The fish community of lake Sone is diverse with 70 species of different size classes. Diversity of fish species is also reflected in the different types of zoogeographical groups. Population dynamics of the Indian Major Carps (IMC), clupeids, silurids and bagrids have been dealt in this paper due to their commercial importance and significant contribution to overall fish production of the lake.
The population of IMC, as a group, fluctuated with a single peak in December during the first session and two peaks, one each in September and January, during the second session. The rich landing during the dry season is essentially due to high catch at low water level (WL) and the monsoon influx could be due to more IMC migration into the Lake from the adjoining rivers. Cirrhinus mrigala portrays two peaks, one each in August and January. Labeo calbasu has rich population in September and little higher population in January. L. gonius exhibits peaks in July and in September. L. rohita depicts peaks in August and in January. P. chola, among the minor carps, is a dominant species in the lake contributing significantly to the total fish production.
The Clupeids of the Lake consists of Gudusia chapra and Hilsa ilisha. Population dynamics of the group behaved uniformly during both the sessions. The seasonal fluctuation of the group is essentially due to G. chapra with peaks in August and in December. H. ilisha is an anadromous migrant into Sone ecosystem in monsoon season only unlike two (monsoon and winter) in other freshwater bodies.
The siluroids show uniform trend in their dynamics. Among the two species in this group, Ompok bimaculatus displays single peak in January while, Wallago attu has maximum population in August and in January .
The bagrids, as a group, shows peaks in October and in February in the first session and, August and November in the second session. Among the bagrids , Mystus vittatus is the most dominant fish which had accounted almost entirely for the dynamics of the whole group and depicted a peak each in July - August and November. Aorichthys seenghala has little high population trend during April, June, January and February months.
Both H' and D follow same pattern of fluctuation with peaks in monsoon season. This lake is highly diverse during monsoon in both sessions with a rising trend beginning from July. It is seen that, species like P.chola, G.chapra, O.bimaculatus and M.vittatus contributed significantly for the high production in monsoon season. During these two sessions, there had been no sign of decreasing trend in diversity under stressed condition However, the decreasing trend in diversity indices during other seasons were mainly due to competition and intense fishing.
Before 1950, lake Sone had not been subjected to much man-made disturbances and it exhibited high species diversity in a stable environment; and, possibly because of this, there had not been evolution of finer specialisation and adaptations which resulted in smaller niches and more species occupying a unit space of habitat. It could also be argued that the high diversity of lake Sone fish communities has not been achieved only by precise niche' diversification in an equilibrium community but rather by highly variable larval recruitment which causes a competition for vacant living spaces in which the first to arrive wins. Since 1950, after the influx of large number of fishermen from adjoining country (Bangladesh) to lake Sone, the biotope had turned into a disturbed environment due to tremendous fishing and agricultural activities, which have been exerting substantial influence on the fish population of the Lake. In view of these, Sone ecosystem of today could be regarded as a disturbed ecosystem but, at the same time, it has been preserving its rich fish diversity as compared to many undisturbed ecosystems where competition and exclusion are more intense.
It may be noted here that the introduction of common carp (Cyprinus carpio) into Sone, about two decades ago, may have significant impact on the diversity of indigenous fish species in future. Aim of this exercise was to transform slow growing fisheries into a rapid one and also to compensate for the loss caused to the fishery of the Lake by the construction of a blind dam and then its replacement by a lock gate (without fish passes and fish ladders) over Kachna (the major outlet) during 1951 and 1964 respectively. These hindrances have reduced the fresh water pulse and might have triggered the deviation of the migratory path of the fishes and take a lengthier route in order to enter Lake Sone. However, a serious effect of this single species introduction leads to inability to maintain genetic diversity inherent in naturally reproducing populations.