e - poz 2 - Central Institute of Brackishwater Aquaculture

The study reveals that the Thanneennukkom barrier commissioned in 1975 across thelake has consi&rably altered the ecology and fishery resources 01 the lake sector south 01the barrage. The mapr changes observed during the post-barrage phase in comparison tothat of the p&anage phase were the following:A marked reduction m the depth due to the accumulation of the silt lrom the incomingrrvers, bringrng down the water holding capacity of the lake.The trdal mfluence reduced from 80 km upstream to 40 km till the barrage dunng thesummerThe Increase m weed growth and a severe restrtctron on the natural flushrng of thepollutants enterrng the sector dunng the summer months when the barrage nmarnrclosedChanges rn spatla1 and temporal varlatrons rn the water and redrment qualrtv - a \tt.rpreductron rn the salrnrty, an elevatron of nutrrent level in the water phase ar~d dl1elevatron of the organlc carbon m the sedrment The changes were more pronounred rnthe southern end of the sector, where an eutrophrcatron tendency exrsted, geared hv thestagnancy consequent to the closure of the barrage durrng December to AprrlChanges rn the composrtion of the plankton and benthic organisms, the salrnc. spec.rtAslargely grvrng way to euryhalrne or freshwater species.An enhancement m the prrmary productron levels at the south end of the scat tor clurrn~the stagnant monthsThe reductron rn the upstream and downstream migration of fish and prawns. The stretrhof the lake above the barrage becoming inaccessible to the euryhalrne mrgratory frsherand prawns lrom the sea, the fish productivity of the sector dwrndled About 13 sucheuryhalrne migratory specres no more contribute to the fishew of this sector. Srntrlarlyabout 16 stenohalrne freshwater species found an entry to the commercral fishery of theSector The pnzed fishery of Macrobrachiurn rosenbergii got reduced to about 35% of rtslandings durrng the pre-barrage phase (196365).