ORIGIN AND DECLINE OF THE ESTUARINE CLAM RANGIA-CUNEATA IN THE NECHESRIVER, TEXAS

The origin and decline of the brackish water calm, Rangia cuneata (Gra y, 1831), in the Neches River were investigated by records of navigati on improvements, salt water encroachment, water quality, and demograph ic data. The origin was probably very recent (since 1900) resulting fr om construction and improvements to the deep water navigation channel. These modifications formed a suitable salinity environment and allowe d the planktonic larvae to be carried upriver from Sabine Lake. By 195 1, after industrialization along the navigation channel, all Rangia be ds located below river km 40.5 had been eliminated by wastewater efflu ents and frequent dredging. In 1971, the Rangia population consisted o f 45 beds located between river km 40.5 and 57.3. The average density was 238 clams/m2 and the total area of the beds was 113, 115 m2. Since 1971, many Rangia beds have disappeared and all remaining beds examin ed exhibited decreased density to <1 to 2 clams/m2 with an increase in average clam size. These changes were due to alterations in the river discharge pattern that decreased the frequency of salt water intrusio n required for spawning and survival of larvae, and natural and cold w eather mortalities. By 1985, the permitted BOD waste load in die lower river had been reduced 96%, but Rangia has not yet recolonized this s ection of the river.