PEe MAR APn 



Figure 30.— Relative abundance of juvenile brown shrimp 

 In the Galveston Estuary, Tex., during 1963 and 1964 

 as Indicated from trawl sampling. 



shrimp were concentrated in the areas of 

 lower salinity of the estuary and, as they 

 grew, migrated into the higher salinity environ- 

 ment. More specifically, juveniles of 41-100 

 mm. (1.61-3.94 in.) collected in 1963 were 

 concentrated in salinities of less than 10.0 

 p.p.t., whereas juveniles 101 to 130 mm. (3.95- 

 5.12 in.) were dispersed uniformly throughout 

 the salinity gradient. Shrimp larger than 

 130 mm. (5,12 in.) were concentrated in 

 salinities between 20 p.p.t. and 30 p.p.t, and 

 were not collected in salinities below 10 p.p.t. 

 In 1964, juveniles from 41 to 70 mm. long 

 (1,61-2,76 in,) were concentrated in salinities 

 below 10 p.p.t,, and those from 71 to 115 mm, 

 (2,77-4,53 in,) were concentrated in salinities 

 between 10 and 20 p.p,t. Juveniles between 

 116 (4,57 in,) and 130 mm. were rather 

 evenly distributed throughout the salinity gra- 

 dient. Those larger than 130 mm. (5.12 in.) 

 were caught mostly in salinities between 10 

 and 20 p.p.t. As in 1963, these larger shrimp 

 were not caught in salinities below 10 p,p.t. 

 During 1964, however, salinities in the estuary 

 seldom exceeded 30 p,p,t. 



It is not known to what extent salinity itself 

 affects or controls the distribution of brown 

 shrimp within the Galveston estuary. The 

 immigration of postlarvae into the peripheral 

 marshes, bayous, and smaller bays and sub- 

 sequent movement of juveniles to the estuary's 

 shore zones and open waters and thence to the 

 lower bays and tidal pass can explain the 

 apparent salinity- size relation. This pattern 

 of movement involves the concentration of 

 small shrimp in the estuary's peripheral areas 

 that are generally less saline than the adjacent 

 bays and the subsequent passage of the larger 

 juveniles toward the Gulf through waters of 

 ever-increasing salinity. 



Bottom Fauna Study--Distribution and 

 Relative Abundance of Rangia Cuneata 



The first phase of an ecological study of 

 the Galveston estuary was to describe the 

 distribution and relative abundance of moUusks 

 within the estuary. Ninety-three species of 



moUusks were collected and identified. A 

 dredge was towed at each of 65 locations 

 once during winter, spring, summer, and 

 fall in 1964. Sampling locations were dis- 

 persed throughout the estuary (see U.S. Fish 

 and Wildlife Service Circular 183, p. 51) to 

 include each of the major types of environ- 

 ment. The composition of bottom sediments 

 and many of the estuary's hydrological char- 

 acteristics were also determined. Detailed 

 evaluation of these data will require con- 

 siderable time, although a preliminary ap- 

 praisal has yielded meaningful results. 



A portion of the data relating to the clam, 

 Rangia cuneata , and the conclusion derived 

 therefrom are presented here as an example 

 of the information we have obtained for each 

 of the 93 species of moUusks in the Galveston 

 estuary. Rangia cuneata Gray is one of the 

 more important components of the estuarine 

 fauna. It inhabits many of the Gulf estuaries 

 where it is an important food for fish, crabs, 

 and shrimp. During 1964, the distribution and 

 relative abundance of this animal in Galveston 

 estuary varied considerably. In the winter it 

 was caught in less than 5,0 percent of the 

 estuary. This figure probably is biased be- 

 cause when water temperatures aredepressed, 

 the clam evidently buries deep into the bottom 

 sediment and out of reach of our collecting 

 gear. During spring and summer, however, 

 numbers of this clam increased and by late 

 summer it was found in about one-third of the 

 entire estuary (fig, 31), 



Significantly, this animal was generally re- 

 stricted to those areas of the estuary that 

 received large volumes of river water or 

 marsh drainage--the areas of low and inter- 

 mediate salinity. The clam was abundant only 

 in the lowest salinity portions of the estuary. 

 It was not collected where the average annual 

 salinity exceeded 18.0 p,p,t, (fig, 32), 



Rangia was found in every major sediment 

 type in the estuary and thus occupied sediments 



AC'HTO OlVffi 



Figure 31, — Distribution and relative abundance of Rangia 

 cuneata In Galveston estuary during late summer 1964. 



35 



