marsh might not be available immediately as 

 nursery area for brown shrimp during a late 

 spring. 



Many types of habitat are available to post- 

 larval and juvenile brown shrimp in an estuary. 

 Some are obviously more suitable than others. 

 In view of the vast physical changes that are 

 occurring to the estuaries, we must know which 

 parts of an estuarine system are most inn- 

 portant. Intensive sampling in the marshes and 

 bayous, along the shoreline (surf zone), near 

 shore, in open bays, and in dredged channels 

 has revealed that small brown shrimp have a 

 decided preference for one part of an estuarine 

 system over ainother (fig. 27). Not unexpectedly, 



Figure 27. — Relative Importance of different types of 

 habitat In the Galveston estuary as nursery areas for 

 juvenile brown shrimp, March-August 1965. 



a concrete bulkhead which drastically altered 

 a habitat formerly similar to the first (fig. 28). 

 Both areas had similar salinity aind water 

 temperatures but differed in the amount of 

 orgcinics in the bottom sediments and water 

 depth at the shoreline. 



Large amounts of organic detritus were 

 present along the natural shoreline in anarrow 

 band adjacent to the vegetation. The bulk- 

 headed shoreline contained little organic 

 detritus and no vegetation, and was deeper. 

 Intensive sampling over a 10-month period 

 along the shorelines and at 15.2 m. aind 30.5 m. 

 from the shorelines of each area (equal 

 sampling effort in each area) disclosed that 

 76 percent of all postlarvae and 64 percent 

 of all juvenile brown shrimp were in the 

 natural, unaltered area. Even more striking, 

 98 percent of all postlarvae and 87 percent 

 of all juvenile white shrimp were caught at 

 the unaltered area. 



Comparison of catches from the shoreline 

 and from stations 15.2 and 30.5 m. from 

 shore for both altered and unaltered areas 

 proved that the natural, unaltered shoreline 

 zone adjacent to the vegetation was by far 

 the preferred habitat for brown and white 

 shrinnp of all sizes. 



Obviously, bulkheading a natural produc- 

 tive shoreline practically destroys its 

 suitability as a nursery area for young 

 shrimp. 



Blue Crab Study 



the edge, or marshes and bay shorelines, 

 of the estuary harbored a denser population 

 of shrimp than elsewhere. Dredged channels 

 were alnnost barren of the smaller shrimp. 



Natural and Altered Habitats for 

 Penaeid Shrimp 



Because bays are the nursery area for 

 commercially important shrimp of the Gulf 

 of Mexico, it is important to evaluate accu- 

 rately the effects of man's increasing activities 

 on the suitability of this environment for young 

 shrimp. This study illustrates one way in 

 which structural modification of a natural 

 shoreline may have detrimental effects. 



Two areas in Clear Lake, a part of the 

 Galveston estuary, were selected for com- 

 parison. One area included an unaltered, 

 vegetated shoreline, and nearby, the other 



Specimens for our study of the life history 

 of the blue crab in the Galveston estuary 

 were obtained from routinely collected trawl 

 samples. Hydrological measurements were 

 obtained concurrently with the trawl sam- 

 ples. Crabs were weighed and measured, 

 and sex was determined. The resulting data 

 were analyzed for length-weight, length- 

 frequency, and size-salinity relations for both 

 sexes. 



Male crabs were heavier at equal lengths 

 (carapace widths) tham the females. Crabs 

 at equal lengths caught in the lower high- 

 salinity bays near the Gulf were in better 

 condition (heavier) than those caught in the 

 upper, low-salinity bays. Smaller crabs were 

 concentrated in low-salinity areas, whereas 

 the larger crabs were caught most frequently 

 in the areas of higher salinity. 



The upper low-salinity bays of the Galves- 

 ton estuary, together with their contiguous 

 marshes and bayou systems, are nursery 

 areas for the juvenile crabs, whereas the 



26 



