Distribution and Density of Juvenile 

 Brown Shrimp 



Because the brown shrimp is the most 

 valuable species harvested commercially from 

 Texas coastal waters, we are devoting con- 

 siderable effort toward study of its inshore 

 life history. Partial evaluation of data col- 

 lected during 1963 and 1964 permits us to 

 describe this shrimp's distribution and rela- 

 tive abundance within the Galveston estuary. 

 We are still processing data collected since 

 January 1965. 



The behavior exhibited by brown shrimp in 

 the estuary was similar in 1963 and 1964. Post- 

 larval brown shrimp, after entering the estuary 

 from the Gulf, nnigrated toward the upper 

 bays, predominately through the Houston Ship 

 Channel, Intracoastal Waterway, and deeper 

 waters of the open bays. Within 2 wk. they 

 were dispersed throughout the estuary and 

 were concentrating in their primary nursery 

 areas--the peripheral marshes, bayous, and 

 smaller bays (fig. 29). They remained in these 

 protected waters fronn 2 to 4 wk., growing 

 rapidly, until they developed into juveniles. 

 Thereafter, they began to disperse throughout 

 the estuary before returning to the Gulf. Dif- 

 ferences between temperature cycles in 1963 

 and 1964 during the time of postlarval influx 

 and initial growth suggest that the duration 

 of this phase of the shrimp's life history is 

 regulated in large part by the water tem- 

 perature. 



The dispersion of juveniles throughout the 

 estuary followed a rather distinct pattern. 



The first juveniles were caught in 1963 during 

 semimonthly trawl sampling in early April in 

 East Bay and Trinity Bay near extensive 

 marshes where postlarvae were concentrated. 

 Within 2 wk. they had dispersed practically 

 throughout the estuary and by late May had 

 begun to return to the Gulf. The same general 

 behavior was evident in 1964 even though 

 trawl sampling was conducted only once each 

 month. During May, when juveniles were 

 most abundant and dispersed throughout the 

 estuary, they were concentrated once again 

 in the shore zones near the nnarshes. Much 

 of the detail of their dispersion, however, 

 was not obtained by the limited monthly 

 sampling. By July of 1963 and 1964, most 

 of the brown shrimp had returned to the 

 Gulf. 



Striking differences in the relative abundance 

 of brown shrimp in the Galveston estuary were 

 evident between 1963 and 1964 (fig. 30). Con- 

 siderably fewer shrimp were caught in 1964, 

 and the subsequent offshore catch was also 

 proportionately lower. This correlation sug- 

 gests the possibility of estimating future pro- 

 duction in the Gulf by determining the density 

 (relative abundance) of the juvenile population 

 harbored in the estuary. Our studies indicate 

 that this method of projection has merit and 

 should be tested over a longer period. Rather 

 intensive sampling would be required in the 

 estuary's shore zones adjacent to thennarshes 

 during April, May, and June. 



The relation between the density of juvenile 

 brown shrimp and salinity during 1963 and 

 1964 was distinct. Generally, the smaller 



^5^, 



8l^«^^^''^>^ 



Figure 29. — Sampling In the estuary's peripheral marshes with device developed by 

 project personnel. Such locations provide critical primary nursery areas for small 

 fish and shrimp. 



34 



