emigration continued heavy through mid-June. 

 It also appears that am early movement of 

 small shrimp to the Gulf started in mid. May 

 of 1966, but peak emigration was not reached 

 until the first week in June. 



When maximunn ennigration from a bay 

 starts early, average size of the shrimp is 

 much smaller than when departure is later. 

 The early movement of small shrimp from the 

 estuary in 1965 might have been caused by 

 the tremendous population density. A similar 

 movement occurred in 1966, however, when 

 shrimp were much less plentiful in the bay. 

 Thus, intrapopulation competition seems un- 

 likely as a cause of early emigration. Flood 

 flows from Trinity River in the spring de- 

 pressed bay salinities lower in 1965 and 1966 

 than in 1963 or 1964. The lower salinity may 

 possibly contribute to earlier return of brown 

 shrimp to the Gulf of Mexico. 



Snnall brown shrimp reside in an extremely 

 complex, constantly changing environment dur- 

 ing their estuarine development. In the Galves- 

 ton estuary, they range from the high-salinity 

 waters of the lower bays to the nearly fresh 

 waters of the bayous and marshes of the upper 

 bays. 



To determine whether these small shrimp 

 nnight be influenced by such environmental 

 variations, their condition (based on length- 

 weight relation) was compared from three 

 areas that are hydrologically different and 

 relatively isolated from each other- -East Bay, 

 Trinity Bay, and upper Galveston Bay south- 

 west of the Houston Ship Channel. Their 

 condition was also compared during the 

 period when they were very numerous in the 

 estuary (April- June) and a later period (July- 

 Decennber) when the population was small. 



Condition differed significantly in the shrimp 

 collected from the three areas during both 

 periods, but the extent of the differences was 

 not constant and it changed with size. The 

 smaller shrimp from Trinity Bay were in 

 better condition than those from East Bay or 

 upper Galveston Bay, whereas the larger 

 shrimp from upper Galveston Bay were in the 

 best condition. Ecological differences in the 

 habitat evidently are reflected in the observed 

 difference in the condition of shrimp. 



Comparison of condition between the two 

 periods did not give clear-cut results. Shrimp 

 of all sizes from Trinity Bay were in better 

 condition during the April-June period of 

 greatest abundance. The condition of shrimp 

 from upper Galveston and East Bays, however, 

 varied between the two periods with size; some 

 size groups were in better condition during 

 the period of greatest abundance and other 

 groups when the population was small. These 

 discrepauncies suggest that the environment 

 and not the density of shrimp affected their 

 condition. 



When postlarvae enter the estuary in the 

 spring, considerable numbers occupy the 



peripheral marshes. Salinity is one of the 

 factors that dictate the type of marsh. Near 

 the Galveston estuary are fresh-water 

 marshes, intermediate marshes of relatively 

 low salinity, transition marshes, and salt 

 marshes where salinity is high. Considerably 

 more small brown shrimp were caught in the 

 marshes where salinity was low than in the 

 high-salinity marshes (fig. 25). 



SAMf>LES WITH SHRIMP 



ALL SAMPLES 



SALINITY l%.) 



Figure 25. — Density of Juvenile brown shrimp In Galveston 

 Bay marshes In relation to salinity, 1965. 



We frequently did not catch small brown 

 shrimp in the low-salinity marsh in the early 

 spring, however. During this spring warming 

 period, this lack of shrimp might be explained 

 by combined effect of salinity and temperature. 

 When temperature is low, salinity must be 

 higher. (See U.S. Fish and Wildlife Service 

 Circular 230, Biological Laboratory, Galves- 

 ton, Tex., fishery research for the year ending 

 June 30, 1964, p. 67.) 



We did not catch postlarval shrimp in 

 marshes where salinity was extremely low 

 (1.0 to 5.9 p.p.t.) until the water temperature 

 reached 23° C, but caught them in marshes 

 where salinity was higher (10.0 to 24.9 p.p.t.) 

 when water temperatures reached 15° C. 

 (fig. 26). Thus, a considerable acreage of 



6 0-9 9 



SALINITY (%.l 



Figure 26. — First occurrence of small brown shrimp in 

 the Galveston estuary marshes during the spring of 

 1965 in relation to water temperature and salinity. 



25 



