A large number of standing bodies are available in India for fishery management purpose. Powai Lake is one of them, situated in metropolitan city of Mumbai with a total area of 210 hectors and mean depth of 3.2 meters. Powai Lake is situated at L 19 O 8' N and L 72 O 54' E, 27 kms away in the North-East of Mumbai city.
This Lake came into existence in the year 1891, when Mumbal Municipality got constructed a monsonary dam of 10 meter height between two hillocks across Powai basin to conserve the rain water for drinking purpose, which was later commonly known as Powai Lake, since it impounded in Powal area. However, the water in the lake was found unpotable because of indiscriminate use of the lake for various purposes by the local people and also due to the addition of domestic waters. This lake was open for the general utilization of the natives and it is leased out to Maharashtra State Angler's Association -Mumbai for angling in addition to conservation.
The conservation of Powai Lake is in the interest of man as its ecological, cultural and touristic value. One of the most important step in the conservation of the lake is to restore the water quality by controlling the pollution through different remedial measures. This lake has been included in the Central Government National Lake Conservation Plan.
Limnological Survey of Powai Lake was under taken from May,1995 to December,1996 with a view to investigate the various changes in its hydrological features durin- pre-monsoon, monsoon, and post-monsoon seasons and correlated the same with the plankton productivity.

MAIN OBJECTIVES OF STUDY.

The main objectives for undertaking the project on the hydrobiolocical studies of Powal Lake:

a) To find out the seasonal hydrobiological variations for ascertaining the limiting factors responsible in favour of declining the lake productivity.

b) To assess the primary productivity of the lake and investigate the possibilities of improving natural productivity both qualitatively and quantitatively.

c) To identify the various constraints for the development and management of Powai Lake and its fisheries and evolve suitable remedial measures.

HYDROLOGY:
To study the hvdrobiology of Powal Lake, first of all a survey of the lake was made and Four random sampling stations were selected. Water samples were collected in every month between 7.30 a.m. to 8.30 a.m. with help of Mayer's Water sampling bottle.

TEMPERATURE:
During the study period, the temperature showed a range of fluctuation. On the basis of surface and sub-surface temperatures, the lake is homothermous throughout the year, except in May,1995. The range of water temperature was 23.625 O C (Feb,96) to 31.50 OC(May,96).

SECCHI DISC TRANSPARENCY:

It is the indicator of the penetration of 1ight into the water column, ranged between 51.25 to 97.25 cm. The lowest value was observed in the month of June,96 due to dense plankton and organic load.

DISSOLVED OXYGEN:

The dissolved oxygen content was found 3.30 to 6.31 mg/I. The lowest beina 3.30 mg/l in the month of June,1995 at sampling station D, but the hichest value recorded was 6.31 m-/l in the month of Au-ust,1996 on all the sampling stations.

FREE CARBON-DIOXIDE

The fluctuation of carbon-dioxide values in pre-monsoon, monsoon and post-monsoon seasons were 1.09 to 1.58, 1.78 to 5.549 and 1.672 to 5.45 mg/l respectively during the study period. The maximum concentrations were seen at sampling station D reciprocating inverse of dissolved oxycen values.

pH:

The pH of lake water was almost alkaline all around the year in the range of 7.133 to 8.128. This range of pH is in favourable for the growth of fish and culture.

SOLIDS

It is the residue after evaporation and drving between 102 to 105 O C. The highest amount of solids have been noticed at the C and D stations.

TOTAL HARDNESS:

The total hardness was found to vary from 104 to 171 ml/I during the study period.

TOTAL ALKALINITY:

It was found to range between 81.250 to 141.00 mg/I. The lesser concentration of total alkalinity in the lake water during rains is mainly due to its dilution with rain water.

CHLORIDES :

The maximum chloride concentration was recorded to be 58.48 mg/l in the month of June,1996 while the minimum was 25.99 mg/l during Oct,1995. This is mainly due to substantial part of sewage and other wastes reaches into the lake directly from surrounding habitats.

PHOSPHATE - PHOSPHORUS:

The seasonal changes in the quantities of soluble phosphate-phosphorus was recorded highest 0.665 mg/l in Jun,1995 and lowest was 0.323 mg/l in Nov,1995.

