Gulf menhaden enter estuaries as larvae. Immigra- 

 tions of larvae occur along the Louisiana and upper 

 Texas coasts from November through April (Gunter 

 1956, Suttkus 1956, Arnold et al. 1960, Fore 1970, 

 Herke 1971, Fore and Baxter 1972, Sabins 1973, 

 Sabins and Truesdale 1974, Tarbox 1974). At the tidal 

 inlets of the Chenier Plain and immediate adjacent 

 areas, peaks of immigration have most frequently 

 occurred in December to March (Herke 1971, Fore and 

 Baxter 1972,Arnoldi 1974). 



Larval and postlarval Gulf menhaden move rapidly 

 to the interior portions of the estuaries. As they increase 

 in size, they spread throughout the estuaries, becoming 

 ubiquitous by the time they have attained juvenile size 

 (about 30 mm or 1.2 in standard length) (Suttkus 

 1956). The young menhaden generally remain in the 

 estuaries for about one year (Combs 1969). Adults 

 move out of the bays and inhabit the nearshore Gulf 

 and adjacent slightly deeper waters throughout the 

 spring and summer. These shallow coastal areas (less 

 than 10 fm) are the focus of the summer fishery, which 

 consists largely of 1- to 2-year-old fish (Reintjes 

 and June 1961, Chapoton 1970). 



Gulf menhaden are euryhaline and inhabit fresh to 

 saline waters (salinities as high as 60%o) (Gunter and 

 Christmas 1960). Copeland and Bechtel (1971) suggest- 

 ed that the marked abundance of these species in 

 extreme upper portions of estuaries is related to low 

 salinities and abundant food sources. Temperature 

 tolerance in juvenile Gulf menhaden is also quite broad, 

 especially in low salinities (Copeland and Bechtel 

 1971). Nevertheless, shock caused by abrupt tempera- 

 ture drops during relatively severe cold weather some- 

 times induces mass mortalities of juvenile menhaden. 



5.6 SHELLFISH 



5.6.1 RANGIA CLAM (Rangia cuneata) 



Rangia is a burrowing clam, but not a very active 

 one. With the exception of short-range burrowing and 

 locomotion by adults, the mass movement of individ- 

 uals occurs during the free-swimming larval stage (a 

 7-day period from fertilization to setting). At that 

 time the principal transportation is provided by water 

 currents. Larv^ stages may occur in all seasons, but 

 are most abundant during the warmer months of the 

 year when water temperatures are above 12° C (57° 

 F) (Hopkins et al. 1973). 



Tarver and Dugas (1973) sampled areas of Lake 

 Pontchartrain and Lake Maurepas (outside the study 

 area), and found rangia on sand and silty clay bot- 

 toms. Tenore et al. (1968), Gooch (1971), and Cain 

 (1972) found larger rangia inhabiting sandy bottom 

 areas. The several explanations are that large-size par- 

 ticles trapped more food, sand substrata facilitated 

 burrowing, and excretions did not accumulate. Hop- 

 kins et al. (1973) reported that along the Texas coast, 

 rangia was often found in muddy substrates, but was 

 also present in combinations of sand, silt, and clay. 



Rangia is usually a dominant species in salinities 

 up to 15%o. Tarver and Dugas (1973) found the high- 

 est concentration of all sizes of rangia adjacent to 

 either a source of fresh or salt water. In those envi- 

 ronments, the clam is subjected to salinity shock, 

 which is an important requirement for reproduction. 



Examples of the sensitivity of rangia to environ- 

 mental change have been reported for the Chenier Plain 

 by several authors. Hopkins et al. (1973) described one 

 example. White Lake, in the southeastern portion of 

 the Mermentau Basin, supported a large rangia popula- 

 tion. Studies by Gunter and Shell (1958) showed many 

 living rangia in this region in 1952. By 1971, Hoese 

 (1972) and his helpers could find no live rangia, 

 although Gooch( 1971) had found a few clams surviving 

 in 1969. Hoese (1972) attributed the disappearance of 

 the White Lake rangia population to the control struc- 

 ture built in 1951 to prevent saltwater intrusion into 

 the lake. It apparently took 19 years for all rangia to 

 die after the construction of the control structure. 



This change could have been avoided by allowing 

 a controlled periodic influx of brackish water. Main- 

 tenance of the rangia populations would have re- 

 quired only a pulse of saline water every few years to 

 a level of about 5%o for less than a month in order to 

 induce reproduction and spawning. 



An abundance of shells in Calcasieu Lake indi- 

 cates the former existence of a substantial population 

 of rangia, but in 1971 and 1972, Hoese (1972) could 

 find no live clams in the lake. Kellog(1905) substan- 

 tiates that rangia were at one time abundant in upper 

 Calcasieu Lake. Hoese (1972) attributed the apparent 

 extermination of rangia in Calcasieu Lake to the 

 higher salinities (15%o to 26%o) caused by the salt- 

 water intrusion through the Lake Charles Ship Chan- 

 nel. 



Pollution may also limit the abundance and 

 spatial distribution of rangia. Thorson (1957) reported 

 that biological waste buildup prevented larval estab- 

 lishment. 



Adult rangia feed on suspended detritus and phy- 

 toplankton by a filter-feeding process in which food 

 particles are captured on the gills. Until the swimming 

 larvae reach the setting stage, they feed on flagellated 

 unicellular algae (Hopkins et al. 1973). 



The reproductive cycle and stages of rangia are 

 strongly linked to environmental parameters. The 

 clams have mature gonads that produce gametes more 

 than half the year, but they do not spawn continu- 

 ously. An individual, though gravid with gametes, will 

 seldom release them until shocked by a sudden change 

 in temperature, salinity, or botli. Changes, not just a 

 favorable level, are necessary to induce spawning (ei- 

 ther up from or down from 15%o). Hopkins et al. 

 (1973) report that a rise from near to 5%o was the 

 best spawning stimulus, and that a temperature rise 

 from 22° C (72° F) to 34° C (93° F) was also sufficient 

 to induce the release of gametes for external fertiliza- 

 tion. 



260 



