Figure 21. — Bottom water temperature (mean and range), river discharge, and salinity in Galveston Bay, 1966. 



night. During the day, bottom tows averaged 

 0.85 shrimp per minute towed, and surface 

 tows, 0.01; at night bottom hauls averaged 

 0.22 shrimp per minute and surface hauls, 

 1.21. Mean lengths of shrimp, regardless of 

 time or depth, were similar. 



White Shrimp 



More white shrimp were caught in bottom 

 tows than in midwater tows during the study. 

 Large numbers of shrinnp were caught in sur- 

 face tows on two sampling dates, but they 

 were absent in the surface tows during the 

 day on all other sampling dates. Statistical 

 tests showed that catch per unit of effort in- 

 creased significantly from surface to bottom 

 during the day. Vertical distribution at night 

 was not determined because ebb tides usually 

 occurred during the day in the fall and winter 

 months. 



Peaks of white shrimp ennigration were 

 correlated closely with rapidly decreasing 

 tennperatures and salinities. The first peak of 

 emigration occurred October 19, when daily 

 mean water temperature dropped from 24° to 

 19° C. (75.2° - 66.2° F.). The next three peaks 

 of emigration also coincided with drops in 

 temperature. For the rest of the season, how- 

 ever, the relation was not clear, probably be- 

 cause temperature changes were not as great 

 and because emigration was more nearly 

 constant. Emigrationoccurred at temperatures 

 between 19° and 8° C. (66.2° - 46.4° F.). 



The mean lengths of shrimp caught on the 

 same date in surface, midwater, and bottom 

 tows, and between sexes, were similar. Size 

 at emigration decreased significantly with, and 



was closely related to, a decrease in water 

 temperature. There was no obvious relation 

 between salinity and size of emigrating 

 shrimp. 



Of 2,964 white shrimp caught in the tidal 

 pass, 1,633 (55.1 percent) were females. The 

 sex ratio did not deviate significantly from a 

 1:1 ratio. 



Character for Identification of Postlarval 

 Penaeid Shrimp 



Studies of the biology of postlarval brown, 

 pink, and white shrimp have been hampered 

 by difficulties in differentiating between 

 species. For postlarval shrimp 10 mm. (0.4 

 inch) total length or less, published descrip- 

 tions, provisional keys, seasonal occurrences, 

 and size differences have been used success- 

 fully to separate postlarvae of the brown and 

 white shrinnp entering Galveston Bay. The 

 problem remained, however, of species dif- 

 ferentiation among shrimp between the lengths 

 of 10 mm. (0. 4 inch) and 25 mm. (1.0 inch). 



In examining postlarvae from Galveston Bay, 

 we noted a morphologic character by which 

 we could distinguish brown from white shrimp 

 at total lengths from 10 to 25 mm. (0.4 - 1.0 

 inch): White shrimp lack spines on the dorsal 

 carina, whereas brown shrimp have spines on 

 the dorsal carina of the sixth abdominal seg- 

 ment. This character is especially valuable in 

 areas such as Galveston Bay where pink shrinnp 

 are rare. 



On the basis of the examination of shrimp 

 of known parentage, we deternnined that the 

 number of spines on the dorsal carina cannot 

 be used to separate postlarvae of brown and 



26 