SILICATES:

Silicate exhibited 3.06 mg/l (Jun,1996) to 27.929 mg/l (Jan,1995)ln the lake throughout the period of study.

NITRATES:

Nitrates of lake water fluctuates between less than 0.120 mg/l in Jan,1996 and 0.625 mg/l in June,1995.

NITRITES :

Nitrite during the period of study fluctuated in between 0.002 mg/l (Dec,1996) and 0.168 mg/l (May,1995)

TURBIDITY :

Turbidity of water fluctuates between 2.275 mg/l in the month of May,1996 and 7.90 in the month of Jun,1995. In monsoon, due to high velocity of wind. which brings bottom sediments to the surface and so increases the turbidity.

PLANKTON:

The qualitative and quantitative estimation of plankton population were investigated during studies period. Plankton of Powai lake showed wide fluctuation both general wise as well as seasonally at all the sampling stations. Numeral percentage of phytoplankton was more than zooplankton during most of the months of study.

PHYTOPLANKTON :

The most common forms of the phytoplankton of the lake were Pediastrum and Navicula sps These forms were present throughout the year. The next form which occurred more consistently were Microcystis sps. The maximum phytoplankton concentration was recorded during the month the January-February. Bacillriophyceae dominated through out the study period with almost 9.40 % to 50.92 % contribution.

ZOOPLANKTON :
The zooplankton maximum was recorded in the month of February and minimum in July. Rotifers were the main dominant group during the study period followed by copepods. It was observed that zooplankton population increased during postmonsoon period as compared to pre-monsoon and monsoon seasons.

DIURNAL VARIATIONS

Diurnal variations of both phytoplankton and zooplankton were studied in different time. Their rhythmic movements are in accordance with changing, light intensity. It is seen that Entemostraca are more influenced toward changing light intensity than rotifers. Rotifers, are seen that they remain concentrated more between 0-3.2 meters and thus their range of migration is limited.

PRIMARY PRODUCTIVITY

During the study period the maximum gross primary productivity observed was 4324.00 mg c/m 3 /day in the month of September,1996, while the minimum value was found to be 600.00 mg c/m 3 /dav in the month of December,1995.

SOIL TEXTURE:

The bottom soil of lake was in general sandy-clay in texture and normal in quality with organic carbon of 0.71 %. Texture of soil studied at all stations with their percentage composition. Recorded Soil was 6.5 i.e. slightly acidic in nature. Its water retention capacity is quite hi-h, hence good for fish culture.

MORPHOEDAPHIC INDEX :

MEI =TDS/Z
It is the ratio of two easily measured parameters like solids [ TDS in mg/1] and the mean depth(Z in meter). The former being and edaphic factor reflecting the nutrient levels prevailing in thesystem, the latter is morphometric factor. The fish yield can be calculated from the equation.

MACROVEGETATION :

A large area of the lake was found to be covered with water hyacinth, Eichhornia crassipes, which was found to be dominating throughout the year.

FISH POPULATION:

Powai Lake has a fairly rich fauna of fishes, but now-a-days due to the high discharge of waste waters, building raw materials, cement and most of the heavy metals like lead of vehicles, silt load from the surrounding area due to quarrying work, it has lost its diverse nature of fish population. The major fish are Indian Carps, Catla Labeo rohita, Cirrhinus mrigala and exotic fishes such as Tilapia sp., Cvprinus. The average catch per day has gone down to 6.55 kg. Such a drastic depletion in the per day catch of fish reflected the poor productivity.

FISHING GEARS:

Fishery aspects of Powal Lake is under the control of Maharashtra State Angling Association and only sport fishing permitted in the lake waters. The various gears used are lure, hook, leader, line, reel etc. The gill and cast nets are used indiscriminately by poachers.

INTERRELATIONSHIPS AND CONCLUSION:

The physico-chemical parameters studies clearly indicate that the main factor presumed to be responsible for the decline of fisheries in Powai Lake are Luxurious growth of aquatic weeds, aquatic pollution, hich lead concentration, predation, over population of exotic species, low fertility, siltation.

RECOMMENDATIONS:

(a) Removal light interrupting floating weeds.

(b) Removal of small low value fishes.

(c) Introduction of predatory fishes like Lates calcarifer, local estuarine fish to check the multiplication of weed fishes and Tilapia mossambica.

(d) Introduction of Fresh water prawns.