarine fisheries are mostly shallow water marine species which can tolerate considerable variations of salinities. Prawns and crabs also constitute a very valuable fishery in estuarine waters.

Seasonal growth of five commercially important fishes at Sofala Bank, Mozambique

An analysis of growth of scad and mackerels on Sofala Bank, Mozambique, was performed using data collected from commercial vessels during the year 1986-87, using the "Compleat Elefan" software package. Updated estimates of growth parameters of Decapterus russelli, D. macrosoma (Carangidae), and Rastrelliger kanagurta (Scombridae) were obtained which take seasonal growth oscillations into account. Preliminary growth parameter estimates are also presented for Selar crumenophthalmus (Carangidae) and Saurida undosquamis (Synodontidae).

Fish fauna of a coastal lagoon in Sri Lanka: distribution and seasonal variation

In this paper results of investigations carried out over the period of September 1977 to August 1978 are presented. These data were collected in the course of a detailed investigation into the biology and the eco-physiology of the young grey mullet and the grey mullet fishery of the Negombo Lagoon. In this paper the distribution and seasonal variation of fin fish species within the Negombo Lagoon are presented.

Studies on the fish ponds at Pitipana, Negombo 1: seasonal and diurnal variation of some hydrobiological factors

The present paper is a part of a detailed investigation on the biology of grey mullet, Mugli cephalus L., and the feasibility of its culture in the brackish waters of Sri Lanka. The authors present the results of investigation on the water quality of five experimental ponds at the Pitipana Brackishwater Fisheries Station, Negombo, carried out over a period of one year, December 1976 to November 1977, together with studies on the primary productivity are presented.

Diel feeding patterns, rate of gastric evacuation and foods of Indian sandwhiting, Sillago sihama in Mulki estuary, west coast of India

Diel feeding chronology of sandwhiting, Sillago sihama was examined from stomach collections taken during the months of April, July and December'99 in Mulki estuary along Dakshina Kannada coast, India. Significant differences in mean stomach content weight were found between several consecutive 3 hour periods with peak fullness occurring in early morning and evening hours. The rate of gastric evacuation of natural food (crustacea, polychaetes and fish) was measured in the field was best described by an exponential model, with an estimated evacuation time of 8.0 h at a temperature of 28.5 ± 1.2°C. Stomach content analysis indicated that this species is a carnivore on a wide range of benthic, epibenthic and planktonic prey. The principal food items of S. sihama were crustaceans, polychaetes and fish. Fishes less than 100 mm TL preferred mainly crustaceans while larger ones depends on polychaetes, crustaceans and fish. The feeding activity of S. sihama was influenced by tidal cycle.

Assessing seasonal precipitation trends in India using parametric and non-parametric statistical techniques

This paper provides an insight into the long-term trends of the four seasonal and annual precipitations in various climatological regions and sub-regions in India. The trends were useful to investigate whether Indian seasonal rainfall is changing in terms of magnitude or location-wise. Trends were assessed over the period of 1954-2003 using parametric ordinary least square fits and non-parametric Mann-Kendall technique. The trend significance was tested at the 95% confidence level. Apart from the trends for individual climatological regions in India and the average for the whole of India, trends were also specifically determined for the possible smaller geographical areas in order to understand how different the trends would be from the bigger spatial scales. The smaller geographical regions consist of the whole southwestern continental state of Kerala. It was shown that there are decreasing trends in the spring and monsoon rainfall and increasing trends in the autumn and winter rainfalls. These changes are not always homogeneous over various regions, even in the very short scales implying a careful regional analysis would be necessary for drawing conclusions regarding agro-ecological or other local projects requiring change in rainfall information. Furthermore, the differences between the trend magnitudes and directions from the two different methods are significantly small and fall well within the significance limit for all the cases investigated in Indian regions (except where noted). © 2010 Springer-Verlag.

Seasonal dynamics in the occurrence and abundance of Pseudo-nitzschia species in the Maheshkhali channel of the Bay of Bengal, Bangladesh

Occurrence and abundance of Pseudo-nitzschia spp. at the mouth of the Maheshkhali channel of the Bay of Bengal, Bangladesh were studied. Plankton and water samples were collected monthly from the sub-surface water during high tide at daytime from June 2000 to May 2001. Four species of Pseudo-nitzschia, namely P. pungens, P. pseudodelicatissima, P. delicatissima and P. australis were identified and among them the first three were most commonly encountered and they varied seasonally. Pseudo-nitzschia delicatissima was the dominant species during the autumn and winter months, whereas P. pungens was dominant during the summer and spring months. Pseudo-nitzschia pseudodelicatissima exhibited its highest abundance level during the summer. Surface water temperature, salinity, nitrate-nitrogen (N03-N) and phosphate-phosphorus (P04-P) were recorded and their relationship with the occurrence and abundance of Pseudo-nitzschia species were studied. At the mouth of the Maheshkhali channel, Pseudo-nitzschia cell density was highest in late autumn (November) when highest salinity (35 o/oo) and P04-P (3.2 mg/l) concentrations and low temperature (23 °C) were recorded.